Commitments and Contingencies (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended				3 Months Ended
	Feb. 28, 2013	Mar. 31, 2013	Mar. 31, 2013 LOC Facilities [Member]	Mar. 31, 2013 Other secured letter of credit facilities [Member]	Mar. 31, 2013 Credit Agreement [Member]	Mar. 31, 2013 Unsecured revolving loan and letter of credit facility [Member]	Mar. 31, 2013 Secured letter of credit facility [Member]
Line of Credit Facility [Abstract]
Maximum borrowing capacity				$ 86.7		$ 300.0	$ 500.0
Line of Credit Facility, Expiration Date					Aug. 18, 2014
Amount outstanding			449.0		100.0
Investments pledged as collateral			524.9
Unfunded Investment Commitments [Abstract]
Investment commitments		807.2
Estimate aggregate consideration to be paid at closing of acquisition of CMG	$ 300
Quota share agreement percentage	100.00%