Income Taxes - Reconciliation of Beginning and Ending Amount of 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 of unrecognized tax benefits at beginning of year	$ 26,428	$ 24,198	$ 23,055
Gross amount of increases in unrecognized tax benefits as a result of positions taken during a prior year	1,169	1,254	18
Gross amount of decreases in unrecognized tax benefits as a result of positions taken during a prior year	(268)	(3)	(43)
Gross amount of increases in unrecognized tax benefits as a result of positions taken during the current year	13,191	8,131	12,011
The amounts of decreases in unrecognized benefits relating to settlements with taxing authorities	0	(6,075)	(10,221)
Reduction in unrecognized tax benefits due to the lapse of applicable statute of limitation	(2,358)	(1,077)	(622)
Balance of unrecognized tax benefits at end of year	$ 38,162	$ 26,428	$ 24,198