Investments - Progression of Credit-Related Portion of OTTI on Investments (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investments Debt And Equity Securities [Abstract]
Beginning balance	$ (2,632)	$ (2,632)	$ (2,632)
Additional credit impairments on:
Previously impaired securities	0	0	0
Securities without previous impairments	(147)	0	0
Total additional credit impairments securities	(147)	0	0
Reductions for securities deemed worthless (realized)	595	0	0
Ending balance	$ (2,184)	$ (2,632)	$ (2,632)