Other Comprehensive Income (Loss) (Changes in Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Equity [Abstract]
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statements of Income, Pre-Tax	$ 122.3	$ (43.9)
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statements of Income, Pre-Tax	0.0	0.1
Amount of losses realized from sales and other recognized in Consolidated Statements of Income, Pre-Tax	18.9	1.5
Amount of credit-related recoveries on prior impairments recognized in the Consolidated Statement of Income, Pre-Tax	(0.1)	(0.3)
Amount of additional impairment losses recognized in the Consolidated Statements of Income, Pre-Tax	0.2	0.0
Net unrealized gains (losses), Pre-Tax	141.3	(42.6)
Amortization of net actuarial losses recognized as net periodic benefit cost, Pre-Tax	1.7	1.7
Net change in market risk, Pre-tax	(1.3)	1.3
Other comprehensive income (loss), Pre-Tax	141.7	(39.6)
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statements of Income, Tax Benefit (Expense)	(25.7)	9.2
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statements of Income, Tax Benefit (Expense)	0.0	(0.1)
Amount of losses realized from sales and other recognized in Consolidated Statements of Income, Tax Benefit (Expense)	(4.3)	(0.7)
Amount of credit-related recoveries on prior impairments recognized in the Consolidated Statement of Income, Tax Benefit (Expense)	0.0	0.1
Amount of additional impairment losses recognized in the Consolidated Statements of Income, Tax Benefit (Expense)	(0.1)	0.0
Net unrealized gains (losses), Tax Benefit (Expense)	(30.1)	8.5
Amortization of net actuarial losses recognized as net periodic benefit cost, Tax Benefit (Expense)	(0.3)	(0.3)
Net change in market risk, Tax Benefit (Expense)	0.3	(0.3)
Other comprehensive income (loss), Tax Benefit (Expense)	(30.1)	7.9
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statements of Income, Net of Tax	96.6	(34.7)
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statements of Income, Net of Tax	0.0	0.0
Amount of losses realized from sales and other recognized in Consolidated Statements of Income, Net of Tax	14.6	0.8
Amount of credit-related recoveries on prior impairments recognized in the Consolidated Statement of Income, Net of Tax	(0.1)	(0.2)
Amount of additional impairment losses recognized in the Consolidated Statements of Income, Net of Tax	0.1	0.0
Total available-for-sale securities	111.2	(34.1)
Amortization of net actuarial losses recognized as net periodic benefit cost, Net of Tax	1.4	1.4
Net change in market risk, Net of Tax	(1.0)	1.0
Other comprehensive income (loss), Net of tax	$ 111.6	$ (31.7)