Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Carrying amounts of consolidated VIE	$ 180,488,000	$ 211,755,000
Common stock, par value	$ 0.01
Common Stock [Member]
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	88,405,632	86,259,684
Common stock, shares outstanding	88,405,632	86,259,684
Preferred Stock [Member]
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	30,000,000	30,000,000
Series G Cumulative Redeemable Preferred Stock
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock dividend rate percentage	6.875%	6.875%
Preferred stock, shares authorized	4,600,000	4,600,000
Preferred stock, shares issued	4,000,000	4,000,000
Preferred stock, shares outstanding	4,000,000	4,000,000
Preferred Stock Liquidation preference	$ 100,000,000	$ 100,000,000
Series H Cumulative Redeemable Preferred Stock
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock dividend rate percentage	6.375%	6.375%
Preferred stock, shares authorized	4,000,000	4,000,000
Preferred stock, shares issued	4,000,000	4,000,000
Preferred stock, shares outstanding	4,000,000	4,000,000
Preferred Stock Liquidation preference	$ 100,000,000	$ 100,000,000