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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) | 12 Months Ended
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Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Schedule Of Accounting Policies [Line Items] |  |  |
Percentage of comparable consolidated amounts | 95.00% | 95.00% | 95.00%
Original maturity of cash equivalent | 90 days |  |
Experience rated commission and other operating costs | $ 300,000 | $ 0 | $ (9,300,000)
Advertising expense | 700,000 | 500,000 | $ 500,000
Premium tax benefit | $ 3,800,000 | $ 4,000,000 |
Assessment Related premium Payment period | 1 year |  |
Assessment Related loss Payment period | 1 year |  |
Land and Office Building [Member] |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |
Estimated useful life of assets | 39 years |  |
Limited Partnership [Member] |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |
Unfunded commitments | $ 0 |  |
Maximum [Member] |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |
Short-term investments maturity period | P1Y |  |
Maximum [Member] | Other Fixed Assets [Member] |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |
Estimated useful life of assets | 7 years |  |
Minimum [Member] |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |
Short-term investments maturity period | P3M |  |
Minimum [Member] | Other Fixed Assets [Member] |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |
Estimated useful life of assets | 3 years |  |

X
- Definition Assessments Related Losses Payment Period Minimum + References No definition available. + Details
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- Definition Increase\(Decrease) in underwriting and certain other operating costs as impacted by adjustments to the experience-rated commission in the year. + References No definition available. + Details
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- Definition Schedule Of Accounting Policies [Line Items] + References No definition available. + Details
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X
- Definition Amount charged to advertising expense for the period, which are expenses incurred with the objective of increasing revenue for a specified brand, product or product line. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 720 -SubTopic 35 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6420018&loc=d3e36677-107848 + Details
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