Acquisitions, Dispositions and Goodwill (Details) (USD $)	3 Months Ended		12 Months Ended					12 Months Ended							12 Months Ended
	Mar. 31, 2015	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 United States Property And Casualty [Member]	Dec. 31, 2011 United States Property And Casualty [Member]	Dec. 31, 2010 United States Property And Casualty [Member]	Dec. 31, 2012 Professional Liability [Member]	Dec. 31, 2011 Professional Liability [Member]	Dec. 31, 2012 Accident And Health [Member]	Dec. 31, 2011 Accident And Health [Member]	Dec. 31, 2012 United States Surety And Credit [Member]	Dec. 31, 2011 United States Surety And Credit [Member]	Dec. 31, 2010 United States Surety And Credit [Member]	Dec. 31, 2012 International [Member]	Dec. 31, 2011 International [Member]
Goodwill [Line Items]
Balance at beginning of period		$ 885,860,000	$ 872,814,000	$ 821,648,000	$ 223,000,000	$ 223,000,000	$ 223,000,000	$ 301,547,000	$ 249,820,000	$ 144,132,000	$ 144,128,000	$ 79,700,000	$ 79,700,000	$ 79,700,000	$ 124,435,000	$ 125,000,000
Future Earnout Payments	5,400,000	6,400,000	13,046,000	51,166,000				12,542,000	51,727,000	(19,000)	4,000				523,000	(565,000)
Balance at end of period			$ 885,860,000	$ 872,814,000	$ 223,000,000	$ 223,000,000	$ 223,000,000	$ 314,089,000	$ 301,547,000	$ 144,113,000	$ 144,132,000	$ 79,700,000	$ 79,700,000	$ 79,700,000	$ 124,958,000	$ 124,435,000