Marketable Securities (Amortized Cost and Fair Value of Marketable Securities) (Details) $ in Millions	Dec. 31, 2019 USD ($)
Schedule of Held-to-maturity Securities [Line Items]
Maturity of one year or less, Amortized Cost	$ 1.8
Maturity between one and five years, Amortized Cost	5.3
Maturity between five and ten years, Amortized Cost	2.8
Maturity greater than ten years, Amortized Cost	6.4
Total, Amortized Cost	16.3
Maturity of one year or less, Fair Value	1.8
Maturity between one and five years, Fair Value	5.4
Maturity between five and ten years, Fair Value	2.9
Maturity greater than ten years, Fair Value	6.9
Total, Fair Value	$ 17.0

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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