Summary of significant accounting policies and basis of presentation - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2020 USD ($) target	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Accounting Policies [Abstract]
Number of operating segments | target	1
Goodwill, impaired, accumulated impairment loss	$ 0
Interest income net of amortization of premium and accretion of discount	11,500,000	$ 34,800,000	$ 30,100,000
Interest expenses	$ 0.0	$ 0.0	$ 15,500,000