Earnings per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Earnings per Share
Income from continuing operations	$ 18,273	$ 13,890	$ 35,277	$ 26,841
Less net income allocated to participating securities:
Nonforfeitable dividends on participating securities	(117)	(91)	(220)	(189)
Total net income allocated to participating securities	(117)	(91)	(220)	(189)
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	17,338	12,981	33,421	25,016
Discontinued operations:
Income from discontinued operations		27		52
Loss on sale of real estate, net		(1,014)		(1,014)
Net loss from discontinued operations		(987)		(962)
Net income available to common stockholders	17,338	11,994	33,421	24,054
Effect of dilutive securities:
Convertible preferred securities	818		1,636
Net income for diluted net income per share	$ 18,156	$ 11,994	$ 35,057	$ 24,054
Shares for basic net income per share	34,597	32,913	34,592	31,645
Effect of dilutive securities:
Stock options (in shares)	24	33	25	34
Convertible preferred securities (in shares)	2,000		2,000
Shares for diluted net income per share	36,621	32,946	36,617	31,679
Basic net income per share (in dollars per share)	$ 0.50	$ 0.36	$ 0.97	$ 0.76
Diluted net income per share (in dollars per share)	$ 0.50	$ 0.36	$ 0.96	$ 0.76