Issuance and Redemption of Preferred Stock (Details) (USD $)	3 Months Ended		1 Months Ended	3 Months Ended		0 Months Ended	1 Months Ended	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2013	Mar. 31, 2012 Class D Cumulative Redeemable Preferred Stock	Mar. 31, 2013 Class D Cumulative Redeemable Preferred Stock	Mar. 31, 2012 Class D Cumulative Redeemable Preferred Stock	Mar. 15, 2012 Class F Cumulative Redeemable Preferred Stock	Feb. 29, 2012 Class F Cumulative Redeemable Preferred Stock	Mar. 31, 2013 Class F Cumulative Redeemable Preferred Stock	Apr. 30, 2012 Class F Cumulative Redeemable Preferred Stock
Issuance and redemption of preferred stock
Shares issued in stock offerings, net of offering costs (in shares)							14,950,000
Preferred stock, dividend rate (as a percent)			7.375%	7.375%			6.625%	6.625%
Redemption amount (in dollars per share)		$ 25	$ 25		$ 25			$ 25
Offering price per share (in dollars per share)							$ 25		$ 25.2863
Preferred stock, purchased by underwriters (in shares)							1,950,000
Underwriting discounts and other offering costs								$ 12,800,000
Net proceeds from preferred stock offering	360,941,000							360,900,000
Redemption of preferred stock	127,500,000							127,500,000
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