Nature of Operations and Summary of Significant Accounting Policies - Net Income (Loss) Per Share and Fair Value (Details) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) $ / shares shares	Jun. 30, 2017 USD ($) $ / shares shares	Mar. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Sep. 30, 2016 USD ($) $ / shares shares	Jun. 30, 2016 USD ($) $ / shares shares	Mar. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) segment $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares
Net Income (loss) Per Common Share
Number of outstanding options not included in computation of Diluted EPS (in shares)									284,000	2,077,000	505,000,000
Numerator
Net income (loss) | $	$ 289,928	$ 53,716	$ 50,477	$ 34,786	$ (270,244)	$ 41,322	$ 40,244	$ 27,092	$ 428,907	$ (161,586)	$ 117,817
Denominator
Denominator for basic earnings per-share weighted average shares	51,811,000	51,833,000	51,751,000	51,820,000	51,757,000	51,627,000	51,418,000	51,218,000	51,804,000	51,505,000	51,077,000
Dilution due to stock options and restricted stock units (in shares)									1,296,000		748,000
Weighted average number of shares for diluted net income per common share	53,140,000	53,080,000	52,977,000	53,202,000	51,757,000	52,471,000	52,194,000	52,014,000	53,100,000	51,505,000	51,825,000
Basic earnings per-share (in dollars per share) | $ / shares	$ 5.60	$ 1.04	$ 0.98	$ 0.67	$ (5.22)	$ 0.80	$ 0.78	$ 0.53	$ 8.28	$ (3.14)	$ 2.31
Diluted earnings per-share (in dollars per share) | $ / shares	$ 5.46	$ 1.01	$ 0.95	$ 0.65	$ (5.22)	$ 0.79	$ 0.77	$ 0.52	$ 8.08	$ (3.14)	$ 2.27
Fair Value of Financial Instruments
Fair value of long-term debt | $	$ 2,698,400				$ 2,566,500				$ 2,698,400	$ 2,566,500
Total long-term debt | $	$ 2,712,350				$ 2,570,904				$ 2,712,350	$ 2,570,904
Segment Reporting
Number of operating segments | segment									3