Other Comprehensive Income (Loss) (Changes in Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Equity [Abstract]
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statement of Income, Pre-Tax	$ 107.7	$ (477.5)
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statement of Income, Pre-Tax	0.8	(1.2)
Amount of (gains) losses realized from sales and other recognized in Consolidated Statement of Income, Pre-Tax	1.1	(3.0)
Amount of credit-related impairments recognized in the Consolidated Statement of Income, Pre-Tax	0.4	0.6
Amount of additional impairment losses recognized in the Consolidated Statement of Income, Pre-Tax	10.3	0.3
Net unrealized gains (losses), Pre-Tax	120.3	(480.8)
Amortization of net actuarial losses recognized as net periodic benefit cost, Pre-Tax	2.0	1.4
Net change in market risk, Pre-tax	(2.7)	8.8
Other comprehensive income (loss), Pre-Tax	119.6	(470.6)
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statement of Income, Tax Benefit (Expense)	(22.6)	100.3
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statement of Income, Tax Benefit (Expense)	(0.1)	0.2
Amount of (gains) losses realized from sales and other recognized in Consolidated Statement of Income, Tax Benefit (Expense)	(1.0)	0.6
Amount of credit-related impairments recognized in the Consolidated Statement of Income, Tax Benefit (Expense)	(0.1)	(0.1)
Amount of additional impairment losses recognized in the Consolidated Statement of Income, Tax Benefit (Expense)	2.2	(0.1)
Net unrealized gains (losses), Tax Benefit (Expense)	(26.0)	100.9
Amortization of net actuarial losses recognized as net periodic benefit cost, Tax Benefit (Expense)	(0.4)	(0.3)
Net change in market risk, Tax Benefit (Expense)	0.6	(1.9)
Other comprehensive income (loss), Tax Benefit (Expense)	(25.8)	98.7
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statement of Income, Net of Tax	85.1	(377.2)
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statement of Income, Net of Tax	0.7	(1.0)
Amount of (gains) losses realized from sales and other recognized in Consolidated Statement of Income, Net of Tax	0.1	(2.4)
Amount of credit-related impairments recognized in the Consolidated Statement of Income, Net of Tax	0.3	0.5
Amount of additional impairment losses recognized in the Consolidated Statement of Income, Net of Tax	8.1	0.2
Total available-for-sale securities	94.3	(379.9)
Amortization of net actuarial losses recognized as net periodic benefit cost, Net of Tax	1.6	1.1
Net change in market risk, Net of Tax	(2.1)	6.9
Other comprehensive income (loss), net of tax	$ 93.8	$ (371.9)