Income Taxes - Carryforwards (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Carryforwards
Valuation allowance against deferred tax assets	$ 36.6	$ 17.2
UK
Carryforwards
Valuation allowance against deferred tax assets	8.1
UK | Capital loss carryforwards
Carryforwards
Tax credit carryforwards	5.1
Germany
Carryforwards
Net operating loss carryforwards	27.0
Valuation allowance against net operating loss carryforwards	9.6
Australia
Carryforwards
Net operating loss carryforwards	15.3
New Zealand
Carryforwards
Net operating loss carryforwards	2.1
Italian
Carryforwards
Net operating loss carryforwards	0.1
Canada
Carryforwards
Net operating loss carryforwards	0.5
State | U.S.
Carryforwards
Net operating loss carryforwards	504.7
Net operating loss carryforwards utilized	248.5
Valuation allowance against net operating loss carryforwards	1.0
Federal and state | U.S.
Carryforwards
Tax credit carryforwards	7.8
Valuation allowance against tax credit carryforwards	0.4
Foreign | U.S.
Carryforwards
Tax credit carryforwards	81.7
Valuation allowance against tax credit carryforwards	$ 17.5