Contract Manufacturing Agreements - Additional Information (Detail)	1 Months Ended	12 Months Ended
	Aug. 31, 2017 USD ($) Right $ / shares shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Long Term Purchase Commitment [Line Items]
Accounts payable		$ 13,215,000	$ 9,304,000
Accrued research and development		19,831,000	44,973,000
Non-cash charge		55,360,000	43,284,000	$ 30,362,000
Prepaid and other long-term assets		20,577,000	$ 9,609,000
Liability-classified Lonza award
Long Term Purchase Commitment [Line Items]
Total financial liabilities		9,200,000
Liability-classified Lonza award | Research and Development Expense
Long Term Purchase Commitment [Line Items]
Non-cash charge		9,200,000
Lonza Manufacturing Services Agreement
Long Term Purchase Commitment [Line Items]
Term of manufacturing commitments	10 years
Lonza Manufacturing Services Agreement | Common Stock
Long Term Purchase Commitment [Line Items]
Number of separate rights to purchase shares of common stock | Right	2
Follow on offering price per share | $ / shares	$ 1.00
Average trading period from date of purchase right	20 days
Lonza Manufacturing Services Agreement | Common Stock | Maximum
Long Term Purchase Commitment [Line Items]
Aggregate market value of shares to be purchased under each right	$ 15,000,000
Number of shares to be purchased under each right | shares	500,000
CMC ICOS Biologics Inc
Long Term Purchase Commitment [Line Items]
Accounts payable		100,000
Accrued manufacturing expenses		2,300,000
Accrued research and development		0
Prepaid manufacturing services		0
Hovione, Limited
Long Term Purchase Commitment [Line Items]
Prepaid and other long-term assets		10,900,000
Hovione, Limited | Maximum
Long Term Purchase Commitment [Line Items]
Cancellable additional purchase commitments		$ 2,900,000