Other Comprehensive Income (Changes in Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Equity [Abstract]
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statements of Income, Pre-Tax	$ 274.6	$ 326.0	$ (206.2)
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statements of Income, Pre-Tax	1.3	1.5	(3.5)
Amount of (gains) losses realized from sales and other recognized in Consolidated Statements of Income, Pre-Tax	(13.5)	(5.0)	2.5
Amount of credit losses recognized in Consolidated Statements of Income, Pre-Tax	1.2	2.1	3.5
Amount of additional impairment losses recognized in Consolidated Statements of Income, Pre-tax	16.6
Unrealized losses (gains) realized with sale of Chaucer business, Pre-Tax		0.1	24.2
Net unrealized gains (losses), Pre-Tax	280.2	324.7	(179.5)
Net gains (losses) arising in the period from net actuarial (gains) losses, Pre-Tax	3.9	20.5	(21.3)
Amortization of net actuarial losses and prior service costs recognized as net periodic benefit cost, Pre-Tax	6.0	11.4	9.7
Pension obligations recognized with sale of Chaucer business, Pre-Tax			21.2
Total pension and postretirement benefits, Pre-Tax	9.9	31.9	9.6
Foreign currency translation recognized during the period, Pre-Tax			(2.2)
Currency translation obligation recognized with sale of Chaucer business, Pre-Tax		0.9	29.4
Total cumulative foreign currency translation adjustment, Pre-Tax		0.9	27.2
Other comprehensive income (loss), Pre-Tax	290.1	357.5	(142.7)
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statements of Income, Tax Benefit (Expense)	(57.7)	(68.5)	43.3
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statements of Income, Tax Benefit (Expense)	(0.2)	(0.4)	0.7
Amount of (gains) losses realized from sales and other recognized in Consolidated Statements of Income, Tax Benefit (Expense)	(6.4)	(13.7)	(9.7)
Amount of credit losses recognized in Consolidated Statements of Income, Tax Benefit (Expense)	(0.3)	(0.4)	(0.7)
Amount of additional impairment losses recognized in Consolidated Statements of Income, Tax Benefit (Expense)	(3.5)
Unrealized losses (gains) realized with sale of Chaucer business, Tax Benefit (Expense)			(5.1)
Net unrealized gains (losses), Tax Benefit (Expense)	(68.1)	(83.0)	28.5
Net gains (losses) arising in the period from net actuarial (gains) losses, Tax Benefit (Expense)	(0.8)	(4.3)	4.3
Amortization of net actuarial losses and prior service costs recognized as net periodic benefit cost, Tax Benefit (Expense)	(1.3)	(2.4)	(2.0)
Pension obligations recognized with sale of Chaucer business, Tax Benefit (Expense)			(4.8)
Total pension and postretirement benefits, Tax Benefit (Expense)	(2.1)	(6.7)	(2.5)
Foreign currency translation recognized during the period, Tax Benefit (Expense)			0.5
Currency translation obligation recognized with sale of Chaucer business, Tax Benefit (Expense)		(0.2)	(6.2)
Total cumulative foreign currency translation adjustment, Tax Benefit (Expense)		(0.2)	(5.7)
Other comprehensive income (loss), Tax Benefit (Expense)	(70.2)	(89.9)	20.3
Net unrealized gains (losses) arising during period for those having no credit losses in Consolidated Statements of Income, Net of Tax	216.9	257.5	(162.9)
Net unrealized gains (losses) arising during period for those having credit losses in Consolidated Statements of Income, Net of Tax	1.1	1.1	(2.8)
Amount of (gains) losses realized from sales and other recognized in Consolidated Statements of Income, Net of Tax	(19.9)	(18.7)	(7.2)
Amount of credit losses recognized in Consolidated Statements of Income, Net of Tax	0.9	1.7	2.8
Amount of additional impairment losses recognized in Consolidated Statements of Income, Net of Tax	13.1
Unrealized losses (gains) realized with sale of Chaucer business, Net of Tax		0.1	19.1
Total available-for-sale securities	212.1	241.7	(151.0)
Net actuarial gains (losses) and prior service costs arising in the period	3.1	16.2	(17.0)
Amortization of net actuarial losses and prior service costs recognized as net periodic benefit cost, Net of Tax	4.7	9.0	7.7
Pension obligations recognized with sale of Chaucer business, Net of Tax			16.4
Total pension and postretirement benefits	7.8	25.2	7.1
Foreign currency translation recognized during the period, Net of Tax			(1.7)
Currency translation obligation recognized with sale of Chaucer business, Net of Tax		0.7	23.2
Total cumulative foreign currency translation adjustment		0.7	21.5
Other comprehensive income, net of tax	$ 219.9	$ 267.6	$ (122.4)