attempt to maintain sufficient inventory in response to seasonal cycles of our business ensuring competitive lead times while managing the risk of inventory obsolescence that may occur due to rapidly changing technology and customer demand. 

Accounts payable increased 14.5% from $42.2 million at December 31, 2012 to $48.3 million at December 31, 2013. Accounts payable will fluctuate due to variations in the timing of the receipt of supplies, inventory and services and our subsequent payments for these purchases. 

## Investing Activities 

Capital expenditures totaled approximately $8.2 million, $12.3 million and $11.8 million for the years ended December 31, 2013, 2012 and 2011, respectively. These expenditures were primarily used to purchase computer hardware, software and manufacturing and test equipment. 

On May 4, 2012, we acquired the NSN BBA business. This acquisition provides us with an established customer base in key markets and complementary, market-focused products and was accounted for as a business combination. Upon acquisition, we received a cash payment of $7.5 million from NSN and recorded a bargain purchase gain of $1.8 million, net of income taxes, subject to customary working capital adjustments between the parties during the three months ended June 30, 2012. We are currently negotiating the final working capital adjustments in accordance with the provisions of the underlying purchase agreement. 

Our combined short-term and long-term investments decreased $78.2 million from $493.2 million at December 31, 2012 to $415.0 million at December 31, 2013. This decrease reflects the impact of our cash needs for share repurchases, shareholder dividends, equipment acquisitions, as well as net realized and unrealized losses and amortization of net premiums on our combined investments, partially offset by additional funds available for investment provided by our operating activities and stock option exercises by our employees. 

We invest all available cash not required for immediate use in operations primarily in securities that we believe bear minimal risk of loss. At December 31, 2013 these investments included corporate bonds of $166.9 million, municipal fixed-rate bonds of $136.3 million and municipal variable rate demand notes of $8.3 million. At December 31, 2012, these investments included corporate bonds of $186.4 million, municipal fixed-rate bonds of $175.1 million and municipal variable rate demand notes of $34.4 million. As of December 31, 2013, our corporate bonds, municipal fixed-rate bonds, and municipal variable rate demand notes were classified as available-for-sale and had a combined duration of 0.95 years with an average credit rating of A+. Because our bond portfolio has a high quality rating and contractual maturities of a short duration, we are able to obtain prices for these bonds derived from observable market inputs, or for similar securities traded in an active market, on a daily basis. 

Our long-term investments decreased 7.1% from $332.7 million at December 31, 2012 to $309.2 million at December 31, 2013. Long-term investments at December 31, 2013 and December 31, 2012 included an investment in a certificate of deposit of $48.3 million, which serves as collateral for our revenue bonds, as discussed below. We have various equity investments included in long-term investments at a cost of $24.7 million and $21.0 million, and with a fair value of $38.5 million and $35.2 million, at December 31, 2013 and December 31, 2012, respectively. 

Long-term investments at December 31, 2013 and 2012 also included $15.1 million and $11.5 million, respectively, related to our deferred compensation plan, and $1.7 million and $1.9 million, respectively, of other investments carried at cost, consisting of interests in two private equity funds and an investment in a privately held telecommunications equipment manufacturer. At December 31, 2012, long-term investments also included $0.5 million of a fixed income bond fund. 

We review our investment portfolio for potential “other-than-temporary” declines in value on an individual investment basis. We assess, on a quarterly basis, significant declines in value which may be considered other-than-temporary and, if necessary, recognize and record the appropriate charge to write-down the carrying value of such investments. In making this assessment, we take into consideration qualitative and quantitative information, including but not limited to the following: the magnitude and duration of historical declines in market prices, credit rating activity, assessments of liquidity, public filings, and statements made by the issuer. We generally begin our identification of potential other-than-temporary impairments by reviewing any security with a fair value that has declined from its original or adjusted cost basis by 25% or more for six or more consecutive months. We then evaluate the individual security based on the previously identified factors to determine the amount of the write-down, if any. As a result of our review, we recorded an other-than-temporary impairment charge of $11 thousand during the fourth quarter of 2013 related to four marketable equity securities. For the years ended December 31, 2013, 2012 and 2011 we recorded charges of $25 thousand, $0.7 million and $68 thousand, respectively, related to the other-than-temporary impairment of certain publicly traded equity securities and our deferred compensation plan assets. 

