Redeemable Preferred Units (Rollforward of Redeemable Preferred Units) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Redeemable Noncontrolling Interest [Line Items]
Balance at December 31, 2017	$ 1,000
Distributions to unitholders and general partner	(347)	$ (242)
Balance at March 31, 2018	1,000
Series A Convertible Preferred Units
Redeemable Noncontrolling Interest [Line Items]
Balance at December 31, 2017	1,000
Net income	16
Distributions to unitholders and general partner	(16)
Balance at March 31, 2018	$ 1,000

Carrying amount, attributable to parent and noncontrolling interests, of an entity's issued and outstanding stock which is not included within permanent equity. Temporary equity is a security with redemption features that are outside the control of the issuer, is not classified as an asset or liability in conformity with GAAP, and is not mandatorily redeemable. Includes any type of security that is redeemable at a fixed or determinable price or on a fixed or determinable date or dates, is redeemable at the option of the holder, or has conditions for redemption which are not solely within the control of the issuer. Includes stock with a put option held by an ESOP and stock redeemable by a holder only in the event of a change in control of the issuer.

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 210.5-02.27(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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