Income Taxes - Components of the Company's Deferred Tax Assets (Liabilities) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Provision for federal or state income taxes	$ 0	$ 0	$ 0
Deferred income tax assets:
Federal U.S. net operating loss carryforward	87,284,000	50,346,000
State net operating loss carryforward	22,809,000	7,551,000
Research and development credit, net	29,750,000	21,284,000
Orphan drug credit, net	22,580,000	21,708,000
Deferred rent	3,118,000	3,385,000
Deferred revenue	3,736,000	2,982,000
Depreciation	0	155,000
Other	6,854,000	5,847,000
Gross deferred income tax assets	176,131,000	113,258,000
Valuation allowance	(172,457,000)	(112,453,000)
Net deferred income tax assets	3,674,000	805,000
Deferred income tax liabilities:
Depreciation	(1,911,000)	0
Prepaid expenditures	(1,763,000)	(805,000)
Gross deferred income tax liabilities	(3,674,000)	(805,000)
Net deferred income tax asset/(liability)	0	$ 0
Operating Loss Carryforwards [Line Items]
Remaining portion of net operating losses	402,100,000
Previously unrecognized excess tax benefits	18,600,000
Previously unrecognized excess tax benefits, tax effected portion	6,500,000
U.S. Federal and State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	415,600,000
U.S. Federal Government
Operating Loss Carryforwards [Line Items]
US Federal tax credits carry forward	52,300,000
Net operating loss for limited use	13,500,000
U.S. Federal Government | Minimum
Operating Loss Carryforwards [Line Items]
Net operating losses limited for use utilized on an annual basis	200,000
U.S. Federal Government | Maximum
Operating Loss Carryforwards [Line Items]
Net operating losses limited for use utilized on an annual basis	1,400,000
Tax Year 2025 To 2037 | U.S. Federal and State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	237,800,000
Indefinite | U.S. Federal and State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 177,800,000