Debt (Details) - USD ($)	3 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2015	Sep. 30, 2014	Jul. 23, 2015	Dec. 31, 2014	Nov. 25, 2014	Oct. 10, 2014	Aug. 14, 2013
Debt Instrument
Total debt	$ 383,300,000	$ 383,300,000			$ 343,100,000
Repayments of Debt		92,900,000	$ 40,200,000
Draw down of debt and revolving line of credit		$ 134,000,000	$ 40,000,000
WTM Bank Facility
Debt Instrument
Total commitment under revolving credit facility								$ 425,000,000
Proceeds from Issuance of Debt	$ 60,000,000
Debt, Weighted Average Interest Rate	3.65%	3.65%
Repayments of Debt	$ 60,000,000
OB Bank Facility [Member]
Debt Instrument
Total commitment under revolving credit facility	65,000,000	$ 65,000,000
Other Debt [Member]
Debt Instrument
Total debt	0	0			1,000,000
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	(200,000)	(200,000)			(300,000)
Total debt	274,800,000	274,800,000			274,700,000
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	95,800,000	95,800,000			68,700,000
Unamortized Debt Issuance Expense	(2,000,000)	(2,000,000)			(1,300,000)
Line of Credit Facility, Fair Value of Amount Outstanding	93,800,000	93,800,000			67,400,000
Total commitment under revolving credit facility	116,000,000	116,000,000
Notional amount	101,000,000	101,000,000					$ 70,000,000
Line of Credit Facility, Remaining Borrowing Capacity	15,000,000	15,000,000
Proceeds from Long-term Lines of Credit		13,000,000
Repayments of Debt	1,300,000	3,900,000
MediaAlpha | MediaAlpha Bank Facility [Member]
Debt Instrument
Long-term Line of Credit	15,000,000	15,000,000			0
Unamortized Debt Issuance Expense	(300,000)	(300,000)			0
Line of Credit Facility, Fair Value of Amount Outstanding	14,700,000	14,700,000			$ 0
Total commitment under revolving credit facility				$ 20,000,000
Notional amount	15,000,000	15,000,000
Line of Credit Facility, Remaining Borrowing Capacity	5,000,000	5,000,000
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
TruBridge [Member] | Tranzact [Member] | Tranzact Bank Facility [Member]
Debt Instrument
Notional amount	$ 31,000,000	$ 31,000,000