Commitment and Contingencies (Details) (USD $)	9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010
Dividends for preferred shares:
Preferred shares, principal amount outstanding	$ 325,000,000	$ 325,000,000
Dividends declared to preferred shareholders	3,300,000
Investment commitments:
Estimated investment commitments	229,600,000
Unsecured revolving loan and letter of credit facility [Member]
Letter of credit and revolving credit facilities:
Line of credit facilitiy, maximum borrowing capacity	300,000,000
Secured letter of credit facility [Member]
Letter of credit and revolving credit facilities:
Line of credit facilitiy, maximum borrowing capacity	500,000,000
Other secured letter of credit facilities [Member]
Letter of credit and revolving credit facilities:
Line of credit facilitiy, maximum borrowing capacity	180,000,000
Credit Agreement [Member]
Letter of credit and revolving credit facilities:
Line of credit facility, amount outstanding	100,000,000
LOC Facilities [Member]
Letter of credit and revolving credit facilities:
Line of credit facility, amount outstanding	621,500,000
Maturity date of facility	2014-08-18
Investments pledged as collateral	644,200,000
Series A non-cumulative preferred shares [Member]
Dividends for preferred shares:
Preferred shares, dividend rate, percentage	8.00%
Preferred shares, principal amount outstanding	200,000,000
Series B non-cumulative preferred shares [Member]
Dividends for preferred shares:
Preferred shares, dividend rate, percentage	7.875%
Preferred shares, principal amount outstanding	$ 125,000,000