Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Net Income Per Common Share
Net income	$ 19,647	$ 17,122	$ 55,127	$ 52,399
Less net income allocated to participating securities:
Nonforfeitable dividends on participating securities	(117)	(123)	(366)	(343)
Income allocated to participating securities	(4)		(4)
Total net income allocated to participating securities	(121)	(123)	(370)	(343)
Less net income allocated to preferred stockholders:
Preferred stock dividends	(818)	(818)	(2,454)	(2,454)
Net income available to common stockholders	18,708	16,181	52,303	49,602
Discontinued operations:
Gain on sale of properties				1,140
Effect of dilutive securities:
Convertible preferred securities	818	818	2,454	2,454
Total effect of dilutive securities	818	818	2,454	2,454
Net income for diluted net income per share	$ 19,526	$ 16,999	$ 54,757	$ 52,056
Shares for basic net income per share	35,341	34,605	35,306	34,596
Effect of dilutive securities:
Stock options (in shares)	11	24	13	24
Convertible preferred securities (in shares)	2,000	2,000	2,000	2,000
Total effect of dilutive securities (in shares)	2,011	2,024	2,013	2,024
Shares for diluted net income per share	37,352	36,629	37,319	36,620
Basic (in dollars per share)	$ 0.53	$ 0.47	$ 1.48	$ 1.43
Diluted (in dollars per share)	$ 0.52	$ 0.46	$ 1.47	$ 1.42