Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income - Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]
Balance at Beginning of Period			$ 4,563.4
Balance at End of Period	$ 4,306.2		4,306.2
Reclassification Adjustment for Amounts Included in Net Income
Reclassification Adjustment for Amounts Included in Net Income	(15.9)	$ (3.5)	(25.7)	$ (7.4)
Income Tax Benefit (Expense)	3.3	0.8	5.4	1.6
Net of Taxes	(12.6)	(2.7)	(20.3)	(5.8)
Changes in Net Unrecognized Postretirement Benefit Costs
Net Unrecognized Postretirement Benefit Costs	0.1	0.7	0.4	1.4
Income Tax Benefit (Expense)	(0.1)	(0.1)	0.4	(0.3)
Net of Taxes	0.0	0.6	0.8	1.1
Changes in Gain (Loss) on Cash Flow Hedges
Gain on Cash Flow Hedges	0.1	0.0	0.2	0.1
Income Tax Benefit (Expense)	0.0	0.0	0.0	0.0
Net of Taxes	0.1	0.0	0.2	0.1
Total Other Comprehensive Income (Loss) Before Income Taxes	190.7	408.0	(175.0)	204.0
Total Income Tax Benefit (Expense)	(38.2)	(85.6)	37.5	(42.8)
Other Comprehensive Income (Loss), Net of Taxes	152.5	322.4	(137.5)	161.2
Net Income	(62.6)	126.1	60.6	190.1
Income Tax Benefit (Expense)	27.1	(33.6)	28.7	(47.3)
Impairment Losses			(7.2)	(19.0)
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax					$ (45.7)
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax					$ (2.3)
Reclassification out of Accumulated Other Comprehensive Income
Changes in Gain (Loss) on Cash Flow Hedges
Net Income	12.5	2.1	19.3	4.6
No Credit Losses Recognized in Condensed Consolidated Statements of Income
Changes in Net Unrealized Gains (Losses) on Investment Securities:
Changes in Net Unrealized Gains (Losses) on Investment Securities	206.7	411.6	(147.5)	213.6
Income Tax Benefit (Expense)	(41.5)	(86.5)	31.2	(44.9)
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	165.2	325.1	(116.3)	168.7
Credit Losses Recognized in Condensed Consolidated Statements of Income
Changes in Net Unrealized Gains (Losses) on Investment Securities:
Changes in Net Unrealized Gains (Losses) on Investment Securities	(0.3)	(0.8)	(2.4)	(3.7)
Income Tax Benefit (Expense)	0.1	0.2	0.5	0.8
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	(0.2)	(0.6)	(1.9)	(2.9)
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member]
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]
Balance at Beginning of Period	441.4		730.6
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax	165.2		(116.3)
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax	(12.6)		(20.3)
Other Comprehensive Income (Loss), Net of Tax	152.6		(136.6)
Balance at End of Period	594.0		594.0
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income
Changes in Gain (Loss) on Cash Flow Hedges
Net Income	12.6	2.7	20.3	5.8
Income Tax Benefit (Expense)	(3.3)	(0.8)	(5.4)	(1.6)
Net Realized Gains on Sales of Investments	19.1	10.5	32.9	26.4
Impairment Losses	(3.2)	(7.0)	(7.2)	(19.0)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	15.9	3.5	25.7	7.4
Accumulated Other-than-Temporary Impairment Attributable to Parent [Member]
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]
Balance at Beginning of Period	(3.8)		(2.1)
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax	(0.2)		(1.9)
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax	0.0		0.0
Other Comprehensive Income (Loss), Net of Tax	(0.2)		(1.9)
Balance at End of Period	(4.0)		(4.0)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]
Balance at Beginning of Period	(44.9)
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax	0.0		0.0
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax			0.8
Other Comprehensive Income (Loss), Net of Tax	0.0		0.8
Balance at End of Period	(44.9)		(44.9)
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] | Reclassification out of Accumulated Other Comprehensive Income
Changes in Gain (Loss) on Cash Flow Hedges
Net Income	0.0	(0.6)	(0.8)	(1.1)
Interest and Other Expenses	(0.1)	(0.7)	(0.4)	(1.4)
Income Tax Benefit (Expense)	(0.1)	0.1	(0.4)	0.3
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]
Balance at Beginning of Period	(2.2)
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax	0.0		0.0
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax	0.1		0.2
Other Comprehensive Income (Loss), Net of Tax	0.1		0.2
Balance at End of Period	(2.1)		(2.1)
Accumulated Other Comprehensive Income
Debt Securities, Available-for-sale, Allowance for Credit Loss [Line Items]
Balance at Beginning of Period	390.5		680.5
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax	165.0		(118.2)
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax	(12.5)		(19.3)
Other Comprehensive Income (Loss), Net of Tax	152.5		(137.5)
Balance at End of Period	543.0		543.0
Changes in Gain (Loss) on Cash Flow Hedges
Other Comprehensive Income (Loss), Net of Taxes	$ 152.5	$ 322.4	$ (137.5)	$ 161.2