Issuance and Redemption of Preferred Stock (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Class of Stock [Line Items]
Underwriting discounts and other offering costs	$ 12,800,000
Proceeds from preferred stock offering, net	360,941,000	0
Redemption of preferred stock	127,500,000	0
Preferred stock redeemed (in shares)	5,100,000
Cumulative Redeemable Preferred Stock Class F [Member]
Class of Stock [Line Items]
Preferred stock, issued date	February 2012
Issued shares of stock (in shares)	14,950,000
Preferred stock, dividend rate percentage (in hundredths)	6.625%
Preferred stock, purchased by Underwriters (in shares)	1,950,000
Redemption amount (in dollars per share)	$ 25
Period when initial dividend payments were made (in days)	37
Dividend paid per initial preferred share of stock (in dollars per share)	$ 0.1702257
Dividend rate per preferred share of stock (in dollars per share)	$ 0.138021
Preferred stock, dividend payment terms	Monthly
Cumulative Redeemable Preferred Stock Class D [Member]
Class of Stock [Line Items]
Preferred stock, issued date	2004
Preferred stock, dividend rate percentage (in hundredths)	7.375%
Redemption amount (in dollars per share)	$ 25
Preferred stock redeemed (in shares)	5,100,000
Preferred stock, dividend payment terms	Monthly
Original issuance costs	$ 3,700,000