Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Face (par) amount of debt instrument at time of issuance.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount, after accumulated amortization, of debt discount.

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Fair value of the amount outstanding under the credit facility.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

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Amount of cash inflow from contractual arrangement with the lender, including but not limited to, letter of credit, standby letter of credit and revolving credit arrangements.

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The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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Represents the price of debt issued as a percentage of its par value.

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Debt (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
Debt (Details) - USD ($) $ in Millions	Nov. 30, 2012	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2012	Dec. 31, 2016	Jul. 23, 2015	Aug. 14, 2013
Debt Instrument
Total debt		$ 284.7			$ 285.9
Notional amount		299.9
Repayments of Debt		1.2	$ 0.0
Draw down of debt and revolving line of credit		$ 0.0	$ 102.5
MediaAlpha Bank Facility [Member]
Debt Instrument
Basis points		1.50%
2012 OBH Senior Notes
Debt Instrument
Proceeds from Issuance of Debt				$ 275.0
WTM Bank Facility
Debt Instrument
Total commitment under revolving credit facility							$ 425.0
OB Bank Facility [Member]
Debt Instrument
Total commitment under revolving credit facility		$ 65.0
One Beacon Holdings Inc [Member] \| 2012 OBH Senior Notes
Debt Instrument
Debt Instrument, Interest Rate, Effective Percentage	4.70%
Proceeds from Issuance of Debt	$ 272.9
Debt Instrument, Interest Rate, Stated Percentage	4.60%
Debt Instrument, Issue Price as Percentage of Par Value	99.90%
OneBeacon [Member] \| 2012 OBH Senior Notes
Debt Instrument
Debt instrument at face value		275.0			275.0
Unamortized original issue discount		(1.8)			(1.8)
Total debt		$ 273.2			$ 273.2
Debt Instrument, Interest Rate, Effective Percentage		4.70%			4.70%
White Mountains \| WTM Bank Facility
Debt Instrument
Total debt		$ 0.0			$ 0.0
MediaAlpha [Member] \| MediaAlpha Bank Facility [Member]
Debt Instrument
Debt instrument at face value		$ 11.7			$ 12.9
Debt Instrument, Interest Rate, Effective Percentage		6.10%			5.70%
Unamortized Debt Issuance Expense		$ (0.2)			$ (0.2)
Line of Credit Facility, Fair Value of Amount Outstanding		11.5			$ 12.7
Total commitment under revolving credit facility						$ 20.0
Notional amount		15.0
Line of Credit Facility, Remaining Borrowing Capacity		5.0
MediaAlpha [Member] \| Term Loan [Member]
Debt Instrument
Repayments of Debt		1.2
Debt Instrument, Redemption, Period One [Member] \| OneBeacon [Member] \| 2012 OBH Senior Notes
Debt Instrument
Total debt		$ 75.0