Lease Accounting (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Leases [Abstract]
Operating lease costs		$ 30.2	$ 27.2	$ 30.8
Mileage credits		18.4	24.8	$ 26.2
Fixed lease payments per year		$ 5.2
Fair value of direct finance lease, current			$ 9.3
Fixed lease interest rate per year		10.25%
Early payment received on direct finance lease	$ 41.0
Direct finance lease receivable		$ 70.0