Marketable Securities (Available-for-Sale Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Available-for-sale Securities, Amortized Cost Basis	$ 16.3	$ 20.3
Available-for-sale Securities	17.1	20.2
Corporate Debt Securities [Member]
Available-for-sale Securities, Amortized Cost Basis	15.8	18.2
Available-for-sale Securities	16.7	18.1
Municipal Bonds [Member]
Available-for-sale Securities, Amortized Cost Basis	0.0	1.5
Available-for-sale Securities	0.0	1.7
Asset-backed Securities [Member]
Available-for-sale Securities, Amortized Cost Basis	0.5	0.6
Available-for-sale Securities	0.3	0.3
Equity Securities [Member]
Available-for-sale Securities, Amortized Cost Basis	0.0	0.0
Available-for-sale Securities	$ 0.1	$ 0.1