Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Net Income Per Common Share
Net income	$ 17,928	$ 18,273	$ 35,480	$ 35,277
Less net income allocated to participating securities:
Nonforfeitable dividends on participating securities	(126)	(117)	(249)	(220)
Total net income allocated to participating securities	(126)	(117)	(249)	(220)
Less net income allocated to preferred stockholders:
Preferred stock dividends	(818)	(818)	(1,636)	(1,636)
Total net income allocated to preferred stockholders	(818)	(818)	(1,636)	(1,636)
Net income available to common stockholders	16,984	17,338	33,595	33,421
Effect of dilutive securities:
Convertible preferred securities	818	818	1,636	1,636
Total effect of dilutive securities	818	818	1,636	1,636
Net income for diluted net income per share	$ 17,802	$ 18,156	$ 35,231	$ 35,057
Shares for basic net income per share	35,299	34,597	35,288	34,592
Effect of dilutive securities:
Stock options (in shares)	12	24	14	25
Convertible preferred securities (in shares)	2,000	2,000	2,000	2,000
Total effect of dilutive securities (in shares)	2,012	2,024	2,014	2,025
Shares for diluted net income per share	37,311	36,621	37,302	36,617
Basic net income per share (in dollars per share)	$ 0.48	$ 0.50	$ 0.95	$ 0.97
Diluted net income per share (in dollars per share)	$ 0.48	$ 0.50	$ 0.94	$ 0.96