Net Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net income									$ 155,076	$ 87,340	$ 85,115
Less income allocated to non-controlling interests									(95)
Less income allocated to participating securities:
Nonforfeitable dividends on participating securities									(357)	(350)	(373)
Income allocated to participating securities									(268)	(12)	(12)
Total net income allocated to participating securities									(625)	(362)	(385)
Net income available to common stockholders	$ 30,645	$ 34,782	$ 68,658	$ 20,271	$ 19,753	$ 20,536	$ 25,273	$ 21,416	154,356	86,978	84,730
Effect of dilutive securities:
Participating securities									625	362	385
Net income for diluted net income per share									$ 154,981	$ 87,340	$ 85,115
Shares for basic net income per share									39,477	39,409	38,388
Effect of dilutive securities: (Shares)
Total effect of dilutive securities (in shares)									362	228	209
Shares for diluted net income per share									39,839	39,637	38,597
Basic (in dollars per share)	$ 0.78	$ 0.88	$ 1.74	$ 0.51	$ 0.50	$ 0.52	$ 0.64	$ 0.54	$ 3.91	$ 2.21	$ 2.21
Diluted (in dollars per share)	$ 0.77	$ 0.88	$ 1.73	$ 0.51	$ 0.50	$ 0.52	$ 0.64	$ 0.54	$ 3.89	$ 2.20	$ 2.21
Stock options
Effect of dilutive securities: (Shares)
Stock options and performance-based stock units (in shares)									3	10	13
Performance-based stock units
Effect of dilutive securities: (Shares)
Stock options and performance-based stock units (in shares)									203	67	27
Participating Securities
Effect of dilutive securities: (Shares)
Participating securities									156	151	169