Income Taxes - Components of net deferred tax assets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Income taxes
Balance carryforwards, gross value	$ 88.8	$ 287.1
Future tax benefits, deferred tax effect	102.5	101.0
Valuation allowance	(2.3)	(2.3)
Domestic tax authority
Income taxes
Tax credit carryforwards, gross value	0.0	0.0
Tax credit carryforwards, deferred tax effect	89.1	75.4
Tax credit carryforward, valuation allowance	0.0	0.0
State and local jurisdiction
Income taxes
Tax credit carryforwards, gross value	0.0	0.0
Operating loss carryforwards, gross value	88.8	287.1
Tax credit carryforwards, deferred tax effect	7.9	8.0
State net operating loss, deferred tax effect	5.5	17.6
Tax credit carryforward, valuation allowance	0.0	0.0
Operating loss carryforwards, valuation allowance	$ (2.3)	$ (2.3)