Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement of Comprehensive Income [Abstract]
Net Income	$ 36.6	$ 4.0	$ 36.3	$ 1.9
Other Comprehensive Income Before Income Taxes:
Unrealized Holding Gains	38.7	124.8	67.2	225.5
Foreign Currency Translation Adjustments	0.7	(0.1)	0.8	0.0
Increase in Net Unrecognized Postretirement Benefit Costs	(0.2)	(16.0)	(0.3)	(14.2)
Loss on Cash Flow Hedges	(4.5)	0.0	(6.5)	0.0
Other Comprehensive Income Before Income Taxes	34.7	108.7	61.2	211.3
Other Comprehensive Income Tax Expense	(12.2)	(38.3)	(21.6)	(74.5)
Other Comprehensive Income	22.5	70.4	39.6	136.8
Total Comprehensive Income	$ 59.1	$ 74.4	$ 75.9	$ 138.7