Condensed Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement of Comprehensive Income [Abstract]
Net Income	$ 122.1	$ 37.6	$ 277.4	$ 91.4
Other Comprehensive Income (Loss) Before Income Taxes:
Unrealized Holding Gains (Losses)	159.3	(60.8)	322.1	(182.2)
Foreign Currency Translation Adjustments	0.0	1.2	0.0	0.3
Decrease in Net Unrecognized Postretirement Benefit Costs	0.0	0.3	0.6	0.6
Gain (Loss) on Cash Flow Hedges	0.0	(0.7)	0.1	0.2
Other Comprehensive Income (Loss) Before Income Taxes	159.3	(60.0)	322.8	(181.1)
Other Comprehensive Income Tax Benefit (Expense)	(33.3)	12.7	(67.7)	38.1
Other Comprehensive Income (Loss)	126.0	(47.3)	255.1	(143.0)
Total Comprehensive Income (Loss)	$ 248.1	$ (9.7)	$ 532.5	$ (51.6)