Debt (Debt Outstanding) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument
Debt	$ 192.7	$ 23.8
Other Debt
Debt Instrument
Debt	1.9	0.0
White Mountains \| WTM Bank Facility
Debt Instrument
Debt	0.0	0.0
NSM Holding Co LLC
Debt Instrument
Debt	178.5	0.0
MediaAlpha
Debt Instrument
Debt	14.2	23.8
MediaAlpha \| MediaAlpha Bank Facility
Debt Instrument
Debt instrument at face value	14.3	23.9
Unamortized Debt Issuance Expense	(0.1)	(0.1)
Debt	$ 14.2	$ 23.8
Effective yield (as a percent)	7.10%	5.60%
Term Loan \| NSM Holding Co LLC \| NSM Bank Facility
Debt Instrument
Debt instrument at face value	$ 180.4	$ 0.0
Unamortized Debt Issuance Expense	$ (3.8)	0.0
Debt		$ 0.0
Effective yield (as a percent)	7.40%
Term Loan \| MediaAlpha \| MediaAlpha Bank Facility
Debt Instrument
Debt instrument at face value	$ 14.3

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

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- Definition

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

+ References

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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.

+ References

No definition available.

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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.

+ References

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- Definition

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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