Debt and standby letters of credit facilities (Details)	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 22, 2017 USD ($)	Sep. 22, 2017 SEK (kr)	Nov. 01, 2016 USD ($)
Debt and standby letters of credit facilities
Long-term debt, total	$ 695,800,000	$ 685,200,000
2017 SEK Subordinated Notes
Debt and standby letters of credit facilities
Debt, at face value	305,000,000.0	295,000,000.0	$ 346,100,000	kr 2,750,000,000.0
Unamortized issuance costs	(3,700,000)	(3,800,000)
Long-term debt, total	$ 301,300,000	$ 291,200,000
Effective rate	4.30%	4.00%
2016 SIG Senior Notes
Debt and standby letters of credit facilities
Debt, at face value	$ 400,000,000.0	$ 400,000,000.0			$ 400,000,000.0
Unamortized issuance costs	(3,400,000)	(3,700,000)
Unamortized discount	(2,100,000)	(2,300,000)
Long-term debt, total	$ 394,500,000	$ 394,000,000.0
Effective rate	4.70%	4.70%