Commitments and Contingencies (Purchase Obligations Narrative) (Details) $ in Billions	12 Months Ended
Dec. 31, 2017 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Term of unrecorded unconditional purchase obligations	Dec. 31, 2036
Unrecorded unconditional purchase obligations	$ 1.8

Amount of the unrecorded obligation to transfer funds in the future for fixed or minimum amounts or quantities of goods or services at fixed or minimum prices (for example, as in take-or-pay contracts or throughput contracts).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 440
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=77896938&loc=d3e25336-109308

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