Net Income Per Common Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Net Income Per Common Share
Income from continuing operations									$ 51,311		$ 49,542		$ 45,595
Less net income allocated to non-controlling interests									(37)		(191)		(191)
Less net income allocated to participating securities:
Non-forfeitable dividends on participating securities									(377)		(342)		(230)
Total net income allocated to participating securities									(377)		(342)		(230)
Less net income allocated to preferred stockholders:
Preferred stock dividends									(3,273)		(5,512)		(13,662)
Preferred stock redemption charge											(3,566)		(2,383)
Total net income allocated to preferred stockholders									(3,273)		(9,078)		(16,045)
Income from continuing operations available to common stockholders									47,624		39,931		29,129
Discontinued operations:
(Loss) gain from discontinued operations											(99)		148
Gain on sale of assets, net									16				310
Net income (loss) from discontinued operations				16	(24)	(25)	(25)	(25)	16		(99)		458
Net income available to common stockholders	11,854	11,583	12,194	12,009	11,656	11,472	11,311	5,393	47,640		39,832		29,587
Effect of dilutive securities:
Convertible preferred securities													40
Total effect of dilutive securities													40
Net income for diluted net income per share									$ 47,640		$ 39,832		$ 29,627
Shares for basic net income per share									30,238		29,194		24,495
Effect of dilutive securities:
Stock options (in shares)									40		28		23
Convertible preferred securities (in shares)													50
Total effect of dilutive securities (in shares)									40		28		73
Shares for diluted net income per share									30,278		29,222		24,568
Basic net income per share (in dollars per share)	$ 0.39	$ 0.38	$ 0.40	$ 0.40	$ 0.39	$ 0.38	$ 0.38	$ 0.20	$ 1.58	[1]	$ 1.36	[1]	$ 1.21	[1]
Diluted net income per share (in dollars per share)	$ 0.39	$ 0.38	$ 0.40	$ 0.40	$ 0.39	$ 0.38	$ 0.37	$ 0.20	$ 1.57	[1]	$ 1.36	[1]	$ 1.21	[1]

[1] NOTE: Computations of per share amounts from continuing operations, discontinued operations and net income are made independently. Therefore, the sum of per share amounts from continuing operations and discontinued operations may not agree with the per share amounts from net income available to common stockholders.