Description of Business and Accounting Policies - Accrued Expenses and Other (Details) - USD ($) $ in Billions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Unredeemed gift certificates	$ 3.0	$ 2.4
Unredeemed gift certificates, period of recognition	2 years

Carrying value as of the balance sheet date of the liability for outstanding gift cards. Retail customers purchase gift cards or gift certificates that can be redeemed at a later date for merchandise or services; those unredeemed represent a liability of the entity because the revenue is being deferred.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.20)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_AccruedLiabilitiesForUnredeeemedGiftCards
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant