Purchase Commitments - Additional Information (Detail)	1 Months Ended	3 Months Ended
	Aug. 31, 2017 USD ($) Right $ / shares shares	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Long Term Purchase Commitment [Line Items]
Prepaid expenses and other current assets		$ 14,569,000	$ 9,010,000
Prepaid and other long-term assets		4,812,000	$ 9,609,000
Lonza Manufacturing Services Agreement
Long Term Purchase Commitment [Line Items]
Term of manufacturing commitments	10 years
Contingently issuable equity awards aggregate fair value		0
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
Average trading period from date of purchase right	20 days
Maximum | Lonza Manufacturing Services Agreement | Common Stock
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
Hovione, Limited
Long Term Purchase Commitment [Line Items]
Prepaid expenses and other current assets		8,200,000
Prepaid and other long-term assets		4,800,000
Hovione, Limited | Maximum
Long Term Purchase Commitment [Line Items]
Cancellable additional purchase commitments		$ 8,600,000