Issuance and Redemption of Preferred Stock (Details) (USD $)	9 Months Ended	1 Months Ended			9 Months Ended	1 Months Ended	9 Months Ended
	Sep. 30, 2012	Apr. 30, 2012 Class F Cumulative Redeemable Preferred Stock	Mar. 31, 2012 Class F Cumulative Redeemable Preferred Stock	Feb. 29, 2012 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, 2012 Class D Cumulative Redeemable Preferred Stock	Sep. 30, 2011 Class D Cumulative Redeemable Preferred Stock
Issuance and redemption of preferred stock
Shares of stock issued		1,400,000		14,950,000
Preferred stock, dividend rate (as a percent)					6.625%	7.375%	7.375%	7.375%
Preferred stock, purchased by underwriters (in shares)				1,950,000
Offering price per share (in dollars per share)		$ 25.2863		$ 25
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	$ 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