Summary of Cumulative Amounts Related to Credit Loss Portion of Other-Than-Temporary Impairment Losses on Debt Securities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investments Debt And Equity Securities [Abstract]
Beginning balance	$ 2,361	$ 2,361	$ 5,154
Additions for credit loss impairments recognized in the current period on securities not previously impaired	150	0	0
Additions for credit loss impairments recognized in the current period on securities previously impaired	0	0	0
Reductions for credit loss impairments previously recognized on securities sold during the period	(150)	0	(2,793)
Ending balance	$ 2,361	$ 2,361	$ 2,361