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, 2016	Dec. 31, 2015	Dec. 31, 2014	Sep. 29, 2015	Aug. 14, 2013
Debt Instrument [Line Items]
Debt issuance costs			$ 1.6
Interest expense on debt			16.1	$ 14.6	$ 14.2
Repayments of debt, principal			404.6	76.1	65.2
Draw down of debt and revolving line of credit			352.5	171.5	133.6
Interest Paid			14.8	12.8	13.0
Expenses related to issuance, including underwriting fees			1.6
Debt			285.9	337.6
OneBeacon Bank Facility [Member]
Debt Instrument [Line Items]
Total commitment under revolving credit facility						$ 65.0
WTM Bank Facility
Debt Instrument [Line Items]
Issuance of debt, net of debt issuance costs			350.0	125.0
Interest expense on debt			1.2	0.1	0.3
Total commitment under revolving credit facility							$ 425.0
Repayments of debt, principal			$ 400.0	$ 75.0
Debt, Weighted Average Interest Rate			3.85%	3.74%
Prior W T M Bank Facility
Debt Instrument [Line Items]
Total commitment under revolving credit facility							$ 375.0
One Beacon US Holdings Inc. | 2012 OBH Senior Notes
Debt Instrument [Line Items]
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%
Effective yield (as a percent)	4.70%
Interest Expense			$ 13.1	$ 13.0	$ 13.0
One Beacon US Holdings Inc. | Senior Notes | 2003 OBH Senior Notes
Debt Instrument [Line Items]
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
MediaAlpha [Member] | MediaAlpha Bank Facility [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate			1.50%
Debt instrument at face value			$ 12.9	$ 15.0
Effective yield (as a percent)			5.70%	5.50%
Total commitment under revolving credit facility				$ 20.0
Term Loan [Member] | MediaAlpha [Member] | MediaAlpha Bank Facility [Member]
Debt Instrument [Line Items]
Total commitment under revolving credit facility			$ 15.0
Repayments of debt, principal			2.1
Revolving Credit Facility [Member] | MediaAlpha [Member] | MediaAlpha Bank Facility [Member]
Debt Instrument [Line Items]
Total commitment under revolving credit facility			5.0
Repayments of debt, principal			$ 2.5