Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		3 Months Ended	6 Months Ended						3 Months Ended		6 Months Ended		3 Months Ended		6 Months Ended		1 Months Ended	3 Months Ended	1 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013 item	Jun. 30, 2012	Jun. 30, 2013 Foreign Exchange Contracts - underwriting activities	Jun. 30, 2012 Foreign Exchange Contracts - underwriting activities	Jun. 30, 2013 Foreign Exchange Contracts - underwriting activities	Jun. 30, 2012 Foreign Exchange Contracts - underwriting activities	Jun. 30, 2013 Foreign Exchange Contracts- investing activities	Jun. 30, 2012 Foreign Exchange Contracts- investing activities	Jun. 30, 2013 Foreign Exchange Contracts- investing activities	Jun. 30, 2012 Foreign Exchange Contracts- investing activities	Jun. 30, 2013 Credit Derivatives	Jun. 30, 2012 Credit Derivatives	Jun. 30, 2013 Credit Derivatives	Jun. 30, 2012 Credit Derivatives	Jun. 30, 2013 Interest Rate Contracts	Jun. 30, 2012 Interest Rate Contracts	Jun. 30, 2013 Interest Rate Contracts	Jun. 30, 2012 Interest Rate Contracts	Jun. 30, 2013 Investment Options and Futures	Jun. 30, 2013 Investment Options and Futures	Jun. 30, 2012 Investment Options and Futures	Jun. 30, 2013 Investment Options and Futures (long)	Dec. 31, 2012 Investment Options and Futures (long)	Jun. 30, 2013 Investment Options and Futures (short)	Dec. 31, 2012 Investment Options and Futures (short)	Jun. 30, 2013 LIBOR Swap	Jun. 30, 2012 LIBOR Swap	Jun. 30, 2013 LIBOR Swap	Jun. 30, 2012 LIBOR Swap	Jun. 30, 2013 ILW Swaps	Jun. 30, 2012 ILW Swaps	Jun. 30, 2013 ILW Swaps	Jun. 30, 2012 ILW Swaps	May 31, 2012 Engineering Swap	Jun. 30, 2013 Japan Wind Swap item	Jun. 30, 2013 Blue Water Re Swap	Jun. 30, 2013 Not Designated as Hedging Instrument Level 2 U.S. Dollars purchased	Dec. 31, 2012 Not Designated as Hedging Instrument Level 2 U.S. Dollars purchased	Jun. 30, 2013 Not Designated as Hedging Instrument Level 2 U.S. Dollars sold	Dec. 31, 2012 Not Designated as Hedging Instrument Level 2 U.S. Dollars sold	Jun. 30, 2013 Not Designated as Hedging Instrument Level 2 Credit Derivatives	Dec. 31, 2012 Not Designated as Hedging Instrument Level 2 Credit Derivatives	Jun. 30, 2013 Not Designated as Hedging Instrument Level 2 Interest Rate Contracts	Dec. 31, 2012 Not Designated as Hedging Instrument Level 2 Interest Rate Contracts	Jun. 30, 2013 Not Designated as Hedging Instrument Level 2 Investment Options and Futures (long)	Dec. 31, 2012 Not Designated as Hedging Instrument Level 2 Investment Options and Futures (long)	Jun. 30, 2013 Not Designated as Hedging Instrument Level 2 Investment Options and Futures (short)	Jun. 30, 2013 Not Designated as Hedging Instrument Level 2 LIBOR Swap	Dec. 31, 2012 Not Designated as Hedging Instrument Level 2 LIBOR Swap	Jun. 30, 2013 Not Designated as Hedging Instrument Level 3 ILW Swaps	Dec. 31, 2012 Not Designated as Hedging Instrument Level 3 ILW Swaps
Derivative instruments
Number of instruments designated as hedging instruments			0
Fair Value, Assets																								$ 0.5	$ 1.0														$ 4.8	$ (1.1)	$ (1.7)	$ 1.6	$ 8.1	$ (0.4)	$ (3.4)	$ (0.2)	$ 0.5	$ 1.0	$ 0.2	$ 0.1	$ (1.7)	$ 2.0	$ 0.1
Fair Value, Liabilities																										0.2	0
Notional Units																																							308.0	165.0	266.5	155.2	333.0	155.1	1,822.0	137.8	0.1	10.0	13.9	100.0	100.0	25.0	3.0
Net income (loss) from derivative instruments	(12.2)	2.1	(7.6)	3.6	(6.1)	(0.9)	(10.2)	2.2	3.6	2.4	6.5	(0.9)	(3.8)	1.5	4.3	2.1	(3.8)	1.0	1.2	1.9	(3.1)	1.6	(0.5)					1.5	(1.6)	1.4	(0.9)	(0.5)	(0.3)	0.6	(0.3)
Number of swaps entered into by the entity																																					4
Fixed-rate payment on swap																																				0.3	2.0	0.5
Maximum recovery under each swap																																				3.0	5.0	5.0
Aggregate maximum recovery under swaps																																					$ 20.0