Other Comprehensive Income (Changes in Other Comprehensive Income) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity [Abstract]
Unrealized gains (losses) arising during period, Pre-Tax	$ (209.7)	$ 77.0	$ 95.9
Amount of realized (gains) losses from sales and other, Pre-tax	2.5	(30.8)	(36.5)
Portion of other-than-temporary impairment losses recognized in earnings, Pre-tax	3.5	3.9	27.9
Unrealized losses (gains) realized with sale of Chaucer business, Pre-tax	24.2
Net unrealized gains (losses), Pre-tax	(179.5)	50.1	87.3
Net gains (losses) arising in the period from net actuarial (gains) losses and prior service costs, Pre-tax	(21.3)	17.0	(43.1)
Amortization of net actuarial losses (gains) and prior service costs recognized as net periodic benefit cost, Pre-tax	9.7	14.0	9.9
Pension obligations recognized with sale of Chaucer business, Pre-tax	21.2
Total pension and postretirement benefits, Pre-tax	9.6	31.0	(33.2)
Foreign currency translation recognized during the period, Pre-tax	(2.2)	3.7	(5.1)
Currency translation obligation recognized with sale of Chaucer business, Pre-tax	29.4
Total cumulative foreign currency translation adjustment, Pre-tax	27.2	3.7	(5.1)
Other comprehensive income (loss), Pre-tax	(142.7)	84.8	49.0
Unrealized gains (losses) arising during period, Tax Benefit (Expense)	44.0	(27.9)	(33.4)
Amount of realized (gains) losses from sales and other, Tax Benefit (Expense)	(9.7)	(1.4)	(8.0)
Portion of other-than-temporary impairment losses recognized in earnings, Tax Benefit (Expense)	(0.7)	(1.4)	(9.8)
Unrealized losses (gains) realized with sale of Chaucer business, Tax Benefit (Expense)	(5.1)
Net unrealized gains, Tax Benefit (Expense)	28.5	(30.7)	(51.2)
Net gains (losses) arising in the period from net actuarial (gains) losses and prior service costs, Tax Benefit (Expense)	4.3	(3.4)	12.8
Amortization of net actuarial losses (gains) and prior service costs recognized as net periodic benefit cost, Tax Benefit (Expense)	(2.0)	(4.6)	(3.5)
Pension obligations recognized with sale of Chaucer business, Tax Benefit (Expense)	(4.8)
Total pension and postretirement benefits, Tax Benefit (Expense)	(2.5)	(8.0)	9.3
Foreign currency translation recognized during the period, Tax Benefit (Expense)	0.5	(1.3)	1.8
Currency translation obligation recognized with sale of Chaucer business, Tax Benefit (Expense)	(6.2)
Total cumulative foreign currency translation adjustment, Tax Benefit (Expense)	(5.7)	(1.3)	1.8
Other comprehensive income (loss), Tax Benefit (Expense)	20.3	(40.0)	(40.1)
Unrealized gains (losses) arising during period, Net of Tax	(165.7)	49.1	62.5
Amount of realized (gains) losses from sales and other, Net of Tax	(7.2)	(32.2)	(44.5)
Portion of other-than-temporary impairment losses recognized in earnings, Net of Tax	2.8	2.5	18.1
Unrealized losses (gains) realized with sale of Chaucer business, Net of Tax	19.1
Total available-for-sale securities	(151.0)	19.4	36.1
Net actuarial losses and prior service costs arising in the period	(17.0)	13.6	(30.3)
Amortization of net actuarial losses (gains) and prior service costs recognized as net periodic benefit cost, Net of Tax	7.7	9.4	6.4
Pension obligations recognized with sale of Chaucer business, Net of Tax	16.4
Total pension and postretirement benefits	7.1	23.0	(23.9)
Amount recognized as cumulative foreign currency translation during the period	(1.7)	2.4	(3.3)
Currency translation obligation recognized with sale of Chaucer business, Net of Tax	23.2
Total cumulative foreign currency translation adjustment	21.5	2.4	(3.3)
Total other comprehensive income (loss), net of tax	$ (122.4)	$ 44.8	$ 8.9