Summary of Significant Accounting Policies (Details) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) $ / shares shares	Jun. 30, 2014 USD ($) $ / shares shares	Jun. 30, 2015 USD ($) item $ / shares shares	Jun. 30, 2014 USD ($) $ / shares shares
Numerator:
Net income	$ 53,657	$ 43,535	$ 54,860	$ 40,039
Income from consolidated joint ventures attributable to non-controlling interests	(2,480)	(1,667)	(4,661)	(3,901)
Preferred stock dividends	(2,300)	(2,300)	(4,600)	(4,600)
Dividends paid on unvested restricted stock compensation	(55)	(97)	(113)	(197)
Undistributed income allocated to unvested restricted stock compensation	(133)	(206)	(96)	(87)
Numerator for basic and diluted income attributable to common stockholders	$ 48,689	$ 39,265	$ 45,390	$ 31,254
Denominator:
Weighted average basic and diluted common shares outstanding (in shares) | shares	207,577	182,604	207,091	181,836
Basic and diluted income attributable to common stockholders per common share (in dollars per share) | $ / shares	$ 0.23	$ 0.22	$ 0.22	$ 0.17
Segment Reporting
Number of operating segments | item			1