Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrecognized Income Tax Benefit [Roll Forward]
Beginning balance, unrecognized tax benefits	$ 26,155	$ 21,834	$ 18,641
Additions based on tax provisions related to the current year	4,143	3,332	9,079
Additions for tax positions due to acquisitions	9,208
Additions based on tax provisions related to the prior year	1,171	2,496	477
Deductions based on settlement/expiration of prior year tax positions	(9,119)	(1,507)	(6,363)
Ending balance, unrecognized tax benefits	$ 31,558	$ 26,155	$ 21,834