Interim financial statements - Additional Information (Details) - USD ($)	Mar. 23, 2021	Mar. 22, 2021	Mar. 31, 2021	Dec. 31, 2020
Short-term Debt [Line Items]
Investments restricted for statutory reserve funds			$ 491,300,000	$ 496,600,000
Restricted cash and cash equivalent			$ 25,600,000	$ 20,000,000.0
Line of credit, conditional increase in maximum borrowing capacity	$ 50,000,000
Line of credit
Short-term Debt [Line Items]
Line of credit facility	350,000,000	$ 200,000,000
Restricted payment	100,000,000	40,000,000
Unsecured debt
Short-term Debt [Line Items]
Indebtedness provision	$ 250,000,000	$ 100,000,000