Nature of Operations and Summary of Significant Accounting Policies - Net Income Per Common Share and Fair Value (Details) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($) $ / shares shares	Sep. 30, 2019 USD ($) $ / shares shares	Jun. 30, 2019 USD ($) $ / shares shares	Mar. 31, 2019 USD ($) $ / shares shares	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, 2019 USD ($) segment $ / shares shares	Dec. 31, 2018 USD ($) segment $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares
Net Income Per Common Share
Number of outstanding units not included in computation of Diluted EPS (in shares)									150,000	207,000	284,000
Numerator:
Net income | $	$ 72,527	$ 91,339	$ 88,052	$ 88,181	$ 54,362	$ 75,859	$ 83,046	$ 67,105	$ 340,099	$ 280,372	$ 428,907
Denominator:
Basic earnings per share weighted average shares (in shares)	50,395,000	50,746,000	51,145,000	51,440,000	51,921,000	52,046,000	52,039,000	51,650,000	50,932,000	51,914,000	51,804,000
Dilution due to stock options and restricted stock units (in shares)									443,000	957,000	1,296,000
Diluted earnings per share weighted average shares	50,796,000	51,129,000	51,477,000	52,098,000	53,033,000	52,913,000	52,981,000	52,556,000	51,375,000	52,871,000	53,100,000
Basic earnings per share (in dollars per share) | $ / shares	$ 1.44	$ 1.80	$ 1.72	$ 1.71	$ 1.05	$ 1.46	$ 1.60	$ 1.30	$ 6.68	$ 5.40	$ 8.28
Diluted earnings per share (in dollars per share) | $ / shares	$ 1.43	$ 1.79	$ 1.71	$ 1.69	$ 1.03	$ 1.43	$ 1.57	$ 1.28	$ 6.62	$ 5.30	$ 8.08
Fair Value of Financial Instruments
Fair value of long-term debt | $	$ 3,049,100				$ 3,157,300				$ 3,049,100	$ 3,157,300
Total long-term debt | $	$ 3,017,523				$ 3,185,438				$ 3,017,523	$ 3,185,438
Segment Reporting
Number of operating segments | segment										3
Number of reporting segments | segment									2