Benefit Obligations (Schedule of Fair Values of Pension Plan Assets) (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets, fair value non-financial receivables		$ 25	$ 20
Pension plan assets, fair value non-financial payables		18	10
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets	[1]	(2,361)	(2,296)
Total liabilities [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		7	3
Swaps [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		7	2
Other [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		0	1
Total plan assets [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets	[2]	(2,368)	(2,299)
Cash and Cash quivalents [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		(5)	(2)
Equities [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		(727)	(195)
Swaps [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		(8)	(2)
US Companies [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		0	(260)
International Companies [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		(72)	(345)
Corporate Debt [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		(776)	(798)
Treasuries, Other Debt [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		(1,459)	(830)
Mortgage Backed Securities [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		(7)	(7)
Registered Investment Companies [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		(60)	(134)
Short-term Investments [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		(96)	(149)
Insurance Contracts [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		(36)	(31)
Other [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		(5)	(24)
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets	[1]	(129)	(668)
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Total liabilities [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Swaps [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Total plan assets [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets	[2]	(129)	(668)
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Cash and Cash quivalents [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		(5)	(2)
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Swaps [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | US Companies [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		0	(260)
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | International Companies [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		(72)	(345)
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Corporate Debt [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Treasuries, Other Debt [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		(48)	(37)
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Mortgage Backed Securities [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Insurance Contracts [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		(4)	(24)
Significant Other Observable Inputs (Level 2) [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets	[1]	(2,232)	(1,628)
Significant Other Observable Inputs (Level 2) [Member] | Total liabilities [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		7	3
Significant Other Observable Inputs (Level 2) [Member] | Swaps [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		7	2
Significant Other Observable Inputs (Level 2) [Member] | Other [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		0	1
Significant Other Observable Inputs (Level 2) [Member] | Total plan assets [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets	[2]	(2,239)	(1,631)
Significant Other Observable Inputs (Level 2) [Member] | Cash and Cash quivalents [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		0	0
Significant Other Observable Inputs (Level 2) [Member] | Swaps [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		(8)	(2)
Significant Other Observable Inputs (Level 2) [Member] | US Companies [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		0	0
Significant Other Observable Inputs (Level 2) [Member] | International Companies [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		0	0
Significant Other Observable Inputs (Level 2) [Member] | Corporate Debt [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		(776)	(798)
Significant Other Observable Inputs (Level 2) [Member] | Treasuries, Other Debt [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		(1,411)	(793)
Significant Other Observable Inputs (Level 2) [Member] | Mortgage Backed Securities [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		(7)	(7)
Significant Other Observable Inputs (Level 2) [Member] | Insurance Contracts [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		(36)	(31)
Significant Other Observable Inputs (Level 2) [Member] | Other [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net assets		$ (1)	$ 0

[1] Total net assets excludes non-financial plan receivables and payables of $25 million and $18 million, respectively, as of December 31, 2017 and $20 million and $10 million, respectively, as of December 31, 2016. Non-financial items include due to/from broker, interest receivables and accrued expenses.

[2] In accordance with ASU 2015-07 (Note 2), certain investments that are measured at fair value using the NAV per share practical expedient have not been classified in the fair value hierarchy. Total investments, at fair value, for the year ended December 31, 2017 excludes investments in common/collective trusts, registered investment companies and short-term investment funds with fair values of $727 million, $60 million and $96 million, respectively. Total investments, at fair value, for the year ended December 31, 2016 excludes investments in common/collective trusts, registered investment companies and short-term investment funds with fair values of $195 million, $134 million and $149 million, respectively.