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Debt and Standby Letter of Credit Facilities (Narrative) (Details) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Nov. 30, 2012 | Dec. 31, 2012 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Sep. 29, 2015 | Aug. 14, 2013
Debt Instrument [Line Items] |  |  |  |  |  |  |
Debt issuance costs |  |  | $ 1.6 |  |  |  |
Interest expense on debt |  |  | 16.1 | $ 14.6 | $ 14.2 |  |
Repayments of debt, principal |  |  | 404.6 | 76.1 | 65.2 |  |
Draw down of debt and revolving line of credit |  |  | 352.5 | 171.5 | 133.6 |  |
Interest Paid |  |  | 14.8 | 12.8 | 13.0 |  |
Expenses related to issuance, including underwriting fees |  |  | 1.6 |  |  |  |
Debt |  |  | 285.9 | 337.6 |  |  |
OneBeacon Bank Facility [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  |  |  |  | $ 65.0 |
WTM Bank Facility |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Issuance of debt, net of debt issuance costs |  |  | 350.0 | 125.0 |  |  |
Interest expense on debt |  |  | 1.2 | 0.1 | 0.3 |  |
Total commitment under revolving credit facility |  |  |  |  |  |  | $ 425.0
Repayments of debt, principal |  |  | $ 400.0 | $ 75.0 |  |  |
Debt, Weighted Average Interest Rate |  |  | 3.85% | 3.74% |  |  |
Prior W T M Bank Facility |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  |  |  |  |  | $ 375.0
One Beacon US Holdings Inc. | 2012 OBH Senior Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Debt instrument at face value | $ 275.0 |  |  |  |  |  |
Percentage of par value at which debt was issued | 99.90% |  |  |  |  |  |
Issuance of debt, net of debt issuance costs | $ 272.9 |  |  |  |  |  |
Interest rate (as a percent) | 4.60% |  |  |  |  |  |
Effective yield (as a percent) | 4.70% |  |  |  |  |  |
Interest Expense |  |  | $ 13.1 | $ 13.0 | $ 13.0 |  |
One Beacon US Holdings Inc. | Senior Notes | 2003 OBH Senior Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Interest rate (as a percent) |  | 5.875% |  |  |  |  |
Debt Instrument, Repurchase Amount |  | $ 269.8 |  |  |  |  |
Aggregate purchase price of senior notes purchased and retired |  | 275.9 |  |  |  |  |
Loss on retirement of debt |  | (6.3) |  |  |  |  |
Write off of deferred issuance costs |  | $ 0.2 |  |  |  |  |
MediaAlpha [Member] | MediaAlpha Bank Facility [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  |  | 1.50% |  |  |  |
Debt instrument at face value |  |  | $ 12.9 | $ 15.0 |  |  |
Effective yield (as a percent) |  |  | 5.70% | 5.50% |  |  |
Total commitment under revolving credit facility |  |  |  | $ 20.0 |  |  |
Term Loan [Member] | MediaAlpha [Member] | MediaAlpha Bank Facility [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  | $ 15.0 |  |  |  |
Repayments of debt, principal |  |  | 2.1 |  |  |  |
Revolving Credit Facility [Member] | MediaAlpha [Member] | MediaAlpha Bank Facility [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Total commitment under revolving credit facility |  |  | 5.0 |  |  |  |
Repayments of debt, principal |  |  | $ 2.5 |  |  |  |

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