Summary of Significant Accounting Policies Deferred Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Deferred Revenue, Current	$ 125	$ 140
Customer Loyalty [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Deferred Revenue, Current	22	18
Deferred Revenue	59	64	$ 69
Deferred Revenue, Additions	17	14
Deferred Revenue, Revenue Recognized	(22)	(19)
Deferred Revenue, Noncurrent	$ 37	46
Prepaid Rentals and Membership Fees [Member]
Revenue Recognition, Multiple-deliverable Arrangements [Line Items]
Deferred Revenue, Current		122
Deferred Revenue		123	$ 125
Deferred Revenue, Additions		1,968
Deferred Revenue, Revenue Recognized		(1,970)
Deferred Revenue, Noncurrent		$ 1