Acquisitions, Dispositions and Goodwill (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013
Goodwill [Line Items]
Balance at beginning of period	$ 885,860
Earnout and other	8,991
Balance at end of period	894,851
U.S. Property & 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	314,089
Earnout and other	9,010
Balance at end of period	323,099
Accident & Health [Member]
Goodwill [Line Items]
Balance at beginning of period	144,113
Earnout and other	0
Balance at end of period	144,113
U.S. Surety & 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	124,958
Earnout and other	(19)
Balance at end of period	$ 124,939