v3.26.1
Other Comprehensive Income (Loss) (Changes in Other Comprehensive Income (Loss)) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Accumulated Other Comprehensive Income (Loss) [Line Items]    
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statements of Income, Pre-Tax $ (93.0) $ 122.3
Net unrealized losses arising during period for those having credit losses in Consolidated Statements of Income,Pre-Tax (0.4) 0.0
Amount of losses realized from sales and other recognized in Consolidated Statements of Income, Pre-Tax 5.8 18.9
Amount of credit-related impairments (recoveries) on prior impairments recognized in the Consolidated Statement of Income, Pre-Tax 0.1 (0.1)
Amount of additional impairment losses recognized in the Consolidated Statements of Income, Pre-Tax 0.4 0.2
Net unrealized gains (losses), Pre-Tax (87.1) 141.3
Amortization of net actuarial losses recognized as net periodic benefit cost, Pre-Tax 237.1 160.1
Net change in market risk, Pre-tax 1.3 (1.3)
Other comprehensive (loss) income, Pre-Tax (84.1) 141.7
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statements of Income, Tax Benefit (Expense) 19.5 (25.7)
Net unrealized losses arising during period for those having credit losses in Consolidated Statements of Income, Tax Benefit (Expense) 0.1 0.0
Amount of losses realized from sales and other recognized in Consolidated Statements of Income, Tax Benefit (Expense) (1.3) (4.3)
Amount of credit-related impairments (recoveries) on prior impairments recognized in the Consolidated Statement of Income, Tax Benefit (Expense) 0.0 0.0
Amount of additional impairment losses recognized in the Consolidated Statements of Income, Tax Benefit (Expense) (0.1) (0.1)
Net unrealized gains, Tax Benefit (Expense) 18.2 (30.1)
Amortization of net actuarial losses recognized as net periodic benefit cost, Tax Benefit (Expense) 50.3 31.9
Net change in market risk, Tax Benefit (Expense) (0.3) 0.3
Other comprehensive (loss) income, Tax Benefit (Expense) 17.6 (30.1)
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statements of Income, Net of Tax (73.5) 96.6
Net unrealized gains arising during period for those having credit losses in Consolidated Statements of Income, Net of Tax (0.3) 0.0
Amount of losses realized from sales and other recognized in Consolidated Statements of Income, Net of Tax 4.5 14.6
Amount of credit-related impairments (recoveries) on prior impairments recognized in the Consolidated Statement of Income, Net of Tax 0.1 (0.1)
Amount of additional impairment losses recognized in the Consolidated Statements of Income, Net of Tax 0.3 0.1
Total available-for-sale securities (68.9) 111.2
Net change in market risk, Net of Tax 1.0 (1.0)
Total other comprehensive income (loss), net of tax (66.5) 111.6
Defined Benefit Pension and Postretirement Plans [Member] | Reclassification out of Accumulated Other Comprehensive Loss [Member]    
Accumulated Other Comprehensive Income (Loss) [Line Items]    
Amortization of net actuarial losses recognized as net periodic benefit cost, Pre-Tax 1.7 1.7
Amortization of net actuarial losses recognized as net periodic benefit cost, Tax Benefit (Expense) (0.3) (0.3)
Amortization of net actuarial losses recognized as net periodic benefit cost, Net of Tax $ 1.4 $ 1.4