Net Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Net Income Per Common Share
Income from continuing operations									$ 73,081	$ 73,399	$ 55,405
Less net income allocated to participating securities:
Nonforfeitable dividends on participating securities									(480)	(465)	(381)
Income allocated to participating securities									(4)	(16)	(2)
Total net income allocated to participating securities									(484)	(481)	(383)
Less net income allocated to preferred stockholders:
Preferred stock dividends									(2,454)	(3,273)	(3,273)
Discontinued operations:
Income from discontinued operations											805
Gain on sale of real estate, net											1,605
Net income from discontinued operations											2,410
Net income available to common stockholders	$ 17,840	$ 18,708	$ 16,984	$ 16,611	$ 20,043	$ 16,181	$ 17,338	$ 16,083	70,143	69,645	54,159
Effect of dilutive securities:
Convertible preferred securities									2,454	3,273
Total effect of dilutive securities									2,454	3,273
Net income for diluted net income per share									$ 72,597	$ 72,918	$ 54,159
Shares for basic net income per share									35,590	34,617	33,111
Effect of dilutive securities:
Stock options (in shares)									13	23	31
Convertible preferred securities (in shares)									1,726	2,000
Total effect of dilutive securities (in shares)									1,739	2,023	31
Shares for diluted net income per share									37,329	36,640	33,142
Basic (in dollars per share)	$ 0.49	$ 0.53	$ 0.48	$ 0.47	$ 0.58	$ 0.47	$ 0.50	$ 0.47	$ 1.97	$ 2.01	$ 1.64
Diluted (in dollars per share)	$ 0.48	$ 0.52	$ 0.48	$ 0.47	$ 0.57	$ 0.46	$ 0.50	$ 0.46	$ 1.94	$ 1.99	$ 1.63