Investments Schedule of Equity Securities (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Schedule of Equity Securities [Abstract]
Equity Securities at Fair Value	$ 568.2
Equity Securities at Fair Value		$ 526.0
Equity Securities without Readily Determinable Fair Value, Impairment Loss, Annual Amount	0.2
Equity Securities without Readily Determinable Fair Value, Upward Price Adjustment, Cumulative Amount	0.0
Equity Securities without Readily Determinable Fair Value, Downward Price Adjustment, Cumulative Amount	0.0
Equity Securities without Readily Determinable Fair Value, Impairment Loss, Cumulative Amount	$ 0.2