Goodwill (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Mar. 31, 2011	Sep. 30, 2011
Goodwill [Line Items]
Balance at beginning of year		$ 821,648,000	$ 821,648,000
Earnout and other	31,300,000	20,000,000	51,727,000
Balance at June 30, 2011	873,375,000		873,375,000
Earnout and other	31,300,000	20,000,000	51,727,000
U.S. Property & Casualty [Member]
Goodwill [Line Items]
Balance at beginning of year		223,000,000	223,000,000
Earnout and other			0
Balance at June 30, 2011	223,000,000		223,000,000
Earnout and other			0
Professional Liability [Member]
Goodwill [Line Items]
Balance at beginning of year		249,820,000	249,820,000
Earnout and other			51,727,000
Balance at June 30, 2011	301,547,000		301,547,000
Earnout and other			51,727,000
Accident & Health [Member]
Goodwill [Line Items]
Balance at beginning of year		144,128,000	144,128,000
Earnout and other			0
Balance at June 30, 2011	144,128,000		144,128,000
Earnout and other			0
U.S. Surety & Credit [Member]
Goodwill [Line Items]
Balance at beginning of year		79,700,000	79,700,000
Earnout and other			0
Balance at June 30, 2011	79,700,000		79,700,000
Earnout and other			0
International [Member]
Goodwill [Line Items]
Balance at beginning of year		125,000,000	125,000,000
Earnout and other			0
Balance at June 30, 2011	125,000,000		125,000,000
Earnout and other			$ 0