Investment Securities - Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Estimated Fair Values of Fixed-Maturity Securities (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed-maturity securities, Amortized Cost	$ 10,432	$ 76,912
Total fixed-maturity securities, Gross Unrealized Gains		15
Total fixed-maturity securities, Gross Unrealized Losses	(10)	(96)
Total fixed-maturity securities, Estimated Fair Value	10,422	76,831
Unrestricted Fixed-Maturity Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed-maturity securities, Amortized Cost	1,402	47,635
Total fixed-maturity securities, Gross Unrealized Losses	(2)	(10)
Total fixed-maturity securities, Estimated Fair Value	1,400	47,625	[1]
Fixed-Maturity Securities Held in Trust [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed-maturity securities, Amortized Cost	9,030	29,277
Total fixed-maturity securities, Gross Unrealized Gains		15
Total fixed-maturity securities, Gross Unrealized Losses	(8)	(86)
Total fixed-maturity securities, Estimated Fair Value	9,022	29,206	[1]
U.S. Government Treasury Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed-maturity securities, Amortized Cost	250	249
Total fixed-maturity securities, Estimated Fair Value	250	249
Obligations of U.S. Government Sponsored Enterprises [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed-maturity securities, Amortized Cost	2,749	59,450
Total fixed-maturity securities, Gross Unrealized Gains		1
Total fixed-maturity securities, Gross Unrealized Losses	(7)	(80)
Total fixed-maturity securities, Estimated Fair Value	2,742	59,371
Corporate Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed-maturity securities, Amortized Cost	3,683	11,415
Total fixed-maturity securities, Gross Unrealized Gains		9
Total fixed-maturity securities, Gross Unrealized Losses	(2)	(9)
Total fixed-maturity securities, Estimated Fair Value	3,681	11,415
Foreign Corporate Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total fixed-maturity securities, Amortized Cost	3,750	5,798
Total fixed-maturity securities, Gross Unrealized Gains		5
Total fixed-maturity securities, Gross Unrealized Losses	(1)	(7)
Total fixed-maturity securities, Estimated Fair Value	$ 3,749	$ 5,796

[1] Balances derived from audited financial statements as of December 31, 2016.