Income Taxes - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)
Operating Loss Carryforwards [Line Items]
Federal and state net operating loss carryforwards relate to deductions for stock option compensation	$ 39.2
Federal Orphan Drug Credits	106.4
Federal [Member]
Operating Loss Carryforwards [Line Items]
NOL	542.7
Federal [Member] | Research and Development [Member]
Operating Loss Carryforwards [Line Items]
Amount of tax credit carryforward	27.3
State [Member]
Operating Loss Carryforwards [Line Items]
NOL	367.4
Federal and state net operating loss carryforwards relate to deductions for stock option compensation	25.2
State [Member] | Research and Development [Member]
Operating Loss Carryforwards [Line Items]
Amount of tax credit carryforward	16.3
State [Member] | Investment [Member]
Operating Loss Carryforwards [Line Items]
Amount of tax credit carryforward	$ 0.8