Other Comprehensive Income (Loss) (Changes in Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Equity [Abstract]
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated, Pre-Tax Statements of Income	$ (1,079.4)	$ (300.1)	$ 274.6
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statements of Income, Pre-Tax	(10.2)	(0.2)	1.3
Amount of (gains) losses realized from sales and other recognized in Consolidated Statements of Income, Pre-Tax	26.9	(3.0)	(13.5)
Amount of credit-related impairments recognized in the Consolidated Statement of Income, Pre-Tax	3.7	0.2	1.2
Amount of additional impairment losses recognized in Consolidated Statements of Income, Pre-Tax	14.8	1.0	16.6
Net unrealized gains (losses),Pre-Tax	(1,044.2)	(302.1)	280.2
Net gains (losses) arising in the period from net actuarial gains (losses), Pre-Tax	(7.2)	(12.4)	3.9
Amortization of net actuarial losses recognized as net periodic benefit cost, Pre-Tax	5.5	3.4	6.0
Total pension and postretirement benefits, Pre-Tax	(1.7)	(9.0)	9.9
Other comprehensive income (loss), Pre-Tax	(1,045.9)	(311.1)	290.1
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated, Tax Benefit (Expense) Statements of Income	226.6	63.0	(57.7)
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statements of Income, Tax Benefit (Expense)	2.2	0.1	(0.2)
Amount of (gains) losses realized from sales and other recognized in Consolidated Statements of Income, Tax Benefit (Expense)	(7.0)	(3.9)	(6.4)
Amount of credit-related impairments recognized in the Consolidated Statement of Income, Tax Benefit (Expense)	(0.8)	(0.1)	(0.3)
Amount of additional impairment losses recognized in Consolidated Statements of Income, Tax Benefit (Expense)	(3.1)	(0.2)	(3.5)
Net unrealized gains (losses), Tax Benefit (Expense)	217.9	58.9	(68.1)
Net gains (losses) arising in the period from net actuarial gains (losses), Tax benefit (Expense)	1.5	2.6	(0.8)
Amortization of net actuarial losses recognized as net periodic benefit cost, Tax Benefit (Expense)	(1.2)	(0.7)	(1.3)
Total pension and postretirement benefits, Tax Benefit (Expense)	0.3	1.9	(2.1)
Other comprehensive income (loss), Tax Benefit (Expense)	218.2	60.8	(70.2)
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statement of Income, Net of Tax	(852.8)	(237.1)	216.9
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statements of Income, Net of Tax	(8.0)	(0.1)	1.1
Amount of (gains) losses realized from sales and other recognized in Consolidated Statements of Income, Net of Tax	19.9	(6.9)	(19.9)
Amount of credit-related impairments recognized in the Consolidated Statement of Income, Net of Tax	2.9	0.1	0.9
Amount of additional impairment losses recognized in Consolidated Statements of Income, Net Of Tax	11.7	0.8	13.1
Total available-for-sale securities	(826.3)	(243.2)	212.1
Net amount arising in the period	(5.7)	(9.8)	3.1
Amortization of net actuarial losses recognized as net periodic benefit cost, Net of Tax	4.3	2.7	4.7
Total pension and postretirement benefits	(1.4)	(7.1)	7.8
Total other comprehensive income (loss), net of tax	$ (827.7)	$ (250.3)	$ 219.9