Accounts Receivable (Narrative) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Accounts Receivable [Line Items]
Unbillable, expected to be collected after one year	$ 1,564	$ 2,011
Commercial Customers [Member]
Accounts Receivable [Line Items]
Unbillable, expected to be collected after one year	$ 172	$ 172

Amount of receivables under long-term contracts that have not been billed and were not billable that are expected to be collected after the next fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 910
-SubTopic 310
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=68065756&loc=d3e49336-109356

Name:	us-gaap_UnbilledReceivablesNotBillableAtBalanceSheetDateAmountExpectedToBeCollectedAfterOneYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant