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Segment Information - Additional Information (Detail) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 USD ($) state Entity | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items] |  |  |
Depreciation | $ | $ 169.9 | $ 137.4 | $ 103.7
Operating Segments | Personal Lines Segment | Agency Channel | Lower Limit |  |  |
Segment Reporting Information [Line Items] |  |  |
Independent insurance agencies and brokerages | Entity | 35,000 |  |
Residential Insurance [Member] | Property Segment |  |  |
Segment Reporting Information [Line Items] |  |  |
Number of states | 41 |  |
National Flood Insurance Program [Member] | Property Segment |  |  |
Segment Reporting Information [Line Items] |  |  |
Number of states | 43 |  |

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- Definition Independent Insurance Agencies and Brokerages + References No definition available. + Details
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- Definition The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 + Details
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