Other Comprehensive Income (Loss) (Changes in Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Equity [Abstract]
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statement of Income, Pre-Tax	$ 84.3	$ 309.7	$ (149.8)	$ 209.4
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statement of Income, Pre-Tax	0.3	14.2	0.3	(7.9)
Amount of (gains) losses realized from sales and other recognized in Consolidated Statement of Income, Pre-Tax	(4.8)	(0.9)	(3.2)	(4.0)
Amount of credit recoveries (losses) recognized in the Consolidated Statement of Income, Pre-Tax		(1.4)		5.0
Amount of additional impairment losses recognized in the Consolidated Statement of Income, Pre-Tax	0.2	0.1	0.2	16.4
Net unrealized gains (losses),Pre-Tax	80.0	321.7	(152.5)	218.9
Amortization of net actuarial losses recognized as net periodic benefit cost, Pre-Tax	0.9	1.5	1.7	3.0
Other comprehensive income (loss),Pre-Tax	80.9	323.2	(150.8)	221.9
Net unrealized losses arising during period for those having no credit losses in Consolidated Statement of Income, Tax Benefit (Expense)	(17.8)	(65.0)	31.4	(44.1)
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statement of Income, Tax Benefit (Expense)	(0.1)	(2.9)	(0.1)	1.8
Amount of (gains) losses realized from sales and other recognized in Consolidated Statement of Income, Tax Benefit (Expense)		(2.4)	(2.2)	(3.6)
Amount of credit losses recognized in the Consolidated Statement of Income, Tax Benefit (Expense)		0.3		(1.1)
Amount of additional impairment losses recognized in the Consolidated Statement of Income, Tax Benefit (Expense)				(3.4)
Net unrealized gains (losses),Tax Benefit (Expense)	(17.9)	(70.0)	29.1	(50.4)
Amortization of net actuarial losses recognized as net periodic benefit cost, Tax Benefit (Expense)	(0.2)	(0.3)	(0.3)	(0.6)
Other comprehensive income (loss),Tax Benefit (Expense)	(18.1)	(70.3)	28.8	(51.0)
Net unrealized losses arising during period for those having no credit losses in Consolidated Statement of Income, Net of Tax	66.5	244.7	(118.4)	165.3
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statement of Income, Net of Tax	0.2	11.3	0.2	(6.1)
Amount of (gains) losses realized from sales and other recognized in Consolidated Statement of Income, Net of Tax	(4.8)	(3.3)	(5.4)	(7.6)
Amount of credit losses recognized in the Consolidated Statement of Income, Net of Tax		(1.1)		3.9
Amount of additional impairment losses recognized in the Consolidated Statement of Income, Net of Tax	0.2	0.1	0.2	13.0
Total available-for-sale securities	62.1	251.7	(123.4)	168.5
Amortization of net actuarial losses recognized as net periodic benefit cost, Net of Tax	0.7	1.2	1.4	2.4
Total other comprehensive income (loss), net of tax	$ 62.8	$ 252.9	$ (122.0)	$ 170.9