Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Aug. 20, 2018
Earnings Per Common Share [Line Items]
Conversion price (in dollars per share)	$ 101.38						$ 101.38
Numerator — basic and diluted
Net income	$ 50,172	$ 48,174	$ 56,747	$ 60,422	$ 64,598	$ 62,645	$ 155,093	$ 187,665
Denominator
Weighted average number of common shares outstanding — basic (in shares)	35,639			36,617			36,073	36,851
Effect of dilutive convertible notes (in shares)	337			21			425	7
Effect of dilutive stock options (in shares)	402			471			436	454
Effect of dilutive restricted shares (in shares)	708			829			774	795
Weighted average number of common shares outstanding — diluted (in shares)	37,086			37,938			37,708	38,107
Earnings per common share — basic (in dollars per share)	$ 1.41			$ 1.65			$ 4.30	$ 5.09
Earnings per common share — diluted (in dollars per share)	$ 1.35			$ 1.59			$ 4.11	$ 4.92
Stock Options And Restricted Shares
Denominator
Antidilutive stock options and restricted shares (in shares)	83			1			50	24
Senior Notes | Convertible Note Due 2023
Earnings Per Common Share [Line Items]
Interest rate (as a percent)	2.00%						2.00%
Conversion price (in dollars per share)									$ 101.38