Income Taxes - Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Federal and state net operating loss carryforwards	$ 259,629	$ 234,316
Share-based compensation	55,280	47,655
Start-up and organizational costs	99	146
Research and development credits	48,805	35,793
Other tax credits	910	910
Accruals and reserves	3,000	2,729
Deferred rent	4,842	5,484
Other deferred tax assets	14,267	8,342
Total deferred tax assets	386,832	335,375
Deferred tax liabilities:
Website and software development costs	(14,685)	(13,202)
Goodwill	(598)	(688)
Intangible assets	(45,035)	(69,241)
Discount on 2021 Notes and 2023 Notes not deductible for tax	(31,450)	(19,374)
Depreciation and amortization	(888)	(2,425)
Total deferred tax liabilities	(92,656)	(104,930)
Net deferred tax assets before valuation allowance	294,176	230,445
Less: valuation allowance	(307,599)	(274,810)
Net deferred tax liabilities	$ (13,423)	$ (44,365)