Marketable Securities (Schedule of Carrying Values of Marketable Securities) (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	$ 15.9	$ 22.3
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated fair value, current	8.5	17.9
Estimated fair value, noncurrent	7.4	4.4
Estimated Fair Value	15.9	22.3
Marketable securities current
Debt Securities, Available-for-Sale [Line Items]
Amortized cost current	8.5	17.9
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated fair value, current	8.5	17.9
U.S. Government bonds
Debt Securities, Available-for-Sale [Line Items]
Amortized cost current	5.9	8.8
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated fair value, current	5.9	8.8
U.S. Corporate bonds
Debt Securities, Available-for-Sale [Line Items]
Amortized cost current	2.5	6.0
Amortized cost, noncurrent	5.9	4.4
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated fair value, current	2.5	6.0
Estimated fair value, noncurrent	5.9	4.4
Non U.S. Corporate bonds
Debt Securities, Available-for-Sale [Line Items]
Amortized cost current		3.0
Amortized cost, noncurrent	1.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated fair value, current		3.0
Estimated fair value, noncurrent	1.5
Investment income receivable
Debt Securities, Available-for-Sale [Line Items]
Amortized cost current	0.1	0.1
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated fair value, current	0.1	$ 0.1
Marketable securities non-current
Debt Securities, Available-for-Sale [Line Items]
Amortized cost, noncurrent	7.4
Gross Unrealized Gains	0.0
Gross Unrealized Losses	0.0
Estimated fair value, noncurrent	$ 7.4

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on 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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