Subsequent Events (Details) (USD $)	12 Months Ended			0 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Feb. 11, 2013 [SeriesFPreferredStockMember]	Dec. 31, 2012 [SeriesCPreferredStockMember]	Feb. 14, 2013 [SeriesCPreferredStockMember]
Subsequent Event [Line Items]
Preferred shares issuance, date				Feb. 11, 2013	May 08, 2003
Preferred shares, shares issued	35,750,000	35,750,000		10,000,000	11,600,000
Preferred Stock, Dividend Rate, Percentage				5.875%	6.75%
Net Proceeds From Issuance Of Preferred Stock And Preference Stock	$ 0	$ 361,722,000	$ 0	$ 242,300,000	$ 280,900,000
Underwriting discounts and commissions				7,700,000	9,100,000
Preference Shares Aggregate Liquidation Value	893,750,000	893,750,000		250,000,000	290,000,000
Distribution on liquidation				$ 25	$ 25.00
Aggregate redemption value						$ 290,000,000
Preferred Stock Redemption Date					Mar. 18, 2013