Debt and Standby Letter of Credit Facilities (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Nov. 30, 2012	Dec. 31, 2012	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Sep. 29, 2015	Oct. 10, 2014	Aug. 14, 2013	May. 31, 2003
Debt Instrument
Interest expense on debt			$ 18.6	$ 15.6	$ 16.2
Draw down of debt and revolving line of credit			195.5	133.6	200.0
Repayments of debt, principal			$ 95.6	66.5	275.0
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate			6.07%
Interest Paid			$ 17.6	15.6	16.2
Debt			$ 442.4	$ 343.1
OneBeacon Bank Facility [Member]
Debt Instrument
Total commitment under revolving credit facility						$ 65.0
WTM Bank Facility
Debt Instrument
Effective yield (as a percent)			3.74%	3.65%
Interest expense on debt			$ 0.1	$ 0.3	$ 0.3
Total commitment under revolving credit facility								$ 425.0
Draw down of debt and revolving line of credit			125.0	65.0
Line of Credit Facility, Fair Value of Amount Outstanding			50.0
Repayments of lines of credit			75.0	65.0
Prior W T M Bank Facility
Debt Instrument
Total commitment under revolving credit facility								$ 375.0
One Beacon US Holdings Inc. | 2012 OBH Senior Notes
Debt Instrument
Debt instrument at face value	$ 275.0
Percentage of par value at which debt was issued	99.90%
Issuance of debt, net of debt issuance costs	$ 272.9
Interest rate (as a percent)	4.60%
Debt issue costs	$ 2.8
Underwriting discount	$ 1.8
Effective yield (as a percent)	4.70%
Interest Expense			13.0
Expenses related to issuance, including underwriting fees	$ 2.8
One Beacon US Holdings Inc. | 2003 OBH Senior Notes
Debt Instrument
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		$ 269.8
Aggregate purchase price of senior notes purchased and retired		275.9
Loss on retirement of debt		(6.3)
Write off of deferred issuance costs		$ 0.2
Tranzact [Member] | Tranzact Bank Facility
Debt Instrument
Interest expense on debt			4.0	0.7
Total commitment under revolving credit facility			116.0
Draw down of debt and revolving line of credit			24.0
Line of Credit Facility, Fair Value of Amount Outstanding			104.7				$ 70.0
Line of Credit Facility, Amount Outstanding			$ 104.7	68.7
Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate			4.73275%
Repayments of lines of credit			$ 13.0
Debt				67.4
MediaAlpha [Member] | MediaAlpha Bank Facility [Member]
Debt Instrument
Total commitment under revolving credit facility			20.0
Line of Credit Facility, Fair Value of Amount Outstanding				$ 0.0
Line of Credit Facility, Amount Outstanding			$ 15.0
Minimum | Tranzact [Member] | Tranzact Bank Facility
Debt Instrument
LIBOR rate			2.50%
Maximum | Tranzact [Member] | Tranzact Bank Facility
Debt Instrument
LIBOR rate			4.50%
Term Loan [Member] | Tranzact [Member] | Tranzact Bank Facility
Debt Instrument
Total commitment under revolving credit facility			$ 101.0
Debt Instrument, Increase (Decrease), Other, Net			$ 31.0
Line of Credit Facility, Collateral			70.0
Repayments of lines of credit			$ 6.5
Term Loan [Member] | MediaAlpha [Member] | MediaAlpha Bank Facility [Member]
Debt Instrument
Total commitment under revolving credit facility			15.0
Revolving Credit Facility [Member] | Tranzact [Member] | Tranzact Bank Facility
Debt Instrument
Total commitment under revolving credit facility			15.0
Revolving Credit Facility [Member] | MediaAlpha [Member] | MediaAlpha Bank Facility [Member]
Debt Instrument
Total commitment under revolving credit facility			$ 5.0