Acquisitions (Narrative) (Details) In Millions, except Per Share data, unless otherwise specified	0 Months Ended	3 Months Ended	9 Months Ended							9 Months Ended
	Jun. 17, 2011 USD ($)	Sep. 30, 2011 USD ($)	Sep. 30, 2011 USD ($)	Sep. 30, 2011 GBP (£)	Jul. 14, 2011 USD ($)	Jul. 14, 2011 GBP (£)	Jun. 30, 2011 Acquisition [Member]	Sep. 30, 2011 Chaucer [Member] USD ($)	Jun. 30, 2011 Chaucer [Member] USD ($)	Sep. 30, 2011 Campania Holding Company [Member] USD ($)	Mar. 31, 2011 Campania Holding Company [Member] USD ($)	Dec. 31, 2010 OneBeacon Insurance Group [Member] USD ($)
Business Acquisition [Line Items]
Per share amount received by shareholders, pence				£ 53.3
Business acquisition, purchase price, settlement loss		$ 11.3	$ 11.3					$ 6.6	$ 4.7
Proceeds from issuance of senior unsecured notes	300
Business acquisition, cost of acquired entity, cash paid					455.0	287.4					24.0
Business acquisition, additional revenue recognized										4.1
Business combination, consideration transferred												23
Business acquisition, additional contingent consideration, at fair value												11
Gain (loss) on foreign exchange		$ 3.0	$ 6.4
Current exchange rate between GBP and US dollars		1.56	1.56				1.6053