Issuance and Redemption of Preferred Stock (Details) (USD $)	9 Months Ended	0 Months Ended	1 Months Ended		9 Months Ended		1 Months Ended	9 Months Ended
	Sep. 30, 2012	Mar. 15, 2012 Class F Cumulative Redeemable Preferred Stock	Apr. 30, 2012 Class F Cumulative Redeemable Preferred Stock	Feb. 29, 2012 Class F Cumulative Redeemable Preferred Stock	Sep. 30, 2013 Class F Cumulative Redeemable Preferred Stock	Sep. 30, 2012 Class F Cumulative Redeemable Preferred Stock	Mar. 31, 2012 Class D Cumulative Redeemable Preferred Stock	Sep. 30, 2013 Class D Cumulative Redeemable Preferred Stock	Sep. 30, 2012 Class D Cumulative Redeemable Preferred Stock
Issuance and redemption of preferred stock
Shares issued in stock offerings, net of offering costs (in shares)			1,400,000	14,950,000
Preferred stock, dividend rate (as a percent)				6.625%	6.625%	6.625%	7.375%	7.375%
Offering price per share (in dollars per share)			$ 25.2863	$ 25
Preferred stock, purchased by underwriters (in shares)				1,950,000
Underwriting discounts and other offering costs						$ 13,800,000
Net proceeds from preferred stock offering	395,377,000					395,400,000
Redemption of preferred stock	127,500,000					127,500,000
Redemption amount (in dollars per share)					$ 25		$ 25
Dividend paid per initial preferred share of stock (in dollars per share)		$ 0.1702257
Period when initial dividend payments were made		37 days
Dividend paid per preferred share of stock (in dollars per share)					$ 0.138021
Preferred stock redeemed (in shares)							5,100,000
Original issuance costs									$ 3,700,000