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Summary of Significant Accounting Policies (Details 5) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Stock-based compensation costs      
Time period of historical volatility of stock used to estimate expected volatility 5 years 5 months 12 days    
Expected dividend yield assumption (as a percent) 0.00%    
Granted (in shares) 1,662,035    
Accrued salaries and wages $ 69,671   $ 84,034
Stock based compensation expense $ 2,100 $ 2,600  
Weighted-average assumptions used in the Black-Scholes option-pricing model      
Risk-free interest rate (as a percent) 1.54%   1.68%
Expected volatility (as a percent) 36.93%   44.80%
Weighted-average expected life (years) 5 years 5 months 12 days   5 years 5 months 12 days
Phantom Share Units (PSUs)      
Stock-based compensation costs      
Accrued salaries and wages $ 7,000   $ 8,200
Total compensation cost related to nonvested awards not yet recognized $ 26,200    
Period for recognition of unrecognized compensation cost 2 years 3 months 18 days    
Stock based compensation expense $ 4,500 1,400  
Amounts paid on cash settled awards $ 5,200 5,500  
Phantom Share Units (PSUs) | Minimum      
Stock-based compensation costs      
Vesting period 3 years    
Phantom Share Units (PSUs) | Maximum      
Stock-based compensation costs      
Vesting period 5 years    
Stock Appreciation Rights (SARs)      
Stock-based compensation costs      
Vesting period 4 years    
Accrued salaries and wages $ 9,100   $ 6,300
Total compensation cost related to nonvested awards not yet recognized $ 8,700    
Period for recognition of unrecognized compensation cost 2 years 11 months 1 day    
Stock based compensation expense $ 4,600 300  
Amounts paid on cash settled awards $ 1,800 $ 500