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	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Investments Debt And Equity Securities [Abstract]
Balance at beginning of period	$ (2,040)	$ (935)
Additional credit loss impairments recognized in the current period on securities previously impaired through OCI		121
Accretion of credit loss impairments previously recognized	(564)	(1,226)
Balance at end of period	$ (2,604)	$ (2,040)