Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income - Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Other Comprehensive Income (Loss) Before Income Taxes:
Reclassification Adjustment for Amounts Included in Net Income	$ (3.9)	$ (12.5)
Unrealized Holding Gains (Losses)	(204.8)	162.8
Foreign Currency Translation Adjustments	0.0	0.0
Net Unrecognized Postretirement Benefit Costs	0.7	0.6
Gain on Cash Flow Hedges	0.1	0.1
Other Comprehensive Income (Loss) Before Income Taxes	(204.0)	163.5
Other Comprehensive Income Tax Benefit (Expense):
Reclassification Adjustment for Amounts Included in Net Income	0.8	2.6
Unrealized Holding Gains (Losses)	43.0	(34.2)
Foreign Currency Translation Adjustments	0.0	0.0
Net Unrecognized Postretirement Benefit Costs	(0.2)	(0.2)
Gain and Loss on Cash Flow Hedges	0.0	0.0
Other Comprehensive Income Tax Benefit (Expense)	42.8	(34.4)
No Credit Losses Recognized in Consolidated Statements of Income
Other Comprehensive Income (Loss) Before Income Taxes:
Unrealized Holding Gains (Losses) on Investment Securities Arising During the Period Before Reclassification Adjustment	(198.0)	175.3
Unrealized Holding Gains (Losses)	(201.9)	162.8
Other Comprehensive Income Tax Benefit (Expense):
Unrealized Holding Gains (Losses) on Investment Securities Arising During the Period Before Reclassification Adjustment	41.6	(36.8)
Credit Losses Recognized in Consolidated Statements of Income
Other Comprehensive Income (Loss) Before Income Taxes:
Unrealized Holding Gains (Losses) on Investment Securities Arising During the Period Before Reclassification Adjustment	(2.9)	0.0
Unrealized Holding Gains (Losses)	(2.9)	0.0
Other Comprehensive Income Tax Benefit (Expense):
Unrealized Holding Gains (Losses) on Investment Securities Arising During the Period Before Reclassification Adjustment	$ 0.6	$ 0.0