Common stock and preferred stock - Additional Information (Detail) $ / shares in Units, $ in Millions	1 Months Ended
	Aug. 31, 2018 USD ($) $ / shares shares	Jul. 31, 2018 USD ($) $ / shares shares	Jan. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2019 vote shares	Dec. 31, 2018 shares
Equity [Abstract]
Common stock, shares authorized (in shares)				125,000,000	125,000,000
Common stock, number of votes (in votes per share) | vote				1
Common stock, shares issued (in shares)				55,368,000	54,738,000
Common stock, shares outstanding (in shares)				55,368,000	54,738,000
Number of shares issued in public offering (in shares)	400,000	3,900,000	300,000
Shares issued (in dollars per share) | $ / shares	$ 238.10	$ 162.50	$ 185.00
Proceeds from public offering of common stock, net of issuance costs | $	$ 100.0	$ 600.6	$ 48.7
Common stock amount attributable to prepayment of joint research activities | $	$ 45.5
Preferred stock, shares authorized (in shares)				5,000,000.0	5,000,000
Preferred Stock, shares issued (in shares)				0	0
Preferred stock, shares outstanding (in shares)				0	0