Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income - Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Other Comprehensive Income (Loss) Before Income Taxes:
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment	$ (119.5)	$ 33.6
Reclassification Adjustment for Amounts Included in Net Income	(1.9)	(5.1)
Unrealized Holding Gains (Losses)	(121.4)	28.5
Foreign Currency Translation Adjustments	(0.9)	0.1
Net Unrecognized Postretirement Benefit Costs	0.3	(0.1)
Gain (Loss) on Cash Flow Hedges During the Period Before Reclassification Adjustment	0.6	(2.0)
Reclassification Adjustment for Amounts Included in Net Income	0.3	0.0
Gain (Loss) on Cash Flow Hedges	0.9	(2.0)
Other Comprehensive Income (Loss) Before Income Taxes	(121.1)	26.5
Other Comprehensive Income Tax Benefit (Expense):
Unrealized Holding Gains and Losses Arising During the Period Before Reclassification Adjustment	25.0	(11.9)
Reclassification Adjustment for Amounts Included in Net Income	0.4	1.8
Unrealized Holding Gains	25.4	(10.1)
Foreign Currency Translation Adjustments	0.2	0.0
Net Unrecognized Postretirement Benefit Costs	0.0	0.0
Gain and Loss on Cash Flow Hedges During the Period Before Reclassification Adjustment	(0.1)	0.7
Reclassification Adjustment for Amounts Included in Net Income	(0.1)	0.0
Gain and Loss on Cash Flow Hedges	(0.2)	0.7
Other Comprehensive Income Tax Benefit (Expense)	$ (25.4)	$ 9.4