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, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments Debt And Equity Securities [Abstract]
Beginning Balance	$ 2,361	$ 2,361	$ 2,361
Additions for Credit Loss Impairments recognized in the current period on securities not previously impaired	0	150	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	0	(150)	0
Ending Balance	$ 2,361	$ 2,361	$ 2,361