Net Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net income									$ 80,872	$ 80,872	$ 155,076	$ 87,340
Less income allocated to non-controlling interests									(346)		(95)
Less income allocated to participating securities:
Non-forfeitable dividends on participating securities									(372)		(357)	(350)
Income allocated to participating securities									(19)		(268)	(12)
Total net income allocated to participating securities									(391)		(625)	(362)
Net income available to common stockholders	$ 12,449	$ 27,080	$ 20,352	$ 20,254	$ 30,645	$ 34,782	$ 68,658	$ 20,271	80,135		154,356	86,978
Effect of dilutive securities:
Participating securities											625	362
Net income for diluted net income per share									$ 80,135		$ 154,981	$ 87,340
Shares for basic net income per share									39,571		39,477	39,409
Effect of dilutive securities: (Shares)
Total effect of dilutive securities (in shares)									188		362	228
Shares for diluted net income per share									39,759		39,839	39,637
Basic (in dollars per share)	$ 0.31	$ 0.68	$ 0.51	$ 0.51	$ 0.78	$ 0.88	$ 1.74	$ 0.51	$ 2.03		$ 3.91	$ 2.21
Diluted (in dollars per share)	$ 0.31	$ 0.68	$ 0.51	$ 0.51	$ 0.77	$ 0.88	$ 1.73	$ 0.51	$ 2.02		$ 3.89	$ 2.20
Stock options
Effect of dilutive securities: (Shares)
Stock options and performance-based stock units (in shares)									4		3	10
Performance-based stock units
Effect of dilutive securities: (Shares)
Stock options and performance-based stock units (in shares)									184		203	67
Participating Securities
Effect of dilutive securities: (Shares)
Participating securities											156	151