Other Comprehensive Income (Loss) (Changes in Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Equity [Abstract]
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statements of Income, Pre-Tax	$ 203.7	$ (1,079.4)	$ (300.1)
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statements of Income, Pre-Tax	3.7	(10.2)	(0.2)
Amount of (gains) losses realized from sales and other recognized in Consolidated Statements of Income, Pre-Tax	9.1	26.9	(3.0)
Amount of credit-related impairments recognized in the Consolidated Statement of Income, Pre-Tax	0.8	3.7	0.2
Amount of additional impairment losses recognized in Consolidated Statements of Income, Pre-Tax	10.3	14.8	1.0
Net unrealized gains (losses),Pre-Tax	227.6	(1,044.2)	(302.1)
Net gains (losses) arising in the period from net actuarial gains (losses), Pre-Tax	3.9	(7.2)	(12.4)
Amortization of net actuarial losses recognized as net periodic benefit cost, Pre-Tax	7.9	5.5	3.4
Total pension and postretirement benefits, Pre-Tax	11.8	(1.7)	(9.0)
Net change in market risk, Pre-tax	(4.6)	21.6	5.3
Other comprehensive income (loss), Pre-Tax	234.8	(1,024.3)	(305.8)
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statement of Income, Tax Benefit (Expense)	(42.7)	226.6	63.0
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statements of Income, Tax Benefit (Expense)	(0.7)	2.2	0.1
Amount of (gains) losses realized from sales and other recognized in Consolidated Statements of Income, Tax Benefit (Expense)	(3.2)	(7.0)	(3.9)
Amount of credit-related impairments recognized in the Consolidated Statement of Income, Tax Benefit (Expense)	(0.2)	(0.8)	(0.1)
Amount of additional impairment losses recognized in Consolidated Statements of Income, Tax Benefit (Expense)	(2.2)	(3.1)	(0.2)
Net unrealized gains (losses), Tax Benefit (Expense)	(49.0)	217.9	58.9
Net gains (losses) arising in the period from net actuarial gains (losses), Tax benefit (Expense)	(0.8)	1.5	2.6
Amortization of net actuarial losses recognized as net periodic benefit cost, Tax Benefit (Expense)	(1.7)	(1.2)	(0.7)
Total pension and postretirement benefits, Tax Benefit (Expense)	(2.5)	0.3	1.9
Net change in market risk, Tax Benefit (Expense)	1.0	(4.6)	(1.1)
Other comprehensive income (loss), Tax Benefit (Expense)	(50.5)	213.6	59.7
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statement of Income, Net of Tax	161.0	(852.8)	(237.1)
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statements of Income, Net of Tax	3.0	(8.0)	(0.1)
Amount of (gains) losses realized from sales and other recognized in Consolidated Statements of Income, Net of Tax	5.9	19.9	(6.9)
Amount of credit-related impairments recognized in the Consolidated Statement of Income, Net of Tax	0.6	2.9	0.1
Amount of additional impairment losses recognized in Consolidated Statements of Income, Net Of Tax	8.1	11.7	0.8
Total available-for-sale securities	178.6	(826.3)	(243.2)
Net amount arising in the period	3.1	(5.7)	(9.8)
Amortization of net actuarial losses recognized as net periodic benefit cost, Net of Tax	6.2	4.3	2.7
Total pension and postretirement benefits	9.3	(1.4)	(7.1)
Net change in market risk, Net of Tax	(3.6)	17.0	4.2
Total other comprehensive income (loss), net of tax	$ 184.3	$ (810.7)	$ (246.1)