Accounting Policies - Taxation (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Accounting Policies [Abstract]
Income tax expense	$ 5.1
Valuation allowance	5.1
Provisional income tax expense (benefit)	2.4
Transition tax for accumulated foreign earnings, provisional liability	10.4
Deferred tax liability, provisional income tax benefit	7.1
Provisional income tax expense (benefit), other	$ 0.9