Investment Securities - Schedule of Investments in Accordance with Fair Value Measurement (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Class of securities:
Total fixed-maturity securities, Estimated Fair Value	$ 3,660	$ 76,831
Total investments fair value	34,440	81,379
Money Market Funds [Member]
Class of securities:
Money market funds	30,780	4,548
U.S. Government Treasury Securities [Member]
Class of securities:
Total fixed-maturity securities, Estimated Fair Value		249
Obligations of U.S. Government Sponsored Enterprises [Member]
Class of securities:
Total fixed-maturity securities, Estimated Fair Value	2,239	59,371
Corporate Debt Securities [Member]
Class of securities:
Total fixed-maturity securities, Estimated Fair Value	1,421	11,415
Foreign Corporate Debt Securities [Member]
Class of securities:
Total fixed-maturity securities, Estimated Fair Value		5,796
Level 1
Class of securities:
Total fixed-maturity securities, Estimated Fair Value	2,416	60,567
Total investments fair value	33,196	65,115
Level 1 | Money Market Funds [Member]
Class of securities:
Money market funds	30,780	4,548
Level 1 | U.S. Government Treasury Securities [Member]
Class of securities:
Total fixed-maturity securities, Estimated Fair Value		249
Level 1 | Obligations of U.S. Government Sponsored Enterprises [Member]
Class of securities:
Total fixed-maturity securities, Estimated Fair Value	995	47,489
Level 1 | Corporate Debt Securities [Member]
Class of securities:
Total fixed-maturity securities, Estimated Fair Value	1,421	7,033
Level 1 | Foreign Corporate Debt Securities [Member]
Class of securities:
Total fixed-maturity securities, Estimated Fair Value		5,796
Level 2
Class of securities:
Total fixed-maturity securities, Estimated Fair Value	1,244	16,264
Total investments fair value	1,244	16,264
Level 2 | Obligations of U.S. Government Sponsored Enterprises [Member]
Class of securities:
Total fixed-maturity securities, Estimated Fair Value	$ 1,244	11,882
Level 2 | Corporate Debt Securities [Member]
Class of securities:
Total fixed-maturity securities, Estimated Fair Value		$ 4,382