Components of U.S. 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	$ 241,881	$ 203,897
Federal and state research tax credit carryforwards	23,917	21,238
Federal Orphan Drug Credit	120,273	96,750
Deferred revenue	29,530	17,538
Stock options	19,992	15,995
Other	9,448	7,529
Net deferred tax assets before valuation allowance	445,041	362,947
Valuation allowance	$ (445,041)	$ (362,947)