Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consolidated Statements of Comprehensive Income [Abstract]
Net Income	$ 36.9	$ 47.7	$ 36.6	$ (0.3)	$ 31.2	$ (16.3)	$ 4.0	$ (2.1)	$ 120.9	$ 16.8	$ 85.7
Unrealized Holding Gains (Losses)									83.8	(2.2)	(177.3)
Foreign Currency Translation Adjustments									1.7	(0.3)	(1.4)
Decrease in Net Unrecognized Postretirement Benefit Costs									3.3	20.5	26.1
Gain on Cash Flow Hedge									(6.7)	1.6	0.0
Other Comprehensive Income (Loss) Before Income Taxes									82.1	19.6	(152.6)
Other Comprehensive Income Tax Benefit (Expense)									(24.7)	(6.9)	54.2
Other Comprehensive Income (Loss)									57.4	12.7	(98.4)
Total Comprehensive Income (Loss)									$ 178.3	$ 29.5	$ (12.7)