Derivative Instruments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended						3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended		9 Months Ended		1 Months Ended		3 Months Ended	1 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 item	Sep. 30, 2012	Sep. 30, 2013 Foreign Exchange Contracts - underwriting activities	Sep. 30, 2012 Foreign Exchange Contracts - underwriting activities	Sep. 30, 2013 Foreign Exchange Contracts - underwriting activities	Sep. 30, 2012 Foreign Exchange Contracts - underwriting activities	Sep. 30, 2013 Foreign Exchange Contracts- investing activities	Sep. 30, 2012 Foreign Exchange Contracts- investing activities	Sep. 30, 2013 Foreign Exchange Contracts- investing activities	Sep. 30, 2012 Foreign Exchange Contracts- investing activities	Sep. 30, 2013 Credit Derivatives	Sep. 30, 2012 Credit Derivatives	Sep. 30, 2013 Credit Derivatives	Sep. 30, 2012 Credit Derivatives	Sep. 30, 2013 Interest Rate Contracts	Sep. 30, 2012 Interest Rate Contracts	Sep. 30, 2013 Interest Rate Contracts	Sep. 30, 2012 Interest Rate Contracts	Sep. 30, 2013 Investment Options and Futures	Sep. 30, 2012 Investment Options and Futures	Sep. 30, 2013 Investment Options and Futures	Sep. 30, 2012 Investment Options and Futures	Sep. 30, 2013 Investment Options and Futures (long)	Dec. 31, 2012 Investment Options and Futures (long)	Sep. 30, 2013 Investment Options and Futures (short)	Dec. 31, 2012 Investment Options and Futures (short)	Sep. 30, 2013 LIBOR Swap	Sep. 30, 2012 LIBOR Swap	Sep. 30, 2013 LIBOR Swap	Sep. 30, 2012 LIBOR Swap	Sep. 30, 2012 UST Contract	Sep. 30, 2012 UST Contract	Sep. 30, 2013 ILW Swaps	Sep. 30, 2012 ILW Swaps	Sep. 30, 2013 ILW Swaps	Sep. 30, 2012 ILW Swaps	Jul. 31, 2013 Florida Wind Swap	Jul. 31, 2013 2012 Engineering Swap	Jun. 30, 2013 Japan Wind Swap item	Jun. 30, 2013 Blue Water Re Swap	Sep. 30, 2013 Level 2 UST Contract	Sep. 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	Sep. 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	Sep. 30, 2013 Not Designated as Hedging Instrument Level 2 Cross-currency	Sep. 30, 2013 Not Designated as Hedging Instrument Level 2 Credit Derivatives	Dec. 31, 2012 Not Designated as Hedging Instrument Level 2 Credit Derivatives	Sep. 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	Sep. 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)	Sep. 30, 2013 Not Designated as Hedging Instrument Level 2 Investment Options and Futures (short)	Sep. 30, 2013 Not Designated as Hedging Instrument Level 2 LIBOR Swap	Dec. 31, 2012 Not Designated as Hedging Instrument Level 2 LIBOR Swap	Sep. 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
Period of future movement in U.S. Treasury rate																																											10 years
Number of instruments designated as hedging instruments			0
Fair Value, Assets																									$ 0.4	$ 1.0																		$ (4.2)	$ (1.1)	$ 5.9	$ 1.6	$ (0.1)	$ 1.1	$ (0.4)	$ 1.9	$ (0.2)	$ 0.9	$ 1.0	$ 0.1	$ (0.4)	$ (1.7)	$ 2.2	$ 0.1
Fair Value, Liabilities																											0.1	0
Notional Units																																												288	165	362	155		204	155	1,566	138	588	10	285	100	100	53	3
Net income (loss) from derivative instruments	(6.7)	0.7	(14.3)	4.3	5.8	4.5	(4.4)	6.7	(2.8)	(1.5)	3.7	(2.4)	(6.4)	(0.7)	(10.7)	1.4	3.4	0.1	4.6	2.0	(0.8)	(0.6)	(2.4)	(1.1)					(0.8)	(1.4)	0.6	(2.3)	0.6	0.6	(5.1)	(0.3)	(5.7)	(0.6)
Number of swaps entered into by the entity																																									4
Fixed-rate payment on swap																																							4.9	0.3	2.0	0.5
Maximum recovery under each swap																																								3.0	5.0	5.0
Aggregate maximum recovery under swaps																																							$ 25.0		$ 20.0