Acquisitions (Narrative) (Details) In Millions, except Per Share data	6 Months Ended						6 Months Ended
	Jun. 30, 2011 USD ($)	Jul. 14, 2011 USD ($)	Jul. 14, 2011 GBP (£)	Jun. 30, 2011 GBP (£)	Sep. 30, 2011 Chaucer [Member] USD ($)	Jun. 30, 2011 Chaucer [Member] USD ($)	Jun. 30, 2011 Campania Holding Company [Member] USD ($)	Mar. 31, 2011 Campania Holding Company [Member] USD ($)	Dec. 31, 2010 OneBeacon Insurance Group [Member] USD ($)
Per share amount received by shareholders				£ 53.3
Business acquisition, purchase price, settlement loss	$ 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							2.8
Business combination, consideration transferred									23
Business acquisition, additional contingent consideration, at fair value									$ 11