Acquisitions, Dispositions and Goodwill (Details) (USD $)	3 Months Ended		12 Months Ended
	Sep. 30, 2011	Mar. 31, 2011	Dec. 31, 2011	Dec. 31, 2010	Sep. 30, 2010
Goodwill [Line Items]
Balance at beginning of period		$ 821,648,000	$ 821,648,000		$ 821,700,000
Earnout and other	31,300,000	20,000,000	51,166,000	(52,000)
Balance at end of period			872,814,000	821,648,000	821,700,000
U.S. Property & Casualty
Goodwill [Line Items]
Balance at beginning of period					223,000,000
Balance at end of period			223,000,000	223,000,000	223,000,000
Professional Liability [Member]
Goodwill [Line Items]
Balance at beginning of period		249,820,000	249,820,000		250,000,000
Earnout and other			51,727,000	(180,000)
Balance at end of period			301,547,000	249,820,000	250,000,000
Accident & Health
Goodwill [Line Items]
Balance at beginning of period		144,128,000	144,128,000		144,000,000
Earnout and other			4,000	128,000
Balance at end of period			144,132,000	144,128,000	144,000,000
U.S. Surety & Credit
Goodwill [Line Items]
Balance at beginning of period					79,700,000
Balance at end of period			79,700,000	79,700,000	79,700,000
International [Member]
Goodwill [Line Items]
Balance at beginning of period		125,000,000	125,000,000		125,000,000
Earnout and other			(565,000)
Balance at end of period			$ 124,435,000		$ 125,000,000