Assets and Liabilities Measured at Fair Value (Schedule of Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Financial assets
Marketable securities, current	$ 8.5	$ 17.9
U.S. Corporate bonds	7.4	4.4
Fair Value, Recurring
Financial assets
Derivatives		1.3
Financial liabilities
Fair value, net asset (liability)	8.9	18.4
Fair Value, Recurring \| Fuel derivative
Financial assets
Derivatives	0.2
Fair Value, Recurring \| Deferred compensation plan liabilities
Financial liabilities
Deferred compensation plan liabilities	(18.7)	(14.7)
Fair Value, Recurring \| Deferred compensation plan assets
Financial assets
Deferred compensation plan assets	11.5	9.5
Fair Value, Recurring \| U.S. Government bonds
Financial assets
Marketable securities, current	5.9	8.8
Fair Value, Recurring \| U.S. Corporate bonds
Financial assets
Marketable securities, current	2.6	6.1
U.S. Corporate bonds	5.9	4.4
Fair Value, Recurring \| Non U.S. Corporate bonds
Financial assets
Marketable securities, current		3.0
U.S. Corporate bonds	1.5
Fair Value, Recurring \| Level 1
Financial assets
Derivatives		0.0
Financial liabilities
Fair value, net asset (liability)	(7.2)	(5.2)
Fair Value, Recurring \| Level 1 \| Fuel derivative
Financial assets
Derivatives	0.0
Fair Value, Recurring \| Level 1 \| Deferred compensation plan liabilities
Financial liabilities
Deferred compensation plan liabilities	(18.7)	(14.7)
Fair Value, Recurring \| Level 1 \| Deferred compensation plan assets
Financial assets
Deferred compensation plan assets	11.5	9.5
Fair Value, Recurring \| Level 1 \| U.S. Government bonds
Financial assets
Marketable securities, current	0.0	0.0
Fair Value, Recurring \| Level 1 \| U.S. Corporate bonds
Financial assets
Marketable securities, current	0.0	0.0
U.S. Corporate bonds	0.0	0.0
Fair Value, Recurring \| Level 1 \| Non U.S. Corporate bonds
Financial assets
Marketable securities, current		0.0
U.S. Corporate bonds	0.0
Fair Value, Recurring \| Level 2
Financial assets
Derivatives		0.0
Financial liabilities
Fair value, net asset (liability)	15.9	22.3
Fair Value, Recurring \| Level 2 \| Fuel derivative
Financial assets
Derivatives	0.0
Fair Value, Recurring \| Level 2 \| Deferred compensation plan liabilities
Financial liabilities
Deferred compensation plan liabilities	0.0	0.0
Fair Value, Recurring \| Level 2 \| Deferred compensation plan assets
Financial assets
Deferred compensation plan assets	0.0	0.0
Fair Value, Recurring \| Level 2 \| U.S. Government bonds
Financial assets
Marketable securities, current	5.9	8.8
Fair Value, Recurring \| Level 2 \| U.S. Corporate bonds
Financial assets
Marketable securities, current	2.6	6.1
U.S. Corporate bonds	5.9	4.4
Fair Value, Recurring \| Level 2 \| Non U.S. Corporate bonds
Financial assets
Marketable securities, current		3.0
U.S. Corporate bonds	1.5
Fair Value, Recurring \| Level 3
Financial assets
Derivatives		1.3
Financial liabilities
Fair value, net asset (liability)	0.2	1.3
Fair Value, Recurring \| Level 3 \| Fuel derivative
Financial assets
Derivatives	0.2
Fair Value, Recurring \| Level 3 \| Deferred compensation plan liabilities
Financial liabilities
Deferred compensation plan liabilities	0.0	0.0
Fair Value, Recurring \| Level 3 \| Deferred compensation plan assets
Financial assets
Deferred compensation plan assets	0.0	0.0
Fair Value, Recurring \| Level 3 \| U.S. Government bonds
Financial assets
Marketable securities, current	0.0	0.0
Fair Value, Recurring \| Level 3 \| U.S. Corporate bonds
Financial assets
Marketable securities, current	0.0	0.0
U.S. Corporate bonds	0.0	0.0
Fair Value, Recurring \| Level 3 \| Non U.S. Corporate bonds
Financial assets
Marketable securities, current		$ 0.0
U.S. Corporate bonds	$ 0.0

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
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