Acquisitions, Dispositions and Goodwill (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012
Goodwill [Line Items]
Balance at beginning of period	$ 872,814
Earnout and other	13,046
Balance at end of period	885,860
United States Property And Casualty [Member]
Goodwill [Line Items]
Balance at beginning of period	223,000
Earnout and other	0
Balance at end of period	223,000
Professional Liability [Member]
Goodwill [Line Items]
Balance at beginning of period	301,547
Earnout and other	12,542
Balance at end of period	314,089
Accident And Health [Member]
Goodwill [Line Items]
Balance at beginning of period	144,132
Earnout and other	(19)
Balance at end of period	144,113
United States Surety And Credit [Member]
Goodwill [Line Items]
Balance at beginning of period	79,700
Earnout and other	0
Balance at end of period	79,700
International [Member]
Goodwill [Line Items]
Balance at beginning of period	124,435
Earnout and other	523
Balance at end of period	$ 124,958