of our valuation allowance against our net deferred income tax assets, primarily resulting in the $8.0 billion benefit in our provision for income taxes.  

 

Defined Benefit Pension Plans  

 

We sponsor defined benefit pension plans for eligible employees and retirees. These plans are closed to new entrants and frozen for future benefit accruals. As of December 31, 2013 , the unfunded benefit obligation for these plans recorded on our Consolidated Balance Sheet was 

$10.1 billion . During 2013 , we contributed $914 million to these plans and recorded $354 million of expense in salaries and related costs on our Consolidated Statement of Operations. In 2014, we estimate we will contribute approximately $925 million to these plans, including $250 million of contributions above the minimum funding requirements, and that our expense will be approximately $230 million. The most critical assumptions impacting our defined benefit pension plan obligations and expenses are the discount rate and the expected long-term rate of return on the plan assets.  

 

Weighted Average Discount Rate. We determine our weighted average discount rate on our measurement date primarily by reference to annualized rates earned on high quality fixed income investments and yield-to-maturity analysis specific to our estimated future benefit payments. We used a weighted average discount rate to value the obligations of 5.01% and 4.11% at December 31, 2013 and 2012 , respectively. Our weighted average discount rate for net periodic pension benefit cost in each of the past three years has varied from the rate selected on our measurement date, ranging from 4.10% to 5.70% between 2013 and 2011.  

 

Expected Long-Term Rate of Return. Our expected long-term rate of return on plan assets is based primarily on plan-specific investment studies using historical market return and volatility data. Modest excess return expectations versus some public market indices are incorporated into the return projections based on the actively managed structure of the investment programs and their records of achieving such returns historically. We also expect to receive a premium for investing in less liquid private markets. We review our rate of return on plan asset assumptions annually. Our annual investment performance for one particular year does not, by itself, significantly influence our evaluation. Our actual historical annualized three and five year rate of return on plan assets for our defined benefit pension plans was approximately 9% and 12% , respectively, as of December 31, 2013 . The investment strategy for our defined benefit pension plan assets is to use a diversified mix of global public and private equity portfolios, public and private fixed income portfolios and private real estate and natural resource investments to earn a long-term investment return that meets or exceeds our annualized return target. Our expected long-term rate of return on assets for net periodic pension benefit cost for the year ended December 31, 2013 was 9%.  

 

Funding. Our funding obligations for qualified defined benefit plans are governed by the Employee Retirement Income Security Act. The