Derivatives (Interest rate cap) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended						1 Months Ended	9 Months Ended	61 Months Ended
	Sep. 30, 2012 Interest Rate Cap		Sep. 30, 2013 Interest Rate Cap Rating		Dec. 31, 2012 Interest Rate Cap		Jun. 30, 2012 Interest Rate Cap		May 31, 2007 Sirius International Group Ltd Preferred Stock	Sep. 30, 2013 Sirius International Group Ltd Interest Rate Cap	Jun. 30, 2022 Sirius International Group Ltd Interest Rate Cap	Sep. 30, 2013 A Barclays Interest Rate Cap		Sep. 30, 2013 Standard & Poor's, AA- Rating [Member] Nordea Bank Finland [Member] Interest Rate Cap
Derivative [Line Items]
Derivative Collateral Right to Reclaim			$ 0	[1]								$ 0	[1]	$ 0	[1]
Preferred Stock, Dividend Rate, Percentage									7.506%
Term of LIBOR loan									3 months
Derivative, Cap Interest Rate										8.30%
Derivative, Cost of Hedge										9.9
basis points									3.20%	3.95%
Non-cumulative perpetual preference shares issued									250.0
years of interest rate cap											5 years
Derivative, fair value, net	9.6	[1]			0	[1]	0	[1]
Purchases	9.9	[1]	9.9	[1]
Realized and unrealized gains (losses)	(0.3)	[1]	(0.3)	[1]
Sales/settlements	$ 0	[1]	$ 0	[1]
S and P Credit Ratings			21

[1] Consists of exchange traded equity index, foreign currency and interest rate futures. Fair value measurements based upon quoted prices for identical instruments that are actively traded.