Shareholders' Equity (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Preferred Shares [Abstract]
Preferred shares, shares issued	34,150,000		34,150,000		35,750,000
Aggregate liquidation value	$ 853,750,000		$ 853,750,000		$ 893,750,000
Net proceeds from issuance of preferred shares			241,265,000	0
Loss on redemption of preferred shares	0	0	9,135,000	0
[SeriesCPreferredStockMember]
Preferred Shares [Abstract]
Aggregate redemption value	290,000,000		290,000,000
Preferred Stock Redemption Date			Mar. 18, 2013
[SeriesFPreferredStockMember]
Preferred Shares [Abstract]
Preferred shares issuance, date			Feb. 14, 2013
Preferred shares, shares issued	10,000,000		10,000,000
Dividend rate, percentage			5.875%
Distribution on liquidation	$ 25		$ 25
Aggregate liquidation value	250,000,000		250,000,000
Net proceeds from issuance of preferred shares			242,300,000
Underwriting discounts and commissions			$ 7,700,000