Summary of Activity Related to Tax Contingencies Recorded As an Offset to Deferred Tax Assets (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Gross tax contingencies, beginning balance	$ 1,070	$ 888	$ 717
Gross increases to tax positions associated with prior periods	0	0	0
Gross increases to current period tax positions	52	182	171
Gross decreases to tax positions associated with prior periods	0	0	0
Settlements	0	0	0
Lapse of statute of limitations	0	0	0
Gross tax contingencies, ending balance	$ 1,122	$ 1,070	$ 888