Significant Transactions (Details 3) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended		62 Months Ended			1 Months Ended		1 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011	Aug. 26, 2010	Nov. 17, 2006	Feb. 28, 2010 Central National	Feb. 26, 2010 Central National	Dec. 31, 2011 Old Lyme	Dec. 30, 2011 Old Lyme
Business acquisition
Cash to be paid on acquisition							$ 5		$ 6
Purchase note acquired									2.1
Gain on acquisition						12.8		7.2
Share Repurchase
Authorized repurchase of common shares (in shares)					1,000,000
Additional authorized repurchase of common shares (in shares)				600,000
Common shares repurchased and retired during the period (in shares)	313,967	677,125	1,411,703
Value of common shares repurchased and retired during the period	$ 114.2	$ 222.0	$ 537.5