Other Comprehensive Income and Accumulated Other Comprehensive Income - Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Other Comprehensive Income (Loss) Before Income Taxes:
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment	$ 33.6	$ 98.2
Reclassification Adjustment for Amounts Included in Net Income	(5.1)	2.5
Unrealized Holding Gains	28.5	100.7
Foreign Currency Translation Adjustments	0.1	0.1
Net Unrecognized Postretirement Benefit Costs Arising During the Period	0.0	(0.8)
Amortization of Net Unrecognized Postretirement Benefit Costs	(0.1)	2.6
Net Unrecognized Postretirement Benefit Costs	0.1	(1.8)
Loss on Cash Flow Hedge	(2.0)	0.0
Other Comprehensive Income Before Income Taxes	26.5	102.6
Other Comprehensive Income Tax Benefit (Expense):
Unrealized Holding Gains and Losses Arising During the Period Before Reclassification Adjustment	(11.9)	(34.6)
Reclassification Adjustment for Amounts Included in Net Income	1.8	(0.9)
Unrealized Holding Gains	(10.1)	(35.5)
Foreign Currency Translation Adjustments	0.0	0.0
Net Unrecognized Postretirement Benefit Costs Arising During the Period	0.0	0.3
Amortization of Net Unrecognized Postretirement Benefit Costs	0.0	(1.0)
Net Unrecognized Postretirement Benefit Costs	0.0	(0.7)
Loss on Cash Flow Hedge	0.7	0.0
Other Comprehensive Income Tax Expense	$ 9.4	$ 36.2