Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Consolidated Statements of Comprehensive Income [Abstract]
Net Income	$ 31.2	$ (16.3)	$ 4.0	$ (2.1)	$ 4.6	$ 37.9	$ 29.7	$ 13.5	$ 16.8	$ 85.7	$ 114.5
Unrealized Holding Gains (Losses)									(2.2)	(177.3)	234.6
Foreign Currency Translation Adjustments									(0.3)	(1.4)	(1.0)
Decrease (Increase) in Net Unrecognized Postretirement Benefit Costs									20.5	26.1	(98.5)
Gain on Cash Flow Hedge									1.6	0.0	0.0
Other Comprehensive Income (Loss) Before Income Taxes									19.6	(152.6)	135.1
Other Comprehensive Income Tax Benefit (Expense)									(6.9)	54.2	(47.7)
Other Comprehensive Income (Loss)									12.7	(98.4)	87.4
Total Comprehensive Income (Loss)									$ 29.5	$ (12.7)	$ 201.9