financial measure) was $3.07 for the year ended December 31, 2013.  

 

Salaries and Related Costs . The increase in salaries and related costs is primarily due to investments in our employees. During the June 2012 quarter, we reached an agreement with the Air Line Pilots Association ("ALPA") that increased pay and benefits for our pilots. Our pilots and substantially all other employees received base pay increases on July 1, 2012 and received additional increases on January 1, 2013. These increases are designed both to recognize changes to the profit sharing program described below and to accelerate the planned 2013 pay increase for non-pilot employees.  

 

Aircraft Maintenance Materials and Outside Repairs. Aircraft maintenance materials and outside repairs consists of costs associated with maintenance of aircraft used in our operations and costs associated with maintenance sales to third parties by our MRO services business. The reduction in aircraft maintenance materials and outside repairs is primarily due to a lower volume of sales to third parties of our MRO services and the timing of maintenance events on our fleet.  

 

Depreciation and Amortization. Depreciation and amortization expense increased year-over-year primarily due to our investment in new B-737900ER and CRJ-900 aircraft, the purchase of aircraft off-lease, and aircraft modifications that upgraded aircraft interiors and enhanced our product offering.  

 

Contracted Services. Contracted services expense increased year-over-year due primarily to the impact of severe winter storms on our operations and costs associated with the 1% increase in capacity.  

 

Landing Fees and Other Rents. Landing fees and other rents increased year-over-year primarily due to our investment in airport facilities.  

 

Profit Sharing. Our broad-based employee profit sharing program provides that, for each year in which we have an annual pre-tax profit, as defined by the terms of the program, we will pay a specified portion of that profit to employees. In determining the amount of profit sharing, the program defines profit as pre-tax profit excluding profit sharing and special items, such as MTM adjustments and restructuring and other items, from pre-tax profit. During the June 2012 quarter, our profit sharing program was modified so that we pay 10% of profits on the first $2.5 billion of annual profits effective with the plan year beginning January 1, 2013 compared to paying 15% of annual profit for the 2012 plan year. Under the program, we will continue to pay 20% of annual profit above $2.5 billion.  

 

Aircraft Rent. Aircraft rent decreased year-over-year due primarily due to the purchase of various aircraft off-lease.  

 

Non-Operating Results   

The decline in interest expense, net is driven by reduced levels of debt and the refinancing of debt obligations at lower interest rates. Our principal amount of debt and capital leases has declined from $14.4 billion at the beginning of 2012 to $9.9 billion at December 31, 2014 .  

 

In each of 2014 and 2012, we extinguished $1.6 billion of existing debt under our secured financing arrangements prior to scheduled maturity. In