Other Comprehensive Income (Loss) And Accumulated Other Comprehensive Income - Reclassification from AOCI to Consolidated Statements of Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income Tax Expense (Benefit)	$ 88.3	$ 124.8	$ (100.2)
Net (Loss) Income	(301.2)	(120.5)	409.9
Stockholders' Equity Attributable to Parent	2,443.2	4,007.7
AOCI, Gain (Loss), Debt Securities, Available-for-sale, with Allowance for Credit Loss, Parent
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	(719.4)	505.8	730.6	$ 439.4
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(1,215.1)	(182.0)	304.4
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(10.1)	(42.8)	(13.2)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(1,225.2)	(224.8)	291.2
Reclassification from AOCI, Current Period, Tax	2.7	11.3	3.6
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	325.9	59.7	(78.7)
Accumulated Net Unrealized Investment Gain (Loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	(2.2)	(3.7)	(2.1)	0.0
OCI, before Reclassifications, Net of Tax, Attributable to Parent	2.0	(1.6)	(2.1)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(0.5)	0.0	0.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	1.5	(1.6)	(2.1)
Reclassification from AOCI, Current Period, Tax	0.1	0.0	0.0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(0.4)	0.4	0.5
Accumulated Defined Benefit Plans Adjustment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	(37.2)	(52.1)	(45.7)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	15.2	(6.5)	4.3
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax				(100.6)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(0.3)	0.1	50.6
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	14.9	(6.4)	54.9
Reclassification from AOCI, Current Period, Tax	0.1	0.0	(13.5)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(4.0)	2.4	(15.3)
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	2.8	(1.9)	(2.3)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	4.7	0.0	0.4
AOCI, Cash Flow Hedge, Cumulative Gain (Loss), after Tax				(2.7)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	0.0	0.4	0.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	4.7	0.4	0.4
Reclassification from AOCI, Current Period, Tax	0.0	(0.1)	0.0
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(1.2)	(0.1)	0.0
Accumulated Other Comprehensive (Loss) Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	(756.0)	448.1	680.5	$ 336.1
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(1,193.2)	(190.1)	307.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(10.9)	(42.3)	37.4
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(1,204.1)	(232.4)	344.4
Reclassification from AOCI, Current Period, Tax	2.9	11.2	(9.9)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	$ 320.3	$ 62.4	$ (93.5)