Fair Value Measurements - Financial Assets and (Liabilities) Required Fair Value Measurement on Recurring Basis (Details) - Recurring Measurement Basis - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Assets, Fair Value Disclosure [Abstract]
Assets	$ 10	$ 23
Liabilities, Fair Value Disclosure [Abstract]
Contingent consideration	122	130
Liabilities	209	163
Interest rate swaps
Assets, Fair Value Disclosure [Abstract]
Derivative asset (less than $1 million for congestion revenue rights in September 30, 2019)		3
Liabilities, Fair Value Disclosure [Abstract]
Derivative liability (less than $1 million for foreign currency forward contracts in September 30,2019)	87	31
Energy derivative
Assets, Fair Value Disclosure [Abstract]
Derivative asset (less than $1 million for congestion revenue rights in September 30, 2019)		7
Foreign currency forward contracts
Assets, Fair Value Disclosure [Abstract]
Derivative asset (less than $1 million for congestion revenue rights in September 30, 2019)	10	12
Liabilities, Fair Value Disclosure [Abstract]
Derivative liability (less than $1 million for foreign currency forward contracts in September 30,2019)	1	2
Congestion revenue rights
Assets, Fair Value Disclosure [Abstract]
Derivative asset (less than $1 million for congestion revenue rights in September 30, 2019)	1	1
Level 1
Assets, Fair Value Disclosure [Abstract]
Assets	0	0
Liabilities, Fair Value Disclosure [Abstract]
Contingent consideration	0	0
Liabilities	0	0
Level 1 \| Interest rate swaps
Assets, Fair Value Disclosure [Abstract]
Derivative asset (less than $1 million for congestion revenue rights in September 30, 2019)		0
Liabilities, Fair Value Disclosure [Abstract]
Derivative liability (less than $1 million for foreign currency forward contracts in September 30,2019)	0	0
Level 1 \| Energy derivative
Assets, Fair Value Disclosure [Abstract]
Derivative asset (less than $1 million for congestion revenue rights in September 30, 2019)		0
Level 1 \| Foreign currency forward contracts
Assets, Fair Value Disclosure [Abstract]
Derivative asset (less than $1 million for congestion revenue rights in September 30, 2019)	0	0
Liabilities, Fair Value Disclosure [Abstract]
Derivative liability (less than $1 million for foreign currency forward contracts in September 30,2019)	0	0
Level 1 \| Congestion revenue rights
Assets, Fair Value Disclosure [Abstract]
Derivative asset (less than $1 million for congestion revenue rights in September 30, 2019)	0	0
Level 2
Assets, Fair Value Disclosure [Abstract]
Assets	10	15
Liabilities, Fair Value Disclosure [Abstract]
Contingent consideration	0	0
Liabilities	87	33
Level 2 \| Interest rate swaps
Assets, Fair Value Disclosure [Abstract]
Derivative asset (less than $1 million for congestion revenue rights in September 30, 2019)		3
Liabilities, Fair Value Disclosure [Abstract]
Derivative liability (less than $1 million for foreign currency forward contracts in September 30,2019)	87	31
Level 2 \| Energy derivative
Assets, Fair Value Disclosure [Abstract]
Derivative asset (less than $1 million for congestion revenue rights in September 30, 2019)		0
Level 2 \| Foreign currency forward contracts
Assets, Fair Value Disclosure [Abstract]
Derivative asset (less than $1 million for congestion revenue rights in September 30, 2019)	10	12
Liabilities, Fair Value Disclosure [Abstract]
Derivative liability (less than $1 million for foreign currency forward contracts in September 30,2019)	0	2
Level 2 \| Congestion revenue rights
Assets, Fair Value Disclosure [Abstract]
Derivative asset (less than $1 million for congestion revenue rights in September 30, 2019)	0	0
Level 3
Assets, Fair Value Disclosure [Abstract]
Assets	0	8
Liabilities, Fair Value Disclosure [Abstract]
Contingent consideration	122	130
Liabilities	122	130
Level 3 \| Interest rate swaps
Assets, Fair Value Disclosure [Abstract]
Derivative asset (less than $1 million for congestion revenue rights in September 30, 2019)		0
Liabilities, Fair Value Disclosure [Abstract]
Derivative liability (less than $1 million for foreign currency forward contracts in September 30,2019)	0	0
Level 3 \| Energy derivative
Assets, Fair Value Disclosure [Abstract]
Derivative asset (less than $1 million for congestion revenue rights in September 30, 2019)		7
Level 3 \| Foreign currency forward contracts
Assets, Fair Value Disclosure [Abstract]
Derivative asset (less than $1 million for congestion revenue rights in September 30, 2019)	0	0
Liabilities, Fair Value Disclosure [Abstract]
Derivative liability (less than $1 million for foreign currency forward contracts in September 30,2019)	0	0
Level 3 \| Congestion revenue rights
Assets, Fair Value Disclosure [Abstract]
Derivative asset (less than $1 million for congestion revenue rights in September 30, 2019)	$ 0	$ 1

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
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Amount of liability recognized arising from contingent consideration in a business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 30
-Section 50
-Paragraph 1
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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Fair value of financial and nonfinancial obligations.

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