Income Taxes - Components of net deferred tax assets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Income taxes
Balance carryforwards, gross value	$ 287.1	$ 275.9
Future tax benefits, deferred tax effect	101.0	70.6
Valuation allowance	(2.3)	(11.4)
Foreign tax authority
Income taxes
Tax credit carryforwards, gross value		0.0
Federal foreign tax credit, deferred tax effect		9.7
Tax credit carryforward, valuation allowance		(9.7)
Domestic tax authority
Income taxes
Tax credit carryforwards, gross value	0.0	0.0
Tax credit carryforwards, deferred tax effect	75.4	39.3
Tax credit carryforward, valuation allowance	0.0	(1.7)
State and local jurisdiction
Income taxes
Tax credit carryforwards, gross value	0.0	0.0
Operating loss carryforwards, gross value	287.1	275.9
Tax credit carryforwards, deferred tax effect	8.0	4.6
State net operating loss, deferred tax effect	17.6	17.0
Tax credit carryforward, valuation allowance	0.0	0.0
Operating loss carryforwards, valuation allowance	$ (2.3)	$ 0.0