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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) | 0 Months Ended | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |
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Nov. 14, 2013 | Apr. 29, 2004 | Dec. 16, 2003 | Aug. 31, 2012 | May 31, 2012 | Jul. 31, 2011 | Dec. 31, 2013 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Sep. 15, 2013 | Jan. 31, 2013
Schedule Of Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Percentage of comparable consolidated amounts |  |  |  |  |  |  |  | 95.00% | 95.00% | 95.00% |  |
Original maturity of cash equivalent |  |  |  |  |  |  |  | 3 months |  |  |  |
Decrease in underwriting and other operating costs | $ 1,500,000 |  |  |  |  |  | $ 1,700,000 |  |  |  |  |
Increased income tax expense | 500,000 |  |  |  |  |  | 600,000 |  |  |  |  |
Net income statement impact | 1,000,000 |  |  |  |  |  | 1,100,000 |  |  |  |  |
Net income statement impact per share | $ 0.05 |  |  |  |  |  | $ 0.06 |  |  |  |  |
Cash received on commutation of reinsurance agreement |  |  |  |  |  |  |  |  |  |  | 14,900,000 | 300,000
Experience rated commission and other operating costs |  |  |  |  |  |  |  | 9,300,000 | 11,100,000 | 5,700,000 |  |
Commutation payment adjusted through offset against balance of funds |  |  |  |  |  |  |  |  |  |  | 17,600,000 |
Advertising expense |  |  |  |  |  |  |  | 491,000 | 444,000 | 421,000 |  |
Premium tax benefit |  |  |  |  |  |  |  | 3,600,000 | 3,400,000 |  |  |
Assessment Related premium Payment period |  |  |  |  |  |  |  | 1 year |  |  |  |
Assessment Related loss Payment period |  |  |  |  |  |  |  | 1 year |  |  |  |
Proceeds from issuance of trust preferred securities |  | 25,000,000 | 10,000,000 |  |  |  |  |  |  |  |  |
Purchase of subordinated debt securities |  | 25,800,000 | 10,300,000 |  |  |  |  |  |  |  |  |
Redemption of debt securities |  |  |  | 12,900,000 | 12,900,000 | 10,300,000 |  |  | 25,780,000 | 10,310,000 |  |
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 | $ 0 |  |  |  |
Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Short-term investments maturity period |  |  |  |  |  |  |  | P1Y |  |  |  |
Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Short-term investments maturity period |  |  |  |  |  |  |  | P3M |  |  |  |
Other Assets [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Estimated useful life of assets |  |  |  |  |  |  |  | 7 years |  |  |  |
Other Assets [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |  |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Estimated useful life of assets |  |  |  |  |  |  |  | 3 years |  |  |  |

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