Marketable Securities (Available-for-sale Securities by Classification in Balance Sheet) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	$ 369.0
Available-for-sale securities, fair value		$ 225.6
Cash and Cash Equivalents [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	328.1
Available-for-sale securities, fair value		43.8
Marketable Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	$ 40.9
Available-for-sale securities, fair value		$ 181.8