Debt and standby letters of credit facilities - 2017 SEK Subordinated Notes and 2016 SIG Senior Notes (Details)				3 Months Ended
	Sep. 22, 2017 USD ($)	Sep. 22, 2017 SEK (kr)	Nov. 01, 2016 USD ($)	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Sep. 22, 2017 SEK (kr)
Debt and standby letters of credit facilities
Interest expense on debt				$ 7,800,000	$ 7,600,000
2017 SEK Subordinated Notes
Debt and standby letters of credit facilities
Debt notes issued	$ 346,100,000			274,200,000		$ 295,000,000	kr 2,750,000,000
Issue price (as a percent)	100.00%	100.00%
Term of redemption following occurrence of Specified Event (in days)	90 days	90 days
Debt issuance costs	$ 4,600,000
Underwriting fees	$ 3,500,000	kr 27,500,000
Foreign currency exchange (losses) gains on remeasurement of debt				$ 20,600,000	$ 11,000,000
Effective rate				4.20%	3.70%
Interest expense on debt				$ 3,000,000	$ 2,800,000
Effective rate				4.20%		4.00%
2016 SIG Senior Notes
Debt and standby letters of credit facilities
Debt notes issued			$ 400,000,000	$ 400,000,000		$ 400,000,000
Issue price (as a percent)			99.209%
Debt issuance costs			$ 5,100,000
Underwriting fees			3,400,000
Interest expense on debt				$ 4,800,000	$ 4,800,000
Net proceeds			$ 392,400,000
Annual interest rate (as a percent)			4.60%
Effective rate				4.70%		4.70%