Condensed Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net Income	$ 129.0	$ 92.2	$ 406.4	$ 183.6
Other Comprehensive Income (Loss) Before Income Taxes:
Unrealized Holding Gains (Losses)	118.3	(49.9)	440.4	(232.1)
Foreign Currency Translation Adjustments	0.0	0.0	0.0	0.3
Decrease (Increase) in Net Unrecognized Postretirement Benefit Costs	(0.4)	0.2	0.2	0.8
Gain on Cash Flow Hedges	0.2	0.9	0.3	1.1
Other Comprehensive Income (Loss) Before Income Taxes	118.1	(48.8)	440.9	(229.9)
Other Comprehensive Income Tax Benefit (Expense)	(24.9)	10.2	(92.6)	48.3
Other Comprehensive Income (Loss)	93.2	(38.6)	348.3	(181.6)
Total Comprehensive Income	$ 222.2	$ 53.6	$ 754.7	$ 2.0