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

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

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Weighted average interest rate of debt outstanding.

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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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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Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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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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Debt (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended			9 Months Ended
Debt (Details) - USD ($) $ in Millions	Nov. 30, 2012	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2012	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Jul. 23, 2015	Aug. 14, 2013
Debt Instrument
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements		11,943			8,208
Total debt		$ 288.5			$ 288.5		$ 337.6
Repayments of Debt					401.8	$ 84.5
Draw down of debt and revolving line of credit					$ 352.5	$ 98.5
MediaAlpha Bank Facility [Member]
Debt Instrument
Debt Instrument, Basis Spread on Variable Rate					1.50%
2012 OBH Senior Notes
Debt Instrument
Proceeds from Issuance of Debt				$ 275.0
WTM Bank Facility
Debt Instrument
Debt Instrument, Interest Rate, Effective Percentage							3.90%
Total commitment under revolving credit facility									$ 425.0
Debt, Weighted Average Interest Rate		3.85%			3.85%
Repayments of Debt		$ 250.0			$ 400.0
Proceeds from Issuance of Debt			$ 250.0		350.0
Term Loan [Member]
Debt Instrument
Repayments of Debt		0.8			0.8
Revolving Credit Facility [Member]
Debt Instrument
Repayments of Debt		0.5			1.0
Proceeds from Issuance of Debt					2.5
OB Bank Facility [Member]
Debt Instrument
Total commitment under revolving credit facility		65.0			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] \| OneBeacon Bank Facility [Member]
Debt Instrument
Long-term Line of Credit		0.0			0.0
OneBeacon [Member] \| 2012 OBH Senior Notes
Debt Instrument
Debt instrument at face value		275.0			275.0		$ 275.0
Unamortized original issue discount		(1.9)			(1.9)		(2.1)
Total debt		$ 273.1			$ 273.1		$ 272.9
Debt Instrument, Interest Rate, Effective Percentage		4.70%			4.70%		4.70%
White Mountains \| WTM Bank Facility
Debt Instrument
Total debt		$ 0.0			$ 0.0		$ 50.0
Tranzact [Member] \| OneBeacon Bank Facility [Member]
Debt Instrument
Long-term Line of Credit							0.0
MediaAlpha [Member] \| MediaAlpha Bank Facility [Member]
Debt Instrument
Total debt		1.5			1.5
Long-term Line of Credit		$ 15.6			$ 15.6		$ 15.0
Debt Instrument, Interest Rate, Effective Percentage		5.80%			5.80%		5.50%
Unamortized Debt Issuance Expense		$ (0.2)			$ (0.2)		$ (0.3)
Line of Credit Facility, Fair Value of Amount Outstanding		15.4			15.4		$ 14.7
Total commitment under revolving credit facility								$ 20.0
Notional amount		14.2			14.2
Line of Credit Facility, Remaining Borrowing Capacity		5.0			5.0
Failure to pay principal and interest [Member] \| OneBeacon [Member] \| 2012 OBH Senior Notes
Debt Instrument
Total debt		$ 75.0			$ 75.0