Income Taxes - Components of net deferred tax assets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Income Taxes
Balance of tax benefit carryforwards, gross	$ 52.4	$ 68.7
Balance of tax benefit carryforwards, deferred tax effect	41.0	15.9
Domestic tax authority
Income Taxes
Tax credit carryforwards, gross	0.0	0.0
Tax credit carryforwards, deferred tax effect	37.1	11.6
State and local jurisdiction
Income Taxes
Tax credit carryforwards, gross	0.0
Tax credit carryforwards, deferred tax effect	0.6
Net operating loss carryforwards, gross	52.4	68.7
Net operating loss carryforwards, deferred tax effect	$ 3.3	$ 4.3