Income Tax Expense - NOL and Tax Credit Carryforwards (Details) - USD ($) $ in Millions		Dec. 31, 2022	Dec. 31, 2021
Federal [Member]
NOL and Tax Credit Carryforwards [Line Items]
NOL Carryforwards	[1]	$ 0.0	$ 177.3
Tax Credit Carryforwards		464.5	416.4
Federal Capital Loss	[1]	35.1	0.0
Tax Credit Carryforwards, Valuation Allowance		0.0
NOL Carryforwards, Valuation Allowance		0.0
State [Member]
NOL and Tax Credit Carryforwards [Line Items]
NOL Carryforwards	[1]	323.0	506.9
Tax Credit Carryforwards	[2]	24.5	24.2
State Capital Loss	[1]	83.2	0.0
Tax Credit Carryforwards, Valuation Allowance		$ 59.6	$ 68.6

[1] Pre-tax amounts.

[2] Net of a $59.6 million valuation allowance as of December 31, 2022 ($68.6 million as of December 31, 2021).