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Loss And Loss Adjustment Expense Reserves Loss And Loss Adjustment Expense Reserves - Additional Information (Detail) $ in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($) data_subset | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Number of subsets of business data reviewed (over) | data_subset | 400 |  |
Reserve development, prior years | $ 25.9 | $ (87.5) | $ (315.1)
Short-duration Insurance Contracts, Accident Year 2016 and 2015 [Member] |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Reserve development, prior years | 64.0 |  |
Short-duration Insurance Contracts, Accident Years 2014 and Prior [Member] |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Reserve development, prior years | (38.0) |  |
2015 |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Reserve development, prior years |  | (56.0) |
2013 |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Reserve development, prior years |  | (51.0) |
2014 |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Reserve development, prior years |  | 19.0 | (239.0)
Commercial Lines Segment |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Reserve development, prior years | 1.0 |  |
Property Segment |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Reserve development, prior years | $ (37.0) |  |
Auto |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Percentage of reserves | 98.00% |  |
Personal Auto | Personal Lines Segment |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Reserve development, prior years | $ 70.0 | (40.0) | $ (217.0)
Personal Lines - Liability | Personal Lines Segment |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Reserve development, prior years |  | (54.0) |
Property | Property Segment |  |  |
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items] |  |  |
Reserve development, prior years |  | $ (52.0) |

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