Other Comprehensive Income (Loss) (Changes in Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Equity [Abstract]
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statement of Income, Pre-Tax	$ (349.7)	$ 84.3	$ (827.2)	$ (149.8)
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statement of Income, Pre-Tax	(5.1)	0.3	(6.3)	0.3
Amount of (gains) losses realized from sales and other recognized in Consolidated Statement of Income, Pre-Tax	19.6	(4.8)	16.6	(3.2)
Amount of credit-related impairments recognized in the Consolidated Statement of Income, Pre-Tax	1.3		1.9
Amount of additional impairment losses recognized in the Consolidated Statement of Income, Pre-Tax	0.2	0.2	0.5	0.2
Net unrealized gains (losses), Pre-Tax	(333.7)	80.0	(814.5)	(152.5)
Amortization of net actuarial losses recognized as net periodic benefit cost, Pre-Tax	1.3	0.9	2.7	1.7
Other comprehensive income (loss), Pre-Tax	(332.4)	80.9	(811.8)	(150.8)
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statement of Income, Tax Benefit (Expense)	73.4	(17.8)	173.7	31.4
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statement of Income, Tax Benefit (Expense)	1.1	(0.1)	1.3	(0.1)
Amount of (gains) losses realized from sales and other recognized in Consolidated Statement of Income, Tax Benefit (Expense)	(4.9)		(4.3)	(2.2)
Amount of credit-related impairments recognized in the Consolidated Statement of Income, Tax Benefit (Expense)	(0.3)		(0.4)
Amount of additional impairment losses recognized in the Consolidated Statement of Income, Tax Benefit (Expense)			(0.1)
Net unrealized gains (losses), Tax Benefit (Expense)	69.3	(17.9)	170.2	29.1
Amortization of net actuarial losses recognized as net periodic benefit cost, Tax Benefit (Expense)	(0.2)	(0.2)	(0.5)	(0.3)
Other comprehensive income (loss), Tax Benefit (Expense)	69.1	(18.1)	169.7	28.8
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statement of Income, Net of Tax	(276.3)	66.5	(653.5)	(118.4)
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statement of Income, Net of Tax	(4.0)	0.2	(5.0)	0.2
Amount of (gains) losses realized from sales and other recognized in Consolidated Statement of Income, Net of Tax	14.7	(4.8)	12.3	(5.4)
Amount of credit-related impairments recognized in the Consolidated Statement of Income, Net of Tax	1.0		1.5
Amount of additional impairment losses recognized in the Consolidated Statement of Income, Net of Tax	0.2	0.2	0.4	0.2
Total available-for-sale securities	(264.4)	62.1	(644.3)	(123.4)
Amortization of net actuarial losses recognized as net periodic benefit cost, Net of Tax	1.1	0.7	2.2	1.4
Total other comprehensive income (loss), net of tax	$ (263.3)	$ 62.8	$ (642.1)	$ (122.0)