Net Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Net income (loss)	$ 22,411	$ 19,647	$ 64,449	$ 55,127
Less net income allocated to participating securities:
Nonforfeitable dividends on participating securities	(85)	(117)	(284)	(366)
Income allocated to participating securities	(5)	(4)	(12)	(4)
Total net income allocated to participating securities	(90)	(121)	(296)	(370)
Less net income allocated to preferred stockholders:
Preferred stock dividends		(818)		(2,454)
Total net income allocated to preferred stockholders		(818)		(2,454)
Net income available to common stockholders	22,321	18,708	64,153	52,303
Effect of dilutive securities:
Participating securities	90		296
Convertible preferred securities		818		2,454
Total effect of dilutive securities	90	818	296	2,454
Net income for diluted net income per share	$ 22,411	$ 19,526	$ 64,449	$ 54,757
Shares for basic net income per share	39,057	35,341	38,161	35,306
Effect of dilutive securities:
Convertible preferred securities (in shares)		2,000		2,000
Total effect of dilutive securities (in shares)	278	2,011	294	2,013
Convertible preferred securities (in shares)		2,000		2,000
Weighted Average Number Diluted Shares Outstanding Adjustment, Total	278	2,011	294	2,013
Shares for diluted net income per share	39,335	37,352	38,455	37,319
Basic (in dollars per share)	$ 0.57	$ 0.53	$ 1.68	$ 1.48
Diluted (in dollars per share)	$ 0.57	$ 0.52	$ 1.68	$ 1.47
Stock options
Effect of dilutive securities:
Stock options and performance-based stock units (in shares)	13	11	13	13
Effect of dilutive securities:
Stock options and performance-based stock units (in shares)	13	11	13	13
Performance-based stock units
Effect of dilutive securities:
Stock options and performance-based stock units (in shares)	108		108
Effect of dilutive securities:
Stock options and performance-based stock units (in shares)	108		108
Participating Securities
Effect of dilutive securities:
Participating securities	$ 157		$ 173