Investments - Progression of Credit-Related Portion of OTTI on Investments (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments Debt And Equity Securities [Abstract]
Beginning balance	$ (2,184)	$ (2,632)	$ (2,632)
Additional credit impairments on:
Previously impaired securities		0	0
Securities without previous impairments		(147)	0
Total additional credit impairments securities		(147)	0
Reductions for securities deemed worthless and realized	314	595
Ending balance	$ (1,870)	$ (2,184)	$ (2,632)