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, 2017	Dec. 31, 2016
Deferred income tax assets:
Net operating loss carryforwards	$ 38,419	$ 65,351
Tax credits	37,305	25,173
Compensation	18,124	39,340
Deferred revenue	22,248	27,303
Other	9,055	6,279
Gross deferred income tax assets	125,151	163,446
Less: valuation allowance	(7,808)	(9,659)
Net deferred income tax assets	117,343	153,787
Deferred income tax liabilities:
Depreciation and amortization	(67,504)	(102,657)
Total deferred income tax liabilities	(67,504)	(102,657)
Net deferred income taxes	49,839	51,130
Deferred income taxes / liabilities included in the balance sheet are:
Deferred income tax asset - noncurrent	66,749	77,479
Deferred income tax liability - noncurrent	(16,910)	(26,349)
Net deferred income taxes	$ 49,839	$ 51,130