Credit Loss Impairments on Fixed Maturity Securities, Non-Credit Portion of Other-Than-Temporary Impairment Loss Recognized in Other Comprehensive Income, and Corresponding Changes (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Balance at beginning of period	$ (3,111)	$ (2,040)
Accretion of credit loss impairments previously recognized	(698)	(1,071)
Balance at end of period	$ (3,809)	$ (3,111)