Unaudited Supplementary Quarterly Financial Information - Unaudited Supplementary Quarterly Financial Information (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Increase in deferred tax valuation allowance	$ 22,000	$ 3,700	$ (13,526)	$ (3,700)	$ (3,000)