Income Taxes - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Contingency [Line Items]
Increase in valuation allowance		$ 60,800	$ 4,800
Total gross deferred tax assets		109,865	47,976
Unrecognized tax benefits		$ 3,934	$ 10,116	$ 4,061	$ 332
Federal tax benefit at statutory rate		34.00%	34.00%	34.00%
Provisional amount related to re-measurement of deferred tax balance, reduction amount		$ 33,000
Scenario, Plan [Member]
Income Tax Contingency [Line Items]
Federal tax benefit at statutory rate	21.00%
Federal [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards		230,000
Federal [Member] | Research and Development Tax Credits [Member]
Income Tax Contingency [Line Items]
Tax credits		$ 860
Tax credit, expiration		Begin to expire in 2029
Federal [Member] | Orphan Drug Credit [Member]
Income Tax Contingency [Line Items]
Tax credits		$ 32,000
Tax credit, expiration		Begins to expire in 2033
State [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards		$ 55,000
Operating loss carryforwards, expiration		Begin to expire in 2029
State [Member] | Research and Development Tax Credits [Member]
Income Tax Contingency [Line Items]
Tax credits		$ 2,460
Foreign [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards		$ 171,000
Operating loss carryforwards, expiration		Begin to expire in 2023