Summary of Significant Accounting Policies - Stock-Based Compensation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Stock-based compensation costs
Compensation benefit related to stock-based compensation, pre tax	$ 1,500	$ 2,000	$ 4,600	$ 6,400
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)			1,573,651
Accrued salaries and wages	80,697		$ 80,697		$ 98,671
Stock based compensation benefit			$ 8,510	$ 10,143
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)			31.23%	36.84%
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	2,900		$ 2,900		7,800
Total compensation cost related to nonvested awards not yet recognized	8,800		$ 8,800
Period for recognition of unrecognized compensation cost			1 year 5 months 12 days
Stock based compensation expense	2,100	1,900	$ 5,700	$ 11,400
Amounts paid on cash settled awards	5,400	7,600	$ 9,900	12,900
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	7,100		$ 7,100		$ 8,000
Total compensation cost related to nonvested awards not yet recognized	4,300		$ 4,300
Period for recognition of unrecognized compensation cost			2 years 8 months 19 days
Stock based compensation benefit		3,000
Stock based compensation expense	400		$ 1,800	4,100
Amounts paid on cash settled awards	$ 1,000	$ 300	$ 2,500	$ 2,600