Income Taxes - Deferred Tax Assets and Liabilities Result from Differences in Timing of Recognition of Certain Income and Expense Items for Tax and Financial Accounting Purposes (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred income tax assets:
Net operating loss carryforwards	$ 25,745	$ 38,419
Tax credits	43,838	37,305
Compensation	15,934	18,124
Deferred revenue	27,587	22,248
Research and development expense deferral	12,631
Other	5,393	9,055
Gross deferred income tax assets	131,128	125,151
Less: valuation allowance	(20,415)	(7,808)
Net deferred income tax assets	110,713	117,343
Deferred income tax liabilities:
Depreciation and amortization	(60,872)	(67,504)
Deferred revenue	(54,508)
Total deferred income tax liabilities	(115,380)	(67,504)
Net deferred income taxes		49,839
Net deferred income taxes	(4,667)
Deferred income taxes / liabilities included in the balance sheet are:
Deferred income tax asset - noncurrent	27,048	66,749
Deferred income tax liability - noncurrent	(31,715)	(16,910)
Net deferred income taxes		$ 49,839
Net deferred income taxes	$ (4,667)