Debt and standby letters of credit facilities - 2017 SEK Subordinated Notes and 2016 SIG Senior Notes (Details)				3 Months Ended		6 Months Ended
	Sep. 22, 2017 USD ($)	Sep. 22, 2017 SEK (kr)	Nov. 01, 2016 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Sep. 22, 2017 SEK (kr)
Debt and standby letters of credit facilities
Interest expense on debt				$ 7,900,000	$ 8,000,000	$ 15,700,000	$ 15,600,000
2017 SEK Subordinated Notes
Debt and standby letters of credit facilities
Debt notes issued	$ 346,100,000			294,200,000		294,200,000		$ 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				$ (19,800,000)	$ 300,000	$ 800,000	$ 11,300,000
Effective rate				4.40%	4.10%	4.30%	3.90%
Interest expense on debt				$ 3,100,000	$ 3,200,000	$ 6,100,000	$ 6,000,000
Effective rate				4.40%		4.40%		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		$ 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	$ 9,600,000	$ 0
Net proceeds			$ 392,400,000
Annual interest rate (as a percent)			4.60%
Effective rate				4.70%		4.70%		4.70%