Other Comprehensive Income (Changes in Other Comprehensive Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Equity [Abstract]
Unrealized gains (losses) arising during period, Pre-Tax	$ 327.5	$ (209.7)	$ 77.0
Amount of realized (gains) losses from sales and other, Pre-tax	(5.0)	2.5	(30.8)
Portion of other-than-temporary impairment losses recognized in earnings, Pre-tax	2.1	3.5	3.9
Unrealized losses (gains) realized with sale of Chaucer business, Pre-tax	0.1	24.2
Net unrealized gains (losses), Pre-tax	324.7	(179.5)	50.1
Net gains (losses) arising in the period from net actuarial (gains) losses, Pre-tax	20.5	(21.3)	17.0
Amortization of net actuarial losses (gains) and prior service costs recognized as net periodic benefit cost, Pre-tax	11.4	9.7	14.0
Pension obligations recognized with sale of Chaucer business, Pre-tax		21.2
Total pension and postretirement benefits, Pre-tax	31.9	9.6	31.0
Foreign currency translation recognized during the period, Pre-tax		(2.2)	3.7
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	3.7
Other comprehensive income (loss), Pre-tax	357.5	(142.7)	84.8
Unrealized gains (losses) arising during period, Tax Benefit (Expense)	(68.9)	44.0	(27.9)
Amount of realized (gains) losses from sales and other, Tax Benefit (Expense)	(13.7)	(9.7)	(1.4)
Portion of other-than-temporary impairment losses recognized in earnings, Tax Benefit (Expense)	(0.4)	(0.7)	(1.4)
Unrealized losses (gains) realized with sale of Chaucer business, Tax Benefit (Expense)		(5.1)
Net unrealized gains, Tax Benefit (Expense)	(83.0)	28.5	(30.7)
Net gains (losses) arising in the period from net actuarial (gains) losses, Tax Benefit (Expense)	(4.3)	4.3	(3.4)
Amortization of net actuarial losses (gains) and prior service costs recognized as net periodic benefit cost, Tax Benefit (Expense)	(2.4)	(2.0)	(4.6)
Pension obligations recognized with sale of Chaucer business, Tax Benefit (Expense)		(4.8)
Total pension and postretirement benefits, Tax Benefit (Expense)	(6.7)	(2.5)	(8.0)
Foreign currency translation recognized during the period, Tax Benefit (Expense)		0.5	(1.3)
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)	(1.3)
Other comprehensive income (loss), Tax Benefit (Expense)	(89.9)	20.3	(40.0)
Unrealized gains (losses) arising during period, Net of Tax	258.6	(165.7)	49.1
Amount of realized (gains) losses from sales and other, Net of Tax	(18.7)	(7.2)	(32.2)
Portion of other-than-temporary impairment losses recognized in earnings, Net of Tax	1.7	2.8	2.5
Unrealized losses (gains) realized with sale of Chaucer business, Net of Tax	0.1	19.1
Total available-for-sale securities	241.7	(151.0)	19.4
Net actuarial gains (losses) and prior service costs arising in the period	16.2	(17.0)	13.6
Amortization of net actuarial losses (gains) and prior service costs recognized as net periodic benefit cost, Net of Tax	9.0	7.7	9.4
Pension obligations recognized with sale of Chaucer business, Net of Tax		16.4
Total pension and postretirement benefits	25.2	7.1	23.0
Foreign currency translation recognized during the period, Net of Tax		(1.7)	2.4
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	2.4
Total other comprehensive income (loss), net of tax	$ 267.6	$ (122.4)	$ 44.8