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Debt and Standby Letter of Credit Facilities (Debt Outstanding) (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Aug. 14, 2013
Debt Instrument [Line Items] |  |  |  |
Debt | $ 285.9 | $ 337.6 |  |
Interest expense on debt | 16.1 | 14.6 | $ 14.2 |
Draw down of debt and revolving line of credit | 352.5 | 171.5 | 133.6 |
WTM Bank Facility |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Total commitment under revolving credit facility |  |  |  | $ 425.0
Interest expense on debt | 1.2 | 0.1 | $ 0.3 |
Prior W T M Bank Facility |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Total commitment under revolving credit facility |  |  |  | $ 375.0
OneBeacon | 2012 OBH Senior Notes |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt instrument at face value | 275.0 | 275.0 |  |
Unamortized original issue discount | (1.8) | (2.1) |  |
Debt | $ 273.2 | $ 272.9 |  |
Effective yield (as a percent) | 4.70% | 4.70% |  |
White Mountains | WTM Bank Facility |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt | $ 0.0 | $ 50.0 |  |
Effective yield (as a percent) |  | 3.90% |  |
MediaAlpha [Member] | MediaAlpha Bank Facility [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Total commitment under revolving credit facility |  | $ 20.0 |  |
Debt instrument at face value | 12.9 | 15.0 |  |
Unamortized Debt Issuance Expense | $ (0.2) | $ (0.3) |  |
Effective yield (as a percent) | 5.70% | 5.50% |  |
Carrying Value | OneBeacon | 2012 OBH Senior Notes |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt | $ 273.2 | $ 272.9 |  |
Carrying Value | MediaAlpha [Member] | MediaAlpha Bank Facility [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt | $ 12.7 | $ 14.7 |  |

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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 Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399 + Details
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- Definition Amount, after accumulated amortization, of debt discount. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400 + Details
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- Definition 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 Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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