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As of December 31, 2013, the estimated future amortization expense of intangible assets is as follows: 

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## 9 Alabama State Industrial Development Authority Financing and Economic Incentives 

In conjunction with an expansion of our Huntsville, Alabama, facility, we were approved for participation in an incentive program offered by the State of Alabama Industrial Development Authority (the “Authority”). Pursuant to the program, on January 13, 1995, the Authority issued $20.0 million of its taxable revenue bonds and loaned the proceeds from the sale of the bonds to ADTRAN. The bonds were originally purchased by AmSouth Bank of Alabama, Birmingham, Alabama (the “Bank”). Wachovia Bank, N.A., Nashville, Tennessee (formerly First Union National Bank of Tennessee) (the “Bondholder”), which was acquired by Wells Fargo & Company on December 31, 2008, purchased the original bonds from the Bank and made further advances to the Authority, bringing the total amount outstanding to $50.0 million. An Amended and Restated Taxable Revenue Bond (“Amended and Restated Bond”) was issued and the original financing agreement was amended. The Amended and Restated Bond bears interest, payable monthly. The interest rate is 5% per annum. The Amended and Restated Bond matures on January 1, 2020. The estimated fair value of the bond using a level 2 valuation technique at December 31, 2013 was approximately $45.4 million, based on a debt security with a comparable interest rate and maturity and a Standard & Poor’s credit rating of A-. We are required to make payments to the Authority in amounts necessary to pay the interest on the Amended and Restated Bond. Included in long-term investments at December 31, 2013 is $48.3 million which is invested in a restricted certificate of deposit. These funds serve as a collateral deposit against the principal of this bond, and we have the right to set-off the balance of the Bond with the collateral deposit in order to reduce the balance of the indebtedness. 

In conjunction with this program, we are eligible to receive certain economic incentives from the state of Alabama that reduce the amount of payroll withholdings that we are required to remit to the state for those employment positions that qualify under the program. For the years ended December 31, 2013, 2012 and 2011, we realized economic incentives related to payroll withholdings totaling $1.3 million, $1.4 million and $1.9 million, respectively. 

We did not make a principal payment for the year-ended December 31, 2013. We made a principal payment of $0.5 million for the year ended December 31, 2012. We anticipate making a principal payment in 2014. At December 31, 2013 and 2012, $0.3 million and $0.5 million, respectively, of the bond debt were classified as a current liability in accounts payable in the Consolidated Balance Sheets at December 31, 2013. 

## 10 Income Taxes 

A summary of the components of the provision for income taxes for the years ended December 31, 2013, 2012 and 2011 is as follows: 

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