Net Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net income (loss)									$ 85,115	$ 73,081	$ 73,399
Less net income allocated to participating securities:
Nonforfeitable dividends on participating securities									(373)	(480)	(465)
Income allocated to participating securities									(12)	(4)	(16)
Total net income allocated to participating securities									(385)	(484)	(481)
Less net income allocated to preferred stockholders:
Preferred stock dividends										(2,454)	(3,273)
Total net income allocated to preferred stockholders										(2,454)	(3,273)
Net income available to common stockholders	$ 20,577	$ 22,321	$ 22,075	$ 19,757	$ 17,840	$ 18,708	$ 16,984	$ 16,611	84,730	70,143	69,645
Effect of dilutive securities:
Participating securities									385
Convertible preferred securities										2,454	3,273
Total effect of dilutive securities									385	2,454	3,273
Net income for diluted net income per share									$ 85,115	$ 72,597	$ 72,918
Shares for basic net income per share									38,388	35,590	34,617
Effect of dilutive securities:
Participating securities									$ 385
Convertible preferred securities (in shares)										1,726	2,000
Total effect of dilutive securities (in shares)									209	1,739	2,023
Shares for diluted net income per share									38,597	37,329	36,640
Basic (in dollars per share)	$ 0.53	$ 0.57	$ 0.58	$ 0.53	$ 0.49	$ 0.53	$ 0.48	$ 0.47	$ 2.21	$ 1.97	$ 2.01
Diluted (in dollars per share)	$ 0.53	$ 0.57	$ 0.58	$ 0.53	$ 0.48	$ 0.52	$ 0.48	$ 0.47	$ 2.21	$ 1.94	$ 1.99
Stock options
Effect of dilutive securities:
Stock options and performance-based stock units (in shares)									13	13	23
Performance-based stock units
Effect of dilutive securities:
Stock options and performance-based stock units (in shares)									27
Participating Securities
Effect of dilutive securities:
Participating securities									$ 169
Effect of dilutive securities:
Participating securities									$ 169