Income Taxes - Reconciliation of Gross Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of year	$ 30,410	$ 27,829	$ 26,463
Increases in tax positions for current year	251	125	39
Increases in tax positions for prior years	904	4,542	1,327
Decreases in tax positions for prior years	(53)	(199)	0
Settlement of tax matters	0	(1,887)	0
Lapse of applicable statutes of limitations	(16,458)	0	0
Balance at end of year	$ 15,054	$ 30,410	$ 27,829