Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Percentage of comparable consolidated amounts		95.00%	95.00%	95.00%
Right-of-use assets		$ 430,000
Operating lease liabilities		$ 430,000
Original maturity of cash equivalent		90 days
Minimum period of underwriting workers’ compensation insurance from which historical loss data utilize		34 years
Experience rated commission and other operating costs		$ 0	$ 200,000	$ 200,000
Commutation payment adjusted through offset against balance of funds withheld and recoverable from reinsurers accounts		8,500,000
Advertising expense		$ 400,000	500,000	$ 500,000
Assessment Related premium Payment period		1 year
Assessment Related loss Payment period		1 year
Buildings [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Estimated useful life of assets		39 years
Minimum [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Short-term investments maturity period		90 days
Minimum [Member] | Other Fixed Assets [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Estimated useful life of assets		3 years
Maximum [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Short-term investments maturity period		1 year
Maximum [Member] | Other Fixed Assets [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Estimated useful life of assets		7 years
ASU 2016-01 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Decrease in net income	$ 390,000
ASU 2016-01 [Member] | Common Shares [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Decrease to diluted earnings per common share	$ 0.02
ASU 2016-01 [Member] | Accumulated Earnings [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Impact of adoption of ASU			$ (615,000)
ASU 2016-02 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Right-of-use assets					$ 400,000
Operating lease liabilities					$ 300,000
Impact of adoption of ASU		$ 1,000
ASU 2016-02 [Member] | Accumulated Earnings [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Impact of adoption of ASU		1,000
ASU 2016-13 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Credit allowance for held-to-maturity fixed income securities		308,000
Credit allowance for reinsurance recoverables		443,000
ASU 2016-13 [Member] | Accumulated Earnings [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Impact of adoption of ASU		$ 593,000