Organization - Additional Information (Detail)	1 Months Ended				3 Months Ended	6 Months Ended
	Sep. 30, 2017 USD ($)	Apr. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)	Mar. 31, 2010 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2018 USD ($) Subsidiary Indication	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Organization [Line Items]
Number of subsidiaries | Subsidiary						2
Cash, cash equivalents and marketable securities						$ 18,088,000		$ 33,529,000
Working capital						2,800,000
Accumulated deficit						594,664,000		$ 586,969,000
Proceeds from sale of equity						518,000	$ 21,035,000
Scenario Forecast
Organization [Line Items]
Milestone payment to be received					$ 0
Collaborations and license agreements, milestone payment					0
Research and development funding received					0
Net proceeds from public offering					0
Proceeds from debt financing					0
Scenario Forecast | PIPE Warrants
Organization [Line Items]
Proceeds from warrant exercises					0
Leerink | Scenario Forecast
Organization [Line Items]
Proceeds from sale of equity					$ 0
EUSA
Organization [Line Items]
Payments received in connection with indications						$ 2,000,000
Eligible number of indications | Indication						3
Percentage of additional proceeds from research and development funding						50.00%
Collaborations and license agreements, expected milestone receivable	$ 4,000,000		$ 2,500,000
EUSA | Maximum
Organization [Line Items]
Payments received in connection with indications						$ 8,000,000
EUSA | Marketing Approval in France, Germany, Italy, Spain and the United Kingdom
Organization [Line Items]
Milestone payment to be received						2,000,000
EUSA | Marketing Approval in Australia, Brazil, New Zealand, South Africa and Switzerland
Organization [Line Items]
Milestone payment to be received						$ 2,000,000
Kyowa Hakko Kirin
Organization [Line Items]
Percentage of sublicense fee payable						30.00%
Kyowa Hakko Kirin | Licensing Agreements
Organization [Line Items]
Collaborations and license agreements, milestone payment				$ 10,000,000
Kyowa Hakko Kirin | Maximum
Organization [Line Items]
Research and development reimbursement payment not required to be paid						$ 20,000,000
CANbridge | Licensing Agreements
Organization [Line Items]
Collaborations and license agreements, expected milestone receivable		$ 1,000,000
CANbridge | Licensing Agreements | Development and Regulatory Milestone Events
Organization [Line Items]
Collaborations and license agreements, expected milestone receivable						$ 2,000,000