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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Schedule Of Accounting Policies [Line Items] |  |  |  |
Percentage of comparable consolidated amounts |  | 95.00% | 95.00% | 95.00%
Provisional amounts an increase in the deferred tax balance |  | $ 1,500,000 |  |
Original maturity of cash equivalent |  | 90 days |  |
Experience rated commission and other operating costs |  | $ 200,000 | $ 200,000 |
Cash received, under reinsurance agreements |  |  | 200,000 |
Commutation payment adjusted through offset against balance of funds |  |  | 25,400,000 |
Advertising expense |  | $ 500,000 | $ 500,000 | $ 700,000
Assessment Related premium Payment period |  | 1 year |  |
Assessment Related loss Payment period |  | 1 year |  |
Buildings [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 |  |
Minimum [Member] |  |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |  |
Short-term investments maturity period |  | 90 days |  |
Minimum [Member] | Other Fixed Assets [Member] |  |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |  |
Estimated useful life of assets |  | 3 years |  |
Maximum [Member] |  |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |  |
Short-term investments maturity period |  | 1 year |  |
Maximum [Member] | Other Fixed Assets [Member] |  |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |  |
Estimated useful life of assets |  | 7 years |  |
ASU 2016-01 [Member] |  |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |  |
Decrease in net income | $ 390,000 |  |  |
ASU 2016-01 [Member] | Common Shares [Member] |  |  |  |
Schedule Of Accounting Policies [Line Items] |  |  |  |
Decrease to diluted earnings per common share | $ 0.02 |  |  |

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