Summary of Significant Accounting Policies - Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Examination [Line Items]
Total amount of unrecognized tax benefits relating to uncertain tax positions	$ 36,229	$ 35,240	$ 54,541	$ 50,668
Amount of unrecognized tax benefits increase	1,000
Unrecognized tax benefits, increase from current period tax positions	7,040	4,227	5,340
Unrecognized tax benefits, increase from prior year tax positions	2,200
Unrecognized tax benefits, decrease resulting from expiration of statute of limitations	8,252	23,448	282
Maximum
Income Tax Examination [Line Items]
Reasonably possible reduction to the balance of unrecognized tax benefits in succeeding 12 months	$ 13,655	$ 12,332	$ 27,485