Acquisitions, Dispositions and Goodwill (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Goodwill [Line Items]
Balance at beginning of period	$ 895,200	$ 885,860
Acquisition	6,996
Earnout and other	3,440	9,340
Balance at end of period	905,636	895,200
U.S. Property & Casualty [Member]
Goodwill [Line Items]
Balance at beginning of period			223,000
Balance at end of period	223,000	223,000	223,000
Professional Liability [Member]
Goodwill [Line Items]
Balance at beginning of period	323,193	314,089
Earnout and other	4,131	9,104
Balance at end of period	327,324	323,193
Accident & Health [Member]
Goodwill [Line Items]
Balance at beginning of period			144,113
Balance at end of period	144,113	144,113	144,113
U.S. Surety & Credit [Member]
Goodwill [Line Items]
Balance at beginning of period	79,700		79,700
Acquisition	6,996
Balance at end of period	86,696		79,700
International [Member]
Goodwill [Line Items]
Balance at beginning of period	125,194	124,958
Earnout and other	(691)	236
Balance at end of period	$ 124,503	$ 125,194