Other Comprehensive Income (Changes in Other Comprehensive Income) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Equity [Abstract]
Unrealized gains (losses) arising during period, Pre-Tax	$ (58.1)	$ 41.0	$ (211.8)	$ 79.0
Amount of realized gains (losses) from sales and other, Pre-Tax		(7.2)	0.2	(11.3)
Portion of other-than-temporary impairment losses recognized in earnings, Pre-Tax	(0.1)	1.8	0.4	2.0
Net unrealized gains (losses), Pre-Tax	(58.2)	35.6	(211.2)	69.7
Net change in net actuarial loss and prior service cost, Pre-Tax	1.1	3.5	(1.8)	7.0
Foreign currency translation recognized during the period, Pre-Tax	(0.6)	(6.9)	(1.2)	0.9
Other comprehensive (loss) income, Pre-Tax	(57.7)	32.2	(214.2)	77.6
Unrealized gains (losses) arising during period, Tax Benefit (Expense)	12.4	(14.5)	44.5	(27.8)
Amount of realized gains (losses) from sales and other, Tax Benefit (Expense)	(3.3)	(0.7)	(5.8)	(2.4)
Portion of other-than-temporary impairment losses recognized in earnings, Tax Benefit (Expense)		(0.6)	(0.1)	(0.7)
Net unrealized gains, Tax Benefit (Expense)	9.1	(15.8)	38.6	(30.9)
Net change in net actuarial loss and prior service cost, Tax Benefit (Expense)	(0.2)	(1.1)	0.4	(2.3)
Foreign currency translation recognized during the period, Tax Benefit (Expense)	0.1	2.4	0.2	(0.3)
Other comprehensive (loss) income, Tax Benefit (Expense)	9.0	(14.5)	39.2	(33.5)
Unrealized gains (losses) arising during period, Net of Tax	(45.7)	26.5	(167.3)	51.2
Amount of realized gains (losses) from sales and other, Net of Tax	(3.3)	(7.9)	(5.6)	(13.7)
Portion of other-than-temporary impairment losses recognized in earnings, Net of Tax	(0.1)	1.2	0.3	1.3
Total available-for-sale securities	(49.1)	19.8	(172.6)	38.8
Net change in net actuarial loss and prior service cost, Net of Tax	0.9	2.4	(1.4)	4.7
Foreign currency translation recognized during the period, Net of Tax	(0.5)	(4.5)	(1.0)	0.6
Total other comprehensive (loss) income, net of tax	$ (48.7)	$ 17.7	$ (175.0)	$ 44.1