Equity (ATM Program) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stockholders Equity [Line Items]
Common Stock, Monetary Value of Shares Authorized	$ 1,700
Units issued		14,129,589	26,885,597
ATM Program
Stockholders Equity [Line Items]
Net proceeds		$ 473	$ 776
Limited Partners Common Units
Stockholders Equity [Line Items]
Units issued	0	13,846,998	26,347,887

Monetary value of the issuance of new units of limited partnership interest in a public offering, net of offering costs. This is a parenthetical disclosure.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 5
-Subparagraph (SAB Topic 4.F)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187171-122770

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration