Investments - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Security Fund	Dec. 31, 2016 USD ($) Security	Dec. 31, 2015 USD ($)
Investments Debt And Equity Securities [Abstract]
Number of securities | Security	26	0
Number of investment funds | Fund	3
Unfunded commitments	$ 2,500,000
Fixed maturities and cash equivalents on deposit with various insurance departments at fair value and amortized cost	6,200,000
Cash equivalents on deposit with another insurance company at fair value and amortized cost	18,300,000
Non credit other than temporary impairment for securities	1,000,000	$ 1,000,000
OTTI charges recognized in net loss	$ 0	$ 147,000	$ 0