Sale-Leaseback Transactions (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016
Sale-Leaseback Transactions
Proceeds from sale-leaseback transactions	$ 265	$ 230

Amount of cash inflow after closing and debt issuance costs received by a seller-lessee in a sale-leaseback recognized in investing activities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 40
-Section 55
-Paragraph 52
-URI http://asc.fasb.org/extlink&oid=28365400&loc=d3e51843-112757

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 40
-Section 55
-Paragraph 50
-URI http://asc.fasb.org/extlink&oid=28365400&loc=d3e51831-112757

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 40
-Section 55
-Paragraph 51
-URI http://asc.fasb.org/extlink&oid=28365400&loc=d3e51840-112757

Name:	us-gaap_SaleLeasebackTransactionNetProceedsInvestingActivities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration