Other Comprehensive Income (Changes in Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Equity [Abstract]
Unrealized gains arising during period, Pre -Tax	$ 23.1	$ 8.1	$ 102.1	$ 290.1
Amount of realized gains from sales and other, Pre-tax	(15.2)	(3.4)	(26.5)	(30.7)
Portion of other-than-temporary impairment losses recognized in earnings, Pre-tax	1.3	0.1	3.3	26.0
Net unrealized gains (losses), Pre-tax	9.2	4.8	78.9	285.4
Amortization of net actuarial loss and prior service cost recognized as net periodic benefit cost, Pre-tax	3.5	2.5	10.5	7.5
Foreign currency translation recognized during the period, Pre-tax	1.7	(2.9)	2.6	(4.9)
Other comprehensive income (loss), Pre-tax	14.4	4.4	92.0	288.0
Unrealized gains arising during period, Tax Benefit (Expense)	(8.0)	(2.9)	(35.8)	(101.5)
Amount of realized gains from sales and other, Tax Benefit (Expense)	3.0	(3.9)	0.6	(3.3)
Portion of other-than-temporary impairment losses recognized in earnings, Tax Benefit (Expense)	(0.5)		(1.2)	(9.1)
Net unrealized gains (losses), Tax Benefit (Expense)	(5.5)	(6.8)	(36.4)	(113.9)
Amortization of net actuarial loss and prior service cost recognized as net periodic benefit cost, Tax Benefit (Expense)	(1.2)	(1.0)	(3.5)	(2.7)
Foreign currency translation recognized during the period, Tax Benefit (Expense)	(0.6)	1.0	(0.9)	1.7
Other comprehensive income (loss), Tax Benefit (Expense)	(7.3)	(6.8)	(40.8)	(114.9)
Unrealized gains arising during period, Net of Tax	15.1	5.2	66.3	188.6
Amount of realized gains from sales and other, Net of Tax	(12.2)	(7.3)	(25.9)	(34.0)
Portion of other-than-temporary impairment losses recognized in earnings, Net of Tax	0.8	0.1	2.1	16.9
Total available-for-sale securities	3.7	(2.0)	42.5	171.5
Amortization of net actuarial loss and prior service cost recognized as net periodic benefit cost, Net of Tax	2.3	1.5	7.0	4.8
Foreign currency translation recognized during the period, Net of Tax	1.1	(1.9)	1.7	(3.2)
Total other comprehensive income (loss), net of tax	$ 7.1	$ (2.4)	$ 51.2	$ 173.1