Debt (Details)	3 Months Ended	6 Months Ended
	Jun. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)	Nov. 25, 2014 USD ($)	Oct. 10, 2014 USD ($)	Aug. 14, 2013 USD ($)
Debt Instrument
Total debt	$ 748,500,000	$ 748,500,000		$ 746,600,000
Repayments of Debt		17,600,000	$ 200,000
Draw down of debt and revolving line of credit		19,500,000	$ 0
WTM Bank Facility
Debt Instrument
Total commitment under revolving credit facility							$ 425,000,000
Proceeds from Issuance of Debt		$ 15,000,000
Debt, Weighted Average Interest Rate	3.65%	3.65%
Repayments of Debt		$ 15,000,000
Other Debt [Member]
Debt Instrument
Total debt	$ 4,800,000	4,800,000		4,800,000
OneBeacon | 2012 OBH Senior Notes
Debt Instrument
Debt instrument at face value	275,000,000	275,000,000		275,000,000
Unamortized original issue discount	(300,000)	(300,000)		(300,000)
Total debt	274,700,000	274,700,000		274,700,000
Sirius Group | SIG Senior Notes
Debt Instrument
Debt instrument at face value	400,000,000	400,000,000		400,000,000
Unamortized original issue discount	(300,000)	(300,000)		(300,000)
Total debt	399,700,000	399,700,000		399,700,000
White Mountains | WTM Bank Facility
Debt Instrument
Total debt	0	0		0
Tranzact [Member] | Tranzact Bank Facility [Member]
Debt Instrument
Long-term Line of Credit	70,600,000	70,600,000		68,700,000
Unamortized Debt Issuance Expense	(1,300,000)	(1,300,000)		(1,300,000)
Line of Credit Facility, Fair Value of Amount Outstanding	69,300,000	69,300,000		$ 67,400,000
Total commitment under revolving credit facility						$ 82,000,000
Notional amount	66,100,000	66,100,000				70,000,000
Line of Credit Facility, Remaining Borrowing Capacity						$ 12,000,000
Proceeds from Long-term Lines of Credit		4,500,000
Repayments of Debt	$ 1,300,000	$ 2,600,000
Sirius International [Member] | Letter of Credit [Member]
Debt Instrument
number of Credit Lines					2
Line of Credit Facility, Fair Value of Amount Outstanding					$ 200,000,000
Nordea Bank Finland
Debt Instrument
Total commitment under revolving credit facility					125,000,000.0
Lloyds Bank plc [Member]
Debt Instrument
Total commitment under revolving credit facility					75,000,000.0
Unsecured Debt [Member] | Nordea Bank Finland
Debt Instrument
Total commitment under revolving credit facility					100,000,000.0
Unsecured Debt [Member] | Lloyds Bank plc [Member]
Debt Instrument
Total commitment under revolving credit facility					$ 25,000,000.0