Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Line Items]
Income tax benefit	$ 0	$ 0	$ 0
Unrecognized tax benefits	0	$ 0	$ 0
Massachusetts [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 265,671,000
Net operating loss carry forwards, expiration year, start	2035
After 2017 [Member] | Maximum [Member]
Income Tax Disclosure [Line Items]
Taxable income on NOL	80.00%
Federal [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 266,485,000
Federal [Member] | Before 2018 [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 119,781,000
Net operating loss carry forwards, expiration year, start	2035
Federal [Member] | After 2017 [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 146,704,000
Federal [Member] | Research and Development Tax Credit [Member]
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 31,828,000
Tax credit carryforwards, expiration year, start	2031
Federal [Member] | Orphan Drug Credit Carryforwards [Member]
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 18,817,000
State [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	265,671,000
State [Member] | Research and Development Tax Credit [Member]
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 6,043,000
Tax credit carryforwards, expiration year, start	2028