Summary of Significant Accounting Policies Deferred Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Other related Revenue	$ 159	$ 144
Customer Loyalty [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Deferred Revenue, Current	17	22
Deferred Revenue	34	59	$ 64
Deferred Revenue, Additions	26	17
Deferred Revenue, Revenue Recognized	(51)	(22)
Deferred Revenue, Noncurrent	$ 17	$ 37