Other Comprehensive Income (Loss) And Accumulated Other Comprehensive Income - Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent, Beginning Balance	$ 3,972.3
Stockholders' Equity Attributable to Parent, Ending Balance	4,563.4	$ 3,972.3
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax	(16.8)	(28.1)	$ 21.9
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	3.6	5.8	4.6
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	(13.2)	(22.3)	(17.3)
Decrease (Increase) in Net Unrecognized Postretirement Benefit Costs	70.2	(7.8)	(6.9)
Tax Effects from Postretirement Benefit Plans	(15.3)	1.7	1.5
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, after Tax	54.9	(6.1)	(5.4)
Gain (Loss) on Cash Flow Hedges	0.4	0.4	1.2
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Tax	0.0	(0.1)	(0.3)
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax	0.4	0.3	0.9
Other Comprehensive Income (Loss) Before Income Taxes	437.9	397.9	(241.5)
Other Comprehensive Income Tax Benefit (Expense)	(93.5)	(83.6)	50.7
Other Comprehensive Income (Loss)	344.4	314.3	(190.8)
OCI, Foreign Currency Transaction and Translation Gain (Loss), Arising During Period, Tax	0.0	0.0	(0.1)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Gain (Loss) Arising During Period, Net of Tax	0.0	0.0	0.2
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Gain (Loss), before Reclassification and Tax	0.0	0.0	0.3
No Credit Losses Recognized in Consolidated Statements of Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment	386.7	433.4	(214.2)
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax	(82.3)	(91.0)	45.0
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	304.4	342.4	(169.2)
Credit Losses Recognized in Consolidated Statements of Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment	(2.6)	0.0	0.0
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, Tax	0.5	0.0	0.0
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax	(2.1)	0.0	0.0
Having No Credit Losses Recognized in Consolidated Statements of Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent, Beginning Balance	439.4	119.3	306.0
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax	304.4	342.4	(169.2)
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax	(13.2)	(22.3)	(17.3)
Other Comprehensive Income (Loss), Net of Tax	291.2	320.1	(186.5)
Stockholders' Equity Attributable to Parent, Ending Balance	730.6	439.4	119.3
Having Credit Losses Recognized in Consolidated Statements of Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent, Beginning Balance	0.0	0.0	0.0
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax	(2.1)	0.0	0.0
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax	0.0	0.0	0.0
Other Comprehensive Income (Loss), Net of Tax	(2.1)	0.0	0.0
Stockholders' Equity Attributable to Parent, Ending Balance	(2.1)	0.0	0.0
Foreign Currency Translation Adjustments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent, Beginning Balance	0.0	0.0	(0.2)
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax	0.0	0.0	(0.1)
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax	0.0	0.0	0.3
Other Comprehensive Income (Loss), Net of Tax	0.0	0.0	0.2
Stockholders' Equity Attributable to Parent, Ending Balance	0.0	0.0	0.0
Net Unrecognized Postretirement Benefit Costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent, Beginning Balance	(100.6)	(94.5)	(89.2)
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax	4.3	(6.1)	(5.4)
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax	50.6	0.0	0.0
Other Comprehensive Income (Loss), Net of Tax	54.9	(6.1)	(5.4)
Stockholders' Equity Attributable to Parent, Ending Balance	(45.7)	(100.6)	(94.5)
Gain (Loss) on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent, Beginning Balance	(2.7)	(3.0)	(4.0)
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax	0.4	0.3	0.9
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax	0.0	0.0	0.0
Other Comprehensive Income (Loss), Net of Tax	0.4	0.3	0.9
Stockholders' Equity Attributable to Parent, Ending Balance	(2.3)	(2.7)	(3.0)
Accumulated Other Comprehensive Income (Loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent, Beginning Balance	336.1	21.8	212.6
Other Comprehensive Income (Loss) Before Reclassification Adjustment, Net of Tax	307.0	336.6	(173.8)
Reclassification Adjustment for Amounts Included in Net Income, Net of Tax	37.4	(22.3)	(17.0)
Other Comprehensive Income (Loss), Net of Tax	344.4	314.3	(190.8)
Stockholders' Equity Attributable to Parent, Ending Balance	680.5	336.1	21.8
Other Comprehensive Income (Loss)	$ 344.4	$ 314.3	(190.8)
Previously Reported | Having No Credit Losses Recognized in Consolidated Statements of Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent, Beginning Balance			269.7
Previously Reported | Having Credit Losses Recognized in Consolidated Statements of Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent, Beginning Balance			0.0
Previously Reported | Foreign Currency Translation Adjustments
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent, Beginning Balance			0.2
Previously Reported | Net Unrecognized Postretirement Benefit Costs
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent, Beginning Balance			(72.2)
Previously Reported | Gain (Loss) on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent, Beginning Balance			(3.3)
Previously Reported | Accumulated Other Comprehensive Income (Loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent, Beginning Balance			194.4
Cumulative Effect of Adoption of New Accounting Standard | Having No Credit Losses Recognized in Consolidated Statements of Income | Cumulative Effect of Adoption of New Accounting Standards
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent, Beginning Balance			36.3
Cumulative Effect of Adoption of New Accounting Standard | Having Credit Losses Recognized in Consolidated Statements of Income | Cumulative Effect of Adoption of New Accounting Standards
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent, Beginning Balance			0.0
Cumulative Effect of Adoption of New Accounting Standard | Foreign Currency Translation Adjustments | Cumulative Effect of Adoption of New Accounting Standards
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent, Beginning Balance			(0.4)
Cumulative Effect of Adoption of New Accounting Standard | Net Unrecognized Postretirement Benefit Costs | Cumulative Effect of Adoption of New Accounting Standards
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent, Beginning Balance			(17.0)
Cumulative Effect of Adoption of New Accounting Standard | Gain (Loss) on Cash Flow Hedges | Cumulative Effect of Adoption of New Accounting Standards
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent, Beginning Balance			(0.7)
Cumulative Effect of Adoption of New Accounting Standard | Accumulated Other Comprehensive Income (Loss) | Cumulative Effect of Adoption of New Accounting Standards
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent, Beginning Balance			$ 18.2