Other Comprehensive Income (Loss) and Accumulated Other Comprehensive Income - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Available for Sale Fixed Maturities with Portion of OTTI Recognized in Earnings	$ 0.0	$ 0.2
Other Net Unrealized Gains on Investments	209.7	269.5
Foreign Currency Translation Adjustments, Net of Income Taxes	(0.5)	0.2
Net Unrecognized Postretirement Benefit Costs, Net of Income Taxes	(89.0)	(72.2)
Accumulated Other Comprehensive Income (Loss), Cumulative Changes in Net Gain (Loss) from Cash Flow Hedges, Effect Net of Tax	(3.3)	(3.3)
Accumulated Other Comprehensive Income	$ 116.9	$ 194.4