Income taxes - Effective Rate Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Rate Reconciliation
Statutory tax rate (as a percent)		21.00%
Tax (expense) benefit at the statutory rate	$ (5.2)	$ 24.2	$ (8.3)
Differences in taxes resulting from:
Tax reserve adjustments	(42.0)	(0.7)	(6.0)
Tax on Safety Reserve	(15.3)
Foreign tax credits	10.8	2.2	6.9
Section 197 Intangible as result of internal restructuring	6.9
Gain on internal restructuring	(9.1)
State taxes expense	(2.9)	(0.7)	(1.1)
Change in valuation allowance	2.3	1.4	55.0
Withholding taxes	(1.9)	(0.8)	(1.4)
Other, net	(1.8)	(4.3)	(0.3)
Total income tax (expense) benefit	(40.4)	(26.4)	7.3
Pre-tax (loss) income	$ 23.7	$ (109.9)	$ 37.9
Swedish Tax Authority
Effective Rate Reconciliation
Statutory tax rate (as a percent)	22.00%	22.00%	22.00%
Sweden
Effective Rate Reconciliation
Statutory tax rate (as a percent)	22.00%
Differences in taxes resulting from:
Tax rate change	$ 15.4
Other
Differences in taxes resulting from:
Tax rate change	0.1	$ (29.7)
Luxembourg
Differences in taxes resulting from:
Tax rate change		0.4	$ (30.6)
Non-Sweden
Differences in taxes resulting from:
Non-Sweden Earnings	2.3	(18.4)	(6.9)
Pre-tax (loss) income	$ 97.9	$ (83.8)	$ 67.3