Issuance and Redemption of Preferred Stock (Details) (USD $)	6 Months Ended		0 Months Ended	1 Months Ended		6 Months Ended		1 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2013	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	Jun. 30, 2013 Class F Cumulative Redeemable Preferred Stock	Jun. 30, 2012 Class F Cumulative Redeemable Preferred Stock	Mar. 31, 2012 Class D Cumulative Redeemable Preferred Stock	Jun. 30, 2013 Class D Cumulative Redeemable Preferred Stock	Jun. 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		$ 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