Other Comprehensive Income (Loss) (Changes in Other Comprehensive Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Equity [Abstract]
Net unrealized losses arising during period for those having no credit losses in Consolidated Statement of Income, Pre-Tax	$ (234.1)	$ (100.3)
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statement of Income, Pre-Tax		(22.1)
Amount of (gains) losses realized from sales and other recognized in Consolidated Statement of Income, Pre-Tax	1.6	(3.1)
Amount of credit losses recognized in the Consolidated Statement of Income, Pre-Tax		6.4
Amount of additional impairment losses recognized in the Consolidated Statement of Income, Pre-Tax		16.3
Net unrealized losses, Pre-Tax	(232.5)	(102.8)
Amortization of net actuarial losses recognized as net periodic benefit cost, Pre-Tax	0.8	1.5
Other comprehensive loss, Pre-Tax	(231.7)	(101.3)
Net unrealized losses arising during period for those having no credit losses in Consolidated Statement of Income, Tax Benefit (Expense)	49.2	20.9
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statement of Income, Tax Benefit (Expense)		4.7
Amount of (gains) losses realized from sales and other recognized in Consolidated Statement of Income, Tax Benefit (Expense)	(2.2)	(1.2)
Amount of credit losses recognized in the Consolidated Statement of Income, Tax Benefit (Expense)		(1.4)
Amount of additional impairment losses recognized in the Consolidated Statement of Income, Tax Benefit (Expense)		(3.4)
Net unrealized losses, Tax Benefit (Expense)	47.0	19.6
Amortization of net actuarial losses recognized as net periodic benefit cost, Tax Benefit (Expense)	(0.1)	(0.3)
Other comprehensive loss, Tax Benefit (Expense)	46.9	19.3
Net unrealized losses arising during period for those having no credit losses in Consolidated Statement of Income, Net of Tax	(184.9)	(79.4)
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statement of Income, Net of Tax		(17.4)
Amount of (gains) losses realized from sales and other recognized in Consolidated Statement of Income, Net of Tax	(0.6)	(4.3)
Amount of credit losses recognized in the Consolidated Statement of Income, Net of Tax		5.0
Amount of additional impairment losses recognized in the Consolidated Statement of Income, Net of Tax		12.9
Total available-for-sale securities	(185.5)	(83.2)
Amortization of net actuarial losses recognized as net periodic benefit cost, Net of Tax	0.7	1.2
Total other comprehensive loss, net of tax	$ (184.8)	$ (82.0)