Income Taxes - Carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Income taxes
Balance carryforwards, gross value	$ 72.6	$ 72.0
Balance carryforwards, deferred tax effect	187.7	104.6
Balance carryforwards, valuation allowance	(1.2)	(1.2)
Federal tax jurisdiction
Income taxes
Tax credit carryforwards, gross value	0.0	0.0
Tax credit carryforwards, deferred tax effect	176.4	91.5
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	72.6	72.0
Tax credit carryforwards, deferred tax effect	6.8	8.7
Operating loss carryforwards, deferred tax effect	4.5	4.4
Tax credit carryforward, valuation allowance	0.0	0.0
Operating loss carryforwards, valuation allowance	$ (1.2)	$ (1.2)