Commitments and Contingencies (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended				6 Months Ended
	Feb. 28, 2013	Jun. 30, 2013	Jun. 30, 2013 LOC Facilities [Member]	Jun. 30, 2013 Other secured letter of credit facilities	Jun. 30, 2013 Credit Agreement	Jun. 30, 2013 Unsecured revolving loan and letter of credit facility	Jun. 30, 2013 Secured letter of credit facility
Line of Credit Facility [Abstract]
Maximum borrowing capacity				$ 113.9		$ 300.0	$ 500.0
Credit facility expiration date					Aug. 18, 2014
Amount outstanding			412.7		100.0
Investments pledged as collateral			481.3
Unfunded Investment Commitments [Abstract]
Investment commitments		751.0
Quota share agreement percentage	100.00%
Estimate aggregate consideration to be paid at closing of acquisition of CMG	$ 300