Debt and Standby Letter of Credit Facilities (Narrative) (Details) (USD $)	12 Months Ended			1 Months Ended	3 Months Ended	5 Months Ended	7 Months Ended	1 Months Ended	3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 31, 2007	Dec. 31, 2013	Dec. 31, 2013	Aug. 13, 2013	Nov. 30, 2012	Dec. 31, 2012	Aug. 14, 2013	Aug. 12, 2011	May 31, 2003	Oct. 10, 2014	Nov. 25, 2014
Debt Instrument
Issuance of debt, net of debt issuance costs	$ 68,600,000	$ 0	$ 271,900,000
Aggregate purchase price of senior notes purchased and retired	(65,000,000)	0	275,900,000
Interest expense on debt	41,900,000	42,500,000	44,800,000
Draw down of revolving line of credit	0	200,000,000	150,000,000
Repayments of debt, principal	66,500,000	275,000,000	75,000,000
Interest Paid	42,000,000	42,600,000	44,600,000
Debt	746,600,000	676,400,000			676,400,000	676,400,000
SIG Senior Notes
Debt Instrument
Debt instrument at face value				400,000,000
Percentage of par value at which debt was issued				99.715%
Issuance of debt, net of debt issuance costs				392,000,000
Interest rate (as a percent)				6.375%
Debt issue costs				3,600,000
Underwriting discount				2,600,000
Effective yield (as a percent)				6.50%
Loss from interest rate hedge recorded in other comprehensive income				(2,400,000)
Unamortized loss remaining in accumulated other comprehensive income	1,000,000
Interest expense on debt	26,200,000	26,200,000	26,200,000
Expenses related to issuance, including underwriting fees				3,600,000
WTM Bank Facility
Debt Instrument
Effective yield (as a percent)	3.65%	3.65%			3.65%	3.65%
Interest expense on debt	300,000					300,000
Total commitment under revolving credit facility										425,000,000
Draw down of revolving line of credit	65,000,000	50,000,000
Repayments of lines of credit					50,000,000
Prior W T M Bank Facility
Debt Instrument
Effective yield (as a percent)		2.83%	3.53%		2.83%	2.83%			3.53%
Interest expense on debt			1,700,000				2,900,000
Total commitment under revolving credit facility											375,000,000
Draw down of revolving line of credit		150,000,000	150,000,000
Repayments of debt, principal			75,000,000
Repayments of lines of credit		225,000,000
One Beacon US Holdings Inc. | 2012 OBH Senior Notes
Debt Instrument
Debt instrument at face value								275,000,000
Percentage of par value at which debt was issued								99.90%
Issuance of debt, net of debt issuance costs								272,900,000
Interest rate (as a percent)								4.60%
Debt issue costs								2,800,000
Underwriting discount								1,800,000
Effective yield (as a percent)								4.70%
Interest Expense	13,000,000
Expenses related to issuance, including underwriting fees								2,800,000
One Beacon US Holdings Inc. | 2003 OBH Senior Notes
Debt Instrument
Interest Expense		13,000,000	16,900,000
Guarantee fee basis points per annum on outstanding principal amount of debt (as a percent)												2.50%
One Beacon US Holdings Inc. | Senior Notes | 2003 OBH Senior Notes
Debt Instrument
Interest rate (as a percent)								5.875%
Debt Instrument, Repurchase Amount		269,800,000			269,800,000	269,800,000
Aggregate purchase price of senior notes purchased and retired									275,900,000
Loss on retirement of debt									(6,300,000)
Write off of deferred issuance costs									200,000
Tranzact [Member] | Tranzact Bank Facility [Member]
Debt Instrument
Interest expense on debt	700,000
Line of Credit Facility, Fair Value of Amount Outstanding	67,400,000	0			0	0							70,000,000
Line of Credit Facility, Amount Outstanding	68,700,000	0			0	0
Line of Credit Facility, Remaining Borrowing Capacity	4,500,000
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate	4.15575%
Sirius International | Letter of Credit [Member]
Debt Instrument
Line of Credit Facility, Fair Value of Amount Outstanding														200,000,000
number of Credit Lines														2
Minimum | Tranzact [Member] | Tranzact Bank Facility [Member]
Debt Instrument
LIBOR rate	2.50%
Maximum | Tranzact [Member] | Tranzact Bank Facility [Member]
Debt Instrument
LIBOR rate	4.50%
Nordea Bank Finland [Member]
Debt Instrument
Total commitment under revolving credit facility														125,000,000
Lloyds Bank plc [Member]
Debt Instrument
Total commitment under revolving credit facility														75,000,000
Unsecured Debt [Member] | Nordea Bank Finland [Member]
Debt Instrument
Total commitment under revolving credit facility														100,000,000
Unsecured Debt [Member] | Lloyds Bank plc [Member]
Debt Instrument
Total commitment under revolving credit facility														$ 25,000,000