Earnings per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Earnings per Share
Income from continuing operations	$ 13,890	$ 13,092	$ 26,841	$ 25,986
Less net income allocated to non-controlling interests		(10)		(21)
Less net income allocated to participating securities:
Nonforfeitable dividends on participating securities	(91)	(91)	(189)	(185)
Total net income allocated to participating securities	(91)	(91)	(189)	(185)
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)
Income from continuing operations available to common stockholders	12,981	12,173	25,016	24,144
Discontinued operations:
Net Income from discontinued operations	27	21	52	43
(Loss) gain on sale of assets, net	(1,014)		(1,014)	16
Net (loss) income from discontinued operations	(987)	21	(962)	59
Net income available to common stockholders	11,994	12,194	24,054	24,203
Net income for diluted net income per share	$ 11,994	$ 12,194	$ 24,054	$ 24,203
Shares for basic net income per share	32,913	30,213	31,645	30,201
Effect of dilutive securities:
Stock options (in shares)	33	45	34	45
Shares for diluted net income per share	32,946	30,258	31,679	30,246
Basic net income per share (in dollars per share)	$ 0.36	$ 0.40	$ 0.76	$ 0.80
Diluted net income per share (in dollars per share)	$ 0.36	$ 0.40	$ 0.76	$ 0.80