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Summary of Significant Accounting Policies Summary of Significant Accounting Policies - LIHC (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Item Effected [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net investment income |  |  |  |  |  |  |  |  | $ 86.8 | $ 60.8 | $ 59.5 |
Income Tax Expense (Benefit) | $ 23.0 | $ 6.7 | $ 6.0 | $ 9.7 | $ 1.0 | $ 1.6 | $ 2.2 | $ (4.6) | $ 45.4 | $ 0.2 | 14.8 |
OneBeacon Ltd. |  |  |  |  |  |  |  |  |  |  |  |
Item Effected [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cumulative Effect on Retained Earnings, Net of Tax |  |  |  |  |  |  |  |  |  |  | 0.9 | $ 0.4
Accounting Standards Update 2014-01 [Member] | Restatement Adjustment [Member] | OneBeacon Ltd. |  |  |  |  |  |  |  |  |  |  |  |
Item Effected [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Net investment income |  |  |  |  |  |  |  |  |  |  | 1.7 | 1.9
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  |  |  | $ (2.3) | $ (2.2)

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