Income taxes - Deferred Tax Inventory (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Deferred income tax assets related to:
Non-U.S. net operating loss carry forwards	$ 249.8	$ 305.1
U.S. federal net operating loss and capital carry forwards	33.8	34.7
Tax credit carry forwards	28.7	18.3
Loss reserve discount	7.3	8.9
Unearned premiums	3.1	1.5
Deferred interest	2.1	0.2
Incentive compensation and benefits accruals	1.9	2.0
Allowance for doubtful accounts	1.4	1.4
Foreign currency translations on investments and other assets	0.8
Net unrealized investment losses		1.8
Other items	2.7	3.6
Total gross deferred income tax assets	331.6	377.5
Valuation allowance	(64.3)	(71.8)
Total adjusted deferred tax asset	267.3	305.7
Deferred income tax liabilities related to:
Safety reserve	261.1	286.6
Intangible assets	28.7	42.0
Deferred acquisition costs	3.1	1.2
Purchase accounting	3.0	2.4
Investment basis differences	2.4	1.1
Net unrealized investment gains	0.3
Foreign currency translations on investments and other assets		7.3
Other items	3.6	3.2
Total deferred income tax liabilities	302.2	343.8
Net deferred tax (liability) asset	(34.9)	$ (38.1)
U.S. subsidiaries
Deferred income tax liabilities related to:
Net deferred tax assets	32.8
Luxembourg subsidiaries
Deferred income tax liabilities related to:
Net deferred tax assets	155.8
United Kingdom subsidiaries
Deferred income tax liabilities related to:
Net deferred tax assets	12.6
Sweden subsidiaries
Deferred income tax liabilities related to:
Deferred tax liability	$ 236.1