Derivatives (Interest rate cap) (Details) - Investments [Domain] $ in Millions	1 Months Ended		3 Months Ended			6 Months Ended
	Jul. 31, 2013 USD ($)	May. 31, 2007 shares	Jun. 30, 2015 USD ($) Rating	Jun. 30, 2014 USD ($)	Sep. 30, 2013	Jun. 30, 2015 USD ($) Rating	Jun. 30, 2014 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Jul. 30, 2013
Interest Rate Cap
Derivative [Line Items]
Derivative collateral right to reclaim			$ 3.3			$ 3.3
Sirius Group Ltd [Member] | Preferred Stock
Derivative [Line Items]
Term of LIBOR loan		3 months
Basis points		3.20%
Sirius Group Ltd [Member] | Interest Rate Cap
Derivative [Line Items]
Basis points						3.95%
Sirius Group | Preferred Stock
Derivative [Line Items]
Preferred Stock, Dividend Rate, Percentage		7.506%
Sirius Group | Interest Rate Cap
Derivative [Line Items]
Derivative, Cap Interest Rate												8.30%
Derivative, Cost of Hedge	$ 9.9
years of interest rate cap					5 years
Derivative, fair value, net			3.3	$ 6.2		$ 3.3	$ 6.2	$ 3.1	$ 4.1	$ 8.2	$ 11.1
Realized and unrealized gains (losses)			$ 0.2	$ (2.0)		$ (0.8)	$ (4.9)
S and P Credit Ratings | Rating			23			23
Sirius Group | Interest Rate Cap | Preferred Stock
Derivative [Line Items]
Non-cumulative perpetual preference shares issued | shares		250,000,000.0
Standard & Poor's, A Rating | Barclays | Interest Rate Cap
Derivative [Line Items]
Derivative collateral right to reclaim			$ 2.3			$ 2.3
Standard & Poor's, AA- Rating | Nordea Bank Finland | Interest Rate Cap
Derivative [Line Items]
Derivative collateral right to reclaim			$ 1.0			$ 1.0