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Discontinued Operations Discontinued Operations (Surplus Note Table) (Details) (Details) - USD ($) $ in Millions |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 23, 2014 | Dec. 31, 2014 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Gain (loss) on sale of discontinued operations, net of tax |  |  | $ 18.2 | $ (1.6) | $ 46.6
Surplus Note |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
RBC Score |  |  | 250.00% |  |
OneBeacon Runoff |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Surplus notes | $ 101.0 |  |  |  |
OneBeacon Runoff | Other long-term investments | Fair Value [Member] |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Surplus notes | $ 64.9 |  |  |  |
OneBeacon Runoff | Surplus Note |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
RBC Score | 250.00% |  |  |  |
Discontinued Operations |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Gain (loss) on sale of discontinued operations, net of tax |  |  |  |  | 46.6
Discontinued Operations | OneBeacon Runoff |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Gain (loss) on sale of discontinued operations, net of tax |  |  | $ 0.3 | (18.8) | $ 46.6
Discontinued Operations | OneBeacon Runoff | Surplus Note |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
RBC Score |  |  | 200.00% |  |
Gain (loss) on sale of discontinued operations, net of tax |  | $ 36.1 |  |  |
Interest rate (as a percent) |  |  | 6.00% |  |
Gain (loss) on sale of discontinued operations, net of tax | $ 36.1 |  |  |  |
Par Value [Member] | Discontinued Operations | OneBeacon Runoff | Surplus Note |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Surplus notes | 101.0 |  |  |  |
Fair Value [Member] | Discontinued Operations | OneBeacon Runoff | Surplus Note |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Surplus notes | 64.9 |  |  |  |
OneBeacon |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Gain (loss) on sale of discontinued operations, net of tax | (36.1) |  | $ (49.5) | (35.9) |
OneBeacon | Seller Priority Surplus Note [Member] |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Gain (loss) on sale of discontinued operations, net of tax | (14.0) |  | (19.9) |  |
OneBeacon | Pari Passu Surplus Note [Member] |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Gain (loss) on sale of discontinued operations, net of tax | (22.1) |  | (29.6) |  |
OneBeacon | Par Value [Member] |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Surplus notes | 101.0 | 101.0 | 101.0 | 101.0 |
OneBeacon | Par Value [Member] | Seller Priority Surplus Note [Member] |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Surplus notes |  |  | 57.9 |  |
OneBeacon | Par Value [Member] | Pari Passu Surplus Note [Member] |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Surplus notes | 43.1 |  | 43.1 |  |
OneBeacon | Par Value [Member] | OneBeacon Runoff | Seller Priority Surplus Note [Member] |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Surplus notes | 57.9 |  |  |  |
OneBeacon | Current market rates on repayments |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Gain (loss) on sale of discontinued operations, net of tax | (6.8) |  | (15.1) | (6.6) |
OneBeacon | Current market rates on repayments | Seller Priority Surplus Note [Member] |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Gain (loss) on sale of discontinued operations, net of tax | 1.6 |  | (0.4) |  |
OneBeacon | Current market rates on repayments | Pari Passu Surplus Note [Member] |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Gain (loss) on sale of discontinued operations, net of tax | (8.4) |  | (14.7) |  |
OneBeacon | Regulatory Approval [Member] |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Gain (loss) on sale of discontinued operations, net of tax | (12.6) |  | (24.2) | (12.6) |
OneBeacon | Regulatory Approval [Member] | Seller Priority Surplus Note [Member] |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Gain (loss) on sale of discontinued operations, net of tax | (4.6) |  | (11.7) |  |
OneBeacon | Regulatory Approval [Member] | Pari Passu Surplus Note [Member] |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Gain (loss) on sale of discontinued operations, net of tax | (8.0) |  | (12.5) |  |
OneBeacon | Liquidity Adjustment [Member] |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Gain (loss) on sale of discontinued operations, net of tax | (16.7) |  | (10.2) | (16.7) |
OneBeacon | Liquidity Adjustment [Member] | Seller Priority Surplus Note [Member] |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Gain (loss) on sale of discontinued operations, net of tax | (11.0) |  | (7.8) |  |
OneBeacon | Liquidity Adjustment [Member] | Pari Passu Surplus Note [Member] |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Gain (loss) on sale of discontinued operations, net of tax | (5.7) |  | (2.4) |  |
OneBeacon | Fair Value [Member] |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Surplus notes | 64.9 | $ 65.1 | 51.5 | $ 65.1 |
OneBeacon | Fair Value [Member] | Seller Priority Surplus Note [Member] |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Surplus notes | 43.9 |  | 38.0 |  |
OneBeacon | Fair Value [Member] | Pari Passu Surplus Note [Member] |  |  |  |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |  |  |  |
Surplus notes | $ 21.0 |  | $ 13.5 |  |

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