Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 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	$ 48,174	$ 56,747	$ 64,598	$ 62,645	$ 104,921	$ 127,243
Denominator
Weighted average number of common shares outstanding — basic (in shares)	36,169		36,960		36,292	36,970
Effect of dilutive convertible notes (in shares)	507		0		470	0
Effect of dilutive stock options (in shares)	444		459		453	445
Effect of dilutive restricted shares (in shares)	732		749		806	778
Weighted average number of common shares outstanding — diluted (in shares)	37,852		38,168		38,021	38,193
Earnings per common share — basic (in dollars per share)	$ 1.33		$ 1.75		$ 2.89	$ 3.44
Earnings per common share — diluted (in dollars per share)	$ 1.27		$ 1.69		$ 2.76	$ 3.33
Stock Options And Restricted Shares
Denominator
Antidilutive stock options and restricted shares (in shares)	54		25		33	38
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