Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income - Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Statement of Comprehensive Income [Abstract]
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, before Tax	$ (0.8)	$ 0.0
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment
Unrealized Holding Gains (Losses) Arising During the Period Before Reclassification Adjustment	98.2	49.7
Reclassification Adjustment for Amounts Included in Net Income	2.5	3.6
Unrealized Holding Gains (Losses)	100.7	53.3
Foreign Currency Translation Adjustments	0.1	(0.9)
Amortization of Net Unrecognized Postretirement Benefit Costs	2.6	5.4
Decrease in Net Unrecognized Postretirement Benefit Costs	1.8	5.4
Other Comprehensive Income Before Income Taxes	102.6	57.8
Other Comprehensive Income Tax Benefit (Expense):
Unrealized Holding Gains and Losses Arising During the Period Before Reclassification Adjustment	(34.6)	(17.5)
Reclassification Adjustment for Amounts Included in Net Income	(0.9)	(1.3)
Unrealized Holding Gains and Losses	(35.5)	(18.8)
Foreign Currency Translation Adjustments	0.0	0.3
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, Tax	0.3	0.0
Amortization of Net Unrecognized Postretirement Benefit Costs	(1.0)	(1.7)
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Tax	(0.7)	(1.7)
Other Comprehensive Income Tax Benefit (Expense)	$ 36.2	$ 20.2