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Income Taxes - Income Tax Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Expected tax (benefit)/expense													$ 0.0	$ 0.0	$ 0.0
Overseas statutory tax rates differential													(17.1)	(41.5)	(19.3)
Base erosion and anti-abuse tax expense													6.0	0.0	0.0
Prior year adjustments													1.4	1.3	3.3
Valuation allowance													7.1	(37.9)	21.0
Impact of unrecognized tax benefits													(12.8)	0.1	(1.9)
Restricted foreign tax credits													0.0	0.7	1.9
Australian non-resident withholding tax													4.4	0.9	0.0
Share-based payments													0.2	(0.9)	0.0
Foreign exchange													0.1	(2.1)	0.2
Non-deductible expenses													0.7	0.4	0.8
Non-taxable items													(0.3)	(0.9)	(0.9)
Impact of changes in statutory tax rates													0.1	64.5	1.0
Total income tax expense/(benefit)	$ 3.2	$ (15.5)	$ (1.5)	$ 3.6	$ (10.2)	$ (9.2)	$ 1.2	$ 2.8	$ (2.6)	$ 4.9	$ 1.3	$ 2.5	(10.2)	(15.4)	$ 6.1
Income tax examination, tax settlement														0.3
Income tax examination, interest accrued	$ 1.8				$ 0.4								$ 1.8	$ 0.4

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