Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Accounting Policies [Line Items]
Percentage of comparable consolidated amounts	95.00%	95.00%	95.00%
Original maturity of cash equivalent	90 days
Monthly redemption of investments notice period	60 days
Experience rated commission and other operating costs	$ 0	$ 300,000	$ 0
Cash received, under reinsurance agreements	200,000
Commutation payment adjusted through offset against balance of funds	25,400,000
Advertising expense	$ 700,000	$ 700,000	$ 500,000
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
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	one year
Maximum [Member] | Other Fixed Assets [Member]
Schedule Of Accounting Policies [Line Items]
Estimated useful life of assets	7 years