Summary of Significant Accounting Policies - Stock-Based Compensation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Stock-based compensation costs
Compensation benefit related to stock-based compensation, pre tax	$ 1,600	$ 2,300	$ 3,100	$ 4,400
Time period of historical volatility of stock used to estimate expected volatility			5 years 4 months 24 days
Dividend yield (as a percent)			0.00%
Granted (in shares)			1,561,035
Accrued salaries and wages	80,744		$ 80,744		$ 98,671
Stock based compensation benefit			$ 4,375	$ 8,036
Weighted-average assumptions used in the Black-Scholes option-pricing model
Risk-free interest rate (as a percent)			1.20%	1.53%
Expected volatility (as a percent)			31.22%	36.86%
Dividend yield (as a percent)			0.00%
Weighted-average expected life			5 years 4 months 24 days	5 years 5 months 12 days
Phantom Share Units (PSUs)
Stock-based compensation costs
Accrued salaries and wages	6,200		$ 6,200		7,800
Total compensation cost related to nonvested awards not yet recognized	12,000		$ 12,000
Period for recognition of unrecognized compensation cost			1 year 6 months 26 days
Stock based compensation expense	600	5,000	$ 3,600	$ 9,500
Amounts paid on cash settled awards	100	100	$ 4,500	5,300
Phantom Share Units (PSUs) | Minimum
Stock-based compensation costs
Vesting period			3 years
Phantom Share Units (PSUs) | Maximum
Stock-based compensation costs
Vesting period			4 years
Stock appreciation rights (SARs)
Stock-based compensation costs
Vesting period			4 years
Accrued salaries and wages	7,700		$ 7,700		$ 8,000
Total compensation cost related to nonvested awards not yet recognized	5,800		$ 5,800
Period for recognition of unrecognized compensation cost			2 years 10 months 21 days
Stock based compensation benefit	500
Stock based compensation expense		2,500	$ 1,400	7,100
Amounts paid on cash settled awards	$ 1,100	$ 500	$ 1,500	$ 2,300