Investment Securities (Investments Held on Deposit or as Collateral) (Details) - USD ($)	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Investment securities
Investments held in trusts	$ 165,900,000	$ 143,600,000
Assets held by insurance regulators	$ 93,500,000	$ 88,000,000
Percentage of investments recorded at fair value	94.00%	91.00%
Surety Product Line [Member] \| OneBeacon
Investment securities
Assets Held in Collateral	$ 153,000,000	$ 137,700,000
Private equity funds
Investment securities
Other than Temporary Impairment Losses, Investments	5,000,000	$ 2,400,000.0
Fixed maturity investments \| White Mountains Life Re
Investment securities
Derivative Collateral Right to Reclaim	$ 0

Carrying amount of cash and investment securities on deposit with state regulatory authorities in connection with capital requirements.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 320
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=28366721&loc=d3e22002-158495

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- Definition

The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e25351-111560

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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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- Definition

Cash and investments held in collateral.

+ References

No definition available.

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- Definition

The amount of collateral held against amounts due to counterparties for over the counter derivative instruments against net derivative instrument positions.

+ References

No definition available.

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- Definition

Investments Held In Trusts

+ References

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- Definition

Represents the percentage of the total investment portfolio recorded at fair value based upon quoted market prices or other observable inputs.

+ References

No definition available.

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