Income Taxes - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($)
Operating Loss Carryforwards [Line Items]
Federal and state net operating loss carryforwards relate to deductions for stock option compensation	$ 36.5
Federal Orphan Drug Credits	62.7
Federal [Member]
Operating Loss Carryforwards [Line Items]
NOL	509.8
Federal [Member] | Research and Development [Member]
Operating Loss Carryforwards [Line Items]
Amount of tax credit carryforward	23.7
State [Member]
Operating Loss Carryforwards [Line Items]
NOL	356.9
Federal and state net operating loss carryforwards relate to deductions for stock option compensation	24.7
State [Member] | Research and Development [Member]
Operating Loss Carryforwards [Line Items]
Amount of tax credit carryforward	9.6
State [Member] | Investment [Member]
Operating Loss Carryforwards [Line Items]
Amount of tax credit carryforward	$ 0.8