Debt and standby letters of credit facilities - 2017 SEK Subordinated Notes and 2016 SIG Senior Notes (Details) kr in Millions, $ in Millions				3 Months Ended
	Sep. 22, 2017 SEK (kr)	Sep. 22, 2017 USD ($)	Nov. 01, 2016 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Sep. 22, 2017 USD ($)
Debt and standby letters of credit facilities
Interest expense on debt				$ 7.6	$ 7.7
2017 SEK Subordinated Notes
Debt and standby letters of credit facilities
Debt notes issued	kr 2,750.0			296.6		$ 307.6	$ 346.1
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							$ 4.6
Underwriting fees	kr 27.5	$ 3.5
Foreign currency exchange (losses) gains on remeasurement of debt				11.0	5.8
Interest expense on debt				$ 2.8	$ 2.9
Annual interest rate (as a percent)				3.70%	3.50%
Effective rate				3.70%		3.80%
2016 SIG Senior Notes
Debt and standby letters of credit facilities
Debt notes issued			$ 400.0	$ 400.0		$ 400.0
Issue price (as a percent)			99.209%
Debt issuance costs			$ 5.1
Underwriting fees			3.4
Interest expense on debt				$ 4.8	$ 4.8
Net proceeds			$ 392.4
Annual interest rate (as a percent)			4.60%
Effective rate			4.70%	4.70%		4.70%