Net Income Per Common Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Net Income Per Common Share
Income from continuing operations									$ 49,673	$ 45,860	$ 44,473
Less income allocated to non-controlling interests									(191)	(191)	(296)
Less income allocated to participating securities
Non-forfeitable dividends on participating securities									(342)	(230)	(139)
Total income allocated to participating securities									(342)	(230)	(139)
Less income allocated to preferred stockholders:
Preferred stock dividends									(5,512)	(13,662)	(15,141)
Preferred stock redemption charge									(3,566)	(2,383)
Allocation of income from preferred stock buyback											626
Total income allocated to preferred stockholders									(9,078)	(16,045)	(14,515)
Income from continuing operations allocable to common stockholders									40,062	29,394	29,523
Add net income from discontinued operations:
Loss from discontinued operations									(230)	(117)	(113)
Gain on sale of assets, net										310
Net (loss) income from discontinued operations	(32)	(30)	(74)	(94)	256	25	(43)	(45)	(230)	193	(113)
Net income available to common stockholders	11,656	11,472	11,311	5,393	9,583	5,571	7,739	6,694	39,832	29,587	29,410
Effect of dilutive securities:
Convertible preferred securities										40	80
Total effect of dilutive securities										40	80
Net income for diluted net income per share (in dollars per share)									$ 39,832	$ 29,627	$ 29,490
Shares for basic net income per share (in shares)									29,194	24,495	23,099
Effect of dilutive securities:
Stock options (in shares)									28	23	8
Convertible preferred securities (in shares)										50	75
Total effect of dilutive securities (in shares)									28	73	83
Shares for diluted net income per share (in shares)									29,222	24,568	23,182
Basic net income per common share (in dollars per share)	$ 0.39	$ 0.38	$ 0.38	$ 0.20	$ 0.37	$ 0.22	$ 0.33	$ 0.29	$ 1.36	$ 1.21	$ 1.27
Diluted net income per common share (in dollars per share)	$ 0.39	$ 0.38	$ 0.37	$ 0.20	$ 0.37	$ 0.22	$ 0.33	$ 0.29	$ 1.36	$ 1.21	$ 1.27