Changes in Liability for Unrecognized Gross Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Contingency [Line Items]
Uncertain tax positions at beginning of year	$ 33,487	$ 30,456	$ 24,779
Additions for tax positions relating to current-year operations	4,332	1,304	3,827
Additions for tax positions relating to prior tax years	1,398	1,997	2,935
Subtractions for tax positions relating to prior tax years	(5,101)	(270)	(963)
Reductions in balance due to settlements	(435)		(122)
Uncertain tax positions at end of year	$ 33,681	$ 33,487	$ 30,456