Purchase Commitments - Additional Information (Detail)	1 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2016 USD ($) Batch	Apr. 30, 2016 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) ManufacturingLine l	Dec. 31, 2015 USD ($)
Long Term Purchase Commitment [Line Items]
Charge				$ 0
Accounts payables	$ 14,546,000			14,546,000	$ 10,279,000
Accrued research and development	23,818,000			23,818,000	24,195,000
Unamortized advance payments recorded in prepaid research and development	7,299,000			7,299,000	16,976,000
Prepaid and other long-term assets	5,214,000			$ 5,214,000	$ 11,993,000
CMC ICOS Biologics Inc
Long Term Purchase Commitment [Line Items]
Number of manufacturing lines | ManufacturingLine				2
Purchase commitment, description				Under the aCSA, the batch price is fixed at $1.0 million, and we are required to purchase twenty batches to be manufactured in 2017 and a further ten batches to be manufactured in 2018 contingent upon the successful delivery of specified services in the aCSA.
Commercial agreement fixed commitment amount	$ 1,000,000
Number of batches to be manufactured, 2017 | Batch	20
Number of batches to be manufactured, 2018 | Batch	10
Purchase obligation services received				$ 33,700,000
Percentage of reduction in cash outlay				50.00%
Term of commercial supply agreement				2 years
Accounts payables	$ 1,500,000			$ 1,500,000
Accrued research and development	4,000,000			4,000,000
CMC ICOS Biologics Inc | Research and Development Expense
Long Term Purchase Commitment [Line Items]
Charge			$ 27,300,000	$ 0
CMC ICOS Biologics Inc | Line A/B
Long Term Purchase Commitment [Line Items]
Purchase commitment, description				(i) the 2,500 liter manufacturing line which has been used since inception of the program to supply clinical drug product, referred to as “Line A/B”
Purchase commitment volume | l				2,500
CMC ICOS Biologics Inc | Line C
Long Term Purchase Commitment [Line Items]
Purchase commitment, description				(ii) the 6x2,000 liter manufacturing line referred to as “Line C” which was intended to satisfy the drug product requirements of our initial commercial launch.
Purchase commitment volume | l				12,000
Hovione, Limited
Long Term Purchase Commitment [Line Items]
Commercial agreement advance payment amount		$ 20,900,000
Unamortized advance payments recorded in prepaid research and development	6,300,000			$ 6,300,000
Prepaid and other long-term assets	$ 5,000,000			$ 5,000,000
Hovione, Limited | Maximum
Long Term Purchase Commitment [Line Items]
Cancellable additional purchase commitments		$ 23,100,000