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Share Capital - Additional Information (Detail)	1 Months Ended			3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 31, 2020 USD ($) $ / shares shares	Aug. 31, 2019 USD ($)	Aug. 31, 2018 USD ($)	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($) shares	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($) shares	Dec. 31, 2019 USD ($) shares	Sep. 30, 2020 SFr / shares	Dec. 31, 2019 SFr / shares shares
Class of Stock [Line Items]
Common stock, shares authorized | shares				115,172,786		115,172,786				103,901,006
Common shares, par value | SFr / shares									SFr 0.03	SFr 0.03
Net cash proceeds from sale of common shares						$ 582,225,000	$ 121,216,000
July 2020 Offering [Member]
Class of Stock [Line Items]
Common shares sold | shares	7,400,000
Net cash proceeds from sale of common shares	$ 484,800,000
Common shares price per share | $ / shares	$ 70.00
Equity issuance costs and stamp tax	$ 32,500,000
Accrued equity issuance costs and stamp taxes				$ 4,900,000		$ 4,900,000
Maximum [Member] | 2019 ATM [Member]
Class of Stock [Line Items]
Aggregate estimated gross proceeds		$ 200,000,000.0
Sales Agreement With Jefferies LLC [Member] | 2018 ATM [Member]
Class of Stock [Line Items]
Common shares sold | shares					1,500,000		2,800,000	2,800,000
Net cash proceeds from sale of common shares					$ 69,400,000		$ 121,900,000	$ 120,600,000
Issuance costs					1,500,000		3,100,000
Equity issuance costs								$ 4,400,000
Payments of stamp taxes related to securities					500,000		700,000
Accrued stamp taxes					$ 600,000		$ 600,000
Sales Agreement With Jefferies LLC [Member] | 2019 ATM [Member]
Class of Stock [Line Items]
Common shares sold | shares				100,000		1,300,000
Net cash proceeds from sale of common shares				$ 9,600,000		$ 91,900,000
Issuance costs				200,000		2,400,000
Payments of stamp taxes related to securities				400,000		400,000
Accrued stamp taxes				$ 600,000		$ 600,000
Sales Agreement With Jefferies LLC [Member] | Maximum [Member] | 2018 ATM [Member]
Class of Stock [Line Items]
Aggregate estimated gross proceeds			$ 125,000,000.0

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