Debt and standby letters of credit facilities (Details)	Mar. 31, 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	$ 664,800,000	$ 685,200,000
2017 SEK Subordinated Notes
Debt and standby letters of credit facilities
Debt, at face value	274,200,000	295,000,000	$ 346,100,000	kr 2,750,000,000
Unamortized issuance costs	(3,600,000)	(3,800,000)
Long-term Debt, Total	$ 270,600,000	$ 291,200,000
Effective rate	4.20%	4.00%
2016 SIG Senior Notes
Debt and standby letters of credit facilities
Debt, at face value	$ 400,000,000	$ 400,000,000			$ 400,000,000
Unamortized issuance costs	(3,600,000)	(3,700,000)
Unamortized discount	(2,200,000)	(2,300,000)
Long-term Debt, Total	$ 394,200,000	$ 394,000,000
Effective rate	4.70%	4.70%