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, 2018 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2018 USD ($) $ / shares shares	Mar. 31, 2018 USD ($) $ / shares shares	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, 2018 USD ($) segment $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares		Dec. 31, 2016 USD ($) $ / shares shares
Net Income (loss) Per Common Share
Number of outstanding units not included in computation of Diluted EPS (in shares)									207,000	284,000		2,077,000
Numerator
Net income (loss) | $	$ 67,105	$ 83,046	$ 75,859	$ 54,362	$ 289,928	$ 53,716	$ 50,477	$ 34,786	$ 280,372	$ 428,907	[1]	$ (161,586)	[1]
Denominator
Basic earnings per share weighted average shares (in shares)	51,650,000	52,039,000	52,046,000	51,921,000	51,811,000	51,833,000	51,751,000	51,820,000	51,914,000	51,804,000	[1]	51,505,000	[1]
Dilution due to stock options and restricted stock units (in shares)									957,000	1,296,000
Diluted earnings per share weighted average shares (in shares)	52,556,000	52,981,000	52,913,000	53,033,000	53,140,000	53,080,000	52,977,000	53,202,000	52,871,000	53,100,000	[1]	51,505,000	[1]
Basic earnings (loss) per-share (in dollars per share) | $ / shares	$ 1.30	$ 1.60	$ 1.46	$ 1.05	$ 5.60	$ 1.04	$ 0.98	$ 0.67	$ 5.40	$ 8.28	[1]	$ (3.14)	[1]
Diluted earnings (loss) per-share (in dollars per share) | $ / shares	$ 1.28	$ 1.57	$ 1.43	$ 1.03	$ 5.46	$ 1.01	$ 0.95	$ 0.65	$ 5.30	$ 8.08	[1]	$ (3.14)	[1]
Fair Value of Financial Instruments
Fair value of long-term debt | $	$ 3,157,300				$ 2,698,400				$ 3,157,300	$ 2,698,400
Total long-term debt | $	$ 3,185,438				$ 2,712,350				$ 3,185,438	$ 2,712,350
Segment Reporting
Number of operating segments | segment									3

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements of this report for additional information.