Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net Income	$ 124.7	$ 129.0	$ 122.1	$ 155.3	$ 6.5	$ 92.2	$ 37.6	$ 53.8	$ 531.1	$ 190.1	$ 120.9
Other Comprehensive Income (Loss) Before Income Taxes:
Unrealized Holding Gains (Losses)									405.3	(236.1)	83.8
Foreign Currency Translation Adjustments									0.0	0.3	1.7
Decrease (Increase) in Net Unrecognized Postretirement Benefit Costs									(7.8)	(6.9)	3.3
Gain (Loss) on Cash Flow Hedges									0.4	1.2	(6.7)
Other Comprehensive Income (Loss) Before Income Taxes									397.9	(241.5)	82.1
Other Comprehensive Income Tax Benefit (Expense)									(83.6)	50.7	(24.7)
Other Comprehensive Income (Loss)									314.3	(190.8)	57.4
Total Comprehensive Income (Loss)									$ 845.4	$ (0.7)	$ 178.3