Investment Securities - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Estimated Fair Values of Fixed-Maturity Securities (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed-maturity securities, Amortized Cost	$ 3,167	$ 3,674
Total fixed-maturity securities, Gross Unrealized Losses	(14)	(14)
Total fixed-maturity securities, Estimated Fair Value	3,153	3,660
Unrestricted Fixed-Maturity Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed-maturity securities, Amortized Cost	2,917	2,151
Total fixed-maturity securities, Gross Unrealized Losses	(13)	(9)
Total fixed-maturity securities, Estimated Fair Value	2,904	2,142	[1]
Fixed-Maturity Securities Held in Trust [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed-maturity securities, Amortized Cost	250	1,523
Total fixed-maturity securities, Gross Unrealized Losses	(1)	(5)
Total fixed-maturity securities, Estimated Fair Value	249	1,518	[1]
Obligations of U.S. Government Sponsored Enterprises [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed-maturity securities, Amortized Cost	2,250	2,249
Total fixed-maturity securities, Gross Unrealized Losses	(8)	(10)
Total fixed-maturity securities, Estimated Fair Value	2,242	2,239
Corporate Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed-maturity securities, Amortized Cost	917	1,425
Total fixed-maturity securities, Gross Unrealized Losses	(6)	(4)
Total fixed-maturity securities, Estimated Fair Value	$ 911	$ 1,421

[1] Balances derived from audited financial statements as of December 31, 2017.