Other Comprehensive Income (Loss) And Accumulated Other Comprehensive Income - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Unrealized Gain (Loss) [Abstract]
Accumulated Other Comprehensive Income	$ (756.0)	$ 448.1
AOCI, Gain (Loss), Debt Securities, Available-for-sale, with Allowance for Credit Loss, Parent
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	325.9	59.7	$ (78.7)
Reclassification from AOCI, Current Period, Tax	2.7	11.3	3.6
Accumulated Net Unrealized Investment Gain (Loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(0.4)	0.4	0.5
Reclassification from AOCI, Current Period, Tax	0.1	0.0	0.0
Accumulated Defined Benefit Plans Adjustment
Debt Securities, Unrealized Gain (Loss) [Abstract]
Net Unrecognized Postretirement Benefit Costs, Net of Income Taxes				$ (100.6)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(4.0)	2.4	(15.3)
Reclassification from AOCI, Current Period, Tax	0.1	0.0	(13.5)
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(1.2)	(0.1)	0.0
Reclassification from AOCI, Current Period, Tax	0.0	(0.1)	0.0
Accumulated Other Comprehensive (Loss) Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	320.3	62.4	(93.5)
Reclassification from AOCI, Current Period, Tax	$ 2.9	$ 11.2	$ (9.9)