Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net Income	$ 37.6	$ 36.6	$ 91.4	$ 36.3
Other Comprehensive Income (Loss) Before Income Taxes:
Unrealized Holding Gains (Losses)	(60.8)	38.7	(182.2)	67.2
Foreign Currency Translation Adjustments	1.2	0.7	0.3	0.8
Decrease (Increase) in Net Unrecognized Postretirement Benefit Costs	0.3	(0.2)	0.6	(0.3)
Gain (Loss) on Cash Flow Hedges	(0.7)	(4.5)	0.2	(6.5)
Other Comprehensive Income (Loss) Before Income Taxes	(60.0)	34.7	(181.1)	61.2
Other Comprehensive Income Tax Benefit (Expense)	12.7	(12.2)	38.1	(21.6)
Other Comprehensive Income (Loss)	(47.3)	22.5	(143.0)	39.6
Total Comprehensive Income (Loss)	$ (9.7)	$ 59.1	$ (51.6)	$ 75.9