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Contingencies and Commitments (Details)
$ in Millions
9 Months Ended
Sep. 30, 2018
USD ($)
agreement
Dec. 31, 2017
USD ($)
Guarantees Letters Of Credit [Abstract]    
The range of expiration dates of guarantees made by the Parent Company less than one year to more than 18 years  
Contingent Contractual Obligations [Line Items]    
Guarantor Obligations, Maximum Exposure, Undiscounted $ 853  
Number of Agreements | agreement 53  
Environmental Remediation Contingency [Domain]    
Contingent Contractual Obligations [Line Items]    
Accrual for Environmental Loss Contingencies $ 5 $ 5
Guarantee Obligations [Member]    
Contingent Contractual Obligations [Line Items]    
Guarantor Obligations, Maximum Exposure, Undiscounted $ 435  
Number of Agreements | agreement 21  
Indemnification Agreement [Member]    
Contingent Contractual Obligations [Line Items]    
Guarantor Obligations, Maximum Exposure, Undiscounted [1] $ 27  
Number of Agreements | agreement [1] 1  
Minimum [Member] | Guarantee Obligations [Member]    
Contingent Contractual Obligations [Line Items]    
Loss Contingency, Estimate of Possible Loss $ 1  
Minimum [Member] | Indemnification Agreement [Member]    
Contingent Contractual Obligations [Line Items]    
Loss Contingency, Estimate of Possible Loss [1] $ 27  
Minimum [Member] | Standby Letters of Credit [Member]    
Contingent Contractual Obligations [Line Items]    
Letter of credit fee percentage paid 1.00%  
Maximum [Member] | Environmental Remediation Contingency [Domain]    
Contingent Contractual Obligations [Line Items]    
Loss Contingency, Estimate of Possible Loss $ 16  
Maximum [Member] | Guarantee Obligations [Member]    
Contingent Contractual Obligations [Line Items]    
Loss Contingency, Estimate of Possible Loss 68  
Maximum [Member] | Indemnification Agreement [Member]    
Contingent Contractual Obligations [Line Items]    
Loss Contingency, Estimate of Possible Loss [1] $ 27  
Maximum [Member] | Standby Letters of Credit [Member]    
Contingent Contractual Obligations [Line Items]    
Letter of credit fee percentage paid 3.00%  
Unsecured Debt [Member] | Financial Standby Letter of Credit [Member]    
Contingent Contractual Obligations [Line Items]    
Guarantor Obligations, Maximum Exposure, Undiscounted $ 348  
Number of Agreements | agreement 6  
Unsecured Debt [Member] | Minimum [Member] | Financial Standby Letter of Credit [Member]    
Contingent Contractual Obligations [Line Items]    
Loss Contingency, Estimate of Possible Loss $ 2  
Unsecured Debt [Member] | Maximum [Member] | Financial Standby Letter of Credit [Member]    
Contingent Contractual Obligations [Line Items]    
Loss Contingency, Estimate of Possible Loss 247  
Secured Debt [Member] | Financial Standby Letter of Credit [Member]    
Contingent Contractual Obligations [Line Items]    
Guarantor Obligations, Maximum Exposure, Undiscounted $ 43  
Number of Agreements | agreement 25  
Secured Debt [Member] | Minimum [Member] | Financial Standby Letter of Credit [Member]    
Contingent Contractual Obligations [Line Items]    
Loss Contingency, Estimate of Possible Loss $ 1  
Secured Debt [Member] | Maximum [Member] | Financial Standby Letter of Credit [Member]    
Contingent Contractual Obligations [Line Items]    
Loss Contingency, Estimate of Possible Loss $ 14  
[1] (1) Excludes normal and customary representations and warranties in agreements for the sale of assets (including ownership in associated legal entities) where the associated risk is considered to be nominal.