Income Taxes - Components of U.S. Deferred Tax Assets and (Liabilities) (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Federal and state net operating loss carryforwards	$ 285,685	$ 241,881
Federal and state research tax credit carryforwards	24,782	23,917
Federal Orphan Drug Credit	112,802	120,273
Deferred revenue	44,588	29,530
Stock options	20,893	19,992
Inventories	10,617	4,350
Operating lease liabilities	3,146	0
Other	7,861	5,098
Total deferred tax assets	510,374	445,041
Deferred tax liabilities:
Operating lease right-of-use assets	(2,793)	0
Total deferred tax liabilities	(2,793)	0
Net deferred tax assets before valuation allowance	507,581	445,041
Valuation allowance	(507,581)	(445,041)
Net deferred tax assets	$ 0	$ 0