Income Taxes - Summary of Changes in Deferred Tax Asset Valuation Allowance (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	$ 88,453	$ 70,629	$ 53,923
Balance at End of Year	68,661	88,453	70,629
Valuation Allowance of Deferred Tax Assets
Movement in Valuation Allowances and Reserves [Roll Forward]
Charged to Costs and Expenses	17,898	17,824	16,706
Write-offs	$ (37,690)	$ 0	$ 0