Income Taxes - Reconciliation of Unrecognized Tax Benefit (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	$ 35,240	$ 54,541	$ 50,668
Additions based on tax positions related to the current year	7,040	4,227	5,340
Additions for tax positions of prior years	3,372	2,048	292
Reductions for tax positions of prior years	(1,171)	(1,948)	(132)
Reductions for audit settlements	0	(180)	(1,345)
Reductions resulting from a lapse of the applicable statute of limitations periods	(8,252)	(23,448)	(282)
Balance at end of year	$ 36,229	$ 35,240	$ 54,541