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Commitments and Contingent Liabilities Contingent Debt (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Guarantor Obligations [Line Items]    
Guarantor Obligations, Maximum Exposure, Undiscounted $ 1,070 $ 1,179
Cortez Pipeline Company    
Guarantor Obligations [Line Items]    
Equity Method Investment, Ownership Percentage 52.98%  
Cortez Expansion Capital Corp [Member]    
Guarantor Obligations [Line Items]    
Percentage of Debt Guaranteed 100.00%  
Revolving Credit Facility [Member] | Cortez Expansion Capital Corp [Member]    
Guarantor Obligations [Line Items]    
Guarantor Obligations, Maximum Exposure, Undiscounted $ 50  
Notes Payable to Banks [Member] | Cortez Expansion Capital Corp [Member]    
Guarantor Obligations [Line Items]    
Guarantor Obligations, Maximum Exposure, Undiscounted 120  
Partnership Interest [Member] | Revolving Credit Facility [Member] | Cortez Pipeline Company    
Guarantor Obligations [Line Items]    
Guarantor Obligations, Maximum Exposure, Undiscounted 50  
Partnership Interest [Member] | Senior Notes [Member] | Cortez Pipeline Company    
Guarantor Obligations [Line Items]    
Guarantor Obligations, Maximum Exposure, Undiscounted $ 100  
Debt Securities [Member] | Cortez Pipeline Company    
Guarantor Obligations [Line Items]    
Equity Method Investment, Ownership Percentage 50.00%