Income Taxes - Carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Income taxes
Balance carryforwards, gross value	$ 62.6	$ 72.6
Balance carryforwards, deferred tax effect	160.7	187.7
Balance carryforwards, valuation allowance	(5.0)	(1.2)
Federal tax jurisdiction
Income taxes
Tax credit carryforwards, gross value	0.0	0.0
Tax credit carryforwards, deferred tax effect	153.0	176.4
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	62.6	72.6
Tax credit carryforwards, deferred tax effect	3.9	6.8
Operating loss carryforwards, deferred tax effect	3.8	4.5
Tax credit carryforward, valuation allowance	(3.9)	0.0
Operating loss carryforwards, valuation allowance	$ (1.1)	$ (1.2)