Net Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net income									$ 95,677	$ 80,872	$ 155,076
Less income allocated to non-controlling interests									(384)	(346)	(95)
Less income allocated to participating securities:
Non-forfeitable dividends on participating securities									(397)	(372)	(357)
Income allocated to participating securities									(25)	(19)	(268)
Total net income allocated to participating securities									(422)	(391)	(625)
Net income available to common stockholders									94,871	80,135	154,356
Effect of dilutive securities:
Participating securities											625
Net income for diluted net income per share									$ 94,871	$ 80,135	$ 154,981
Shares for basic net income per share									39,179	39,571	39,477
Effect of dilutive securities: (Shares)
Total effect of dilutive securities (in shares)									85	188	362
Shares for diluted net income per share									39,264	39,759	39,839
Basic (in dollars per share)	$ 0.45	$ 0.31	$ 0.05	$ 1.60	$ 0.31	$ 0.68	$ 0.51	$ 0.51	$ 2.42	$ 2.03	$ 3.91
Diluted (in dollars per share)	$ 0.45	$ 0.31	$ 0.05	$ 1.60	$ 0.31	$ 0.68	$ 0.51	$ 0.51	$ 2.42	$ 2.02	$ 3.89
Stock options
Effect of dilutive securities: (Shares)
Stock options and performance-based stock units (in shares)										4	3
Performance-based stock units
Effect of dilutive securities: (Shares)
Stock options and performance-based stock units (in shares)									85	184	203
Participating Securities
Effect of dilutive securities: (Shares)
Participating securities											156