Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Consolidated Statements of Comprehensive Income [Abstract]
Net Income	$ 6.5	$ 92.2	$ 37.6	$ 53.8	$ 36.9	$ 47.7	$ 36.6	$ (0.3)	$ 190.1	$ 120.9	$ 16.8
Unrealized Holding Gains (Losses)									(236.1)	83.8	(2.2)
Foreign Currency Translation Adjustments									0.3	1.7	(0.3)
Decrease (Increase) in Net Unrecognized Postretirement Benefit Costs									(6.9)	3.3	20.5
Gain on Cash Flow Hedge									1.2	(6.7)	1.6
Other Comprehensive Income (Loss) Before Income Taxes									(241.5)	82.1	19.6
Other Comprehensive Income Tax Benefit (Expense)									50.7	(24.7)	(6.9)
Other Comprehensive Income (Loss)									(190.8)	57.4	12.7
Total Comprehensive Income (Loss)									$ (0.7)	$ 178.3	$ 29.5