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Retirement Benefits (Obligations and Funded Status) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Change in benefit obligation:    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement $ 0 $ 0
Change in plan assets:    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement 0 0
Pension Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Service Cost 0 0
Change in benefit obligation:    
Defined Benefit Plan, Interest Cost 3 3
Change in plan assets:    
Defined Benefit Plan, Plan Assets, Amount 43  
Defined Benefit Plan, Plan Assets, Amount 45 43
Amounts recognized in the consolidated balance sheets consist of:    
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position   (9)
Amounts recognized in accumulated other comprehensive loss (pre-tax basis) consist of:    
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax   0
Other Postretirement Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Service Cost 1 1
Change in benefit obligation:    
Defined Benefit Plan, Benefit Obligation 208 198
Defined Benefit Plan, Interest Cost 5 7
Defined Benefit Plan, Benefits Paid (Deprecated 2017-01-31) (16) (16)
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) 10 18
Defined Benefit Plan, Benefit Obligation 208 208
Change in plan assets:    
Defined Benefit Plan, Plan Assets, Amount 270 241
Defined Benefit Plan, Plan Assets, Increase (Decrease) for Actual Return (Loss) 28 35
Defined Benefit Plan, Plan Assets, Contributions by Employer 9 10
Defined Benefit Plan, Plan Assets, Amount 291 270
Defined Benefit Plan, Funded (Unfunded) Status of Plan (83) (62)
Amounts recognized in the consolidated balance sheets consist of:    
Assets for Plan Benefits, Defined Benefit Plan 108 88
Liability, Defined Benefit Plan, Current (2) (2)
Liability, Defined Benefit Plan, Noncurrent (23) (24)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position 83 62
Amounts recognized in accumulated other comprehensive loss (pre-tax basis) consist of:    
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Reclassification Adjustment, before Tax (18) (5)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax 21 40
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax 3 35
Defined Benefit Plan, Plan Assets, Benefits Paid (16) (16)
Funded Plans [Member] | Pension Benefits    
Change in benefit obligation:    
Defined Benefit Plan, Benefit Obligation 52 1
Defined Benefit Plan, Interest Cost 2 2
Defined Benefit Plan, Benefits Paid (Deprecated 2017-01-31) (2) (1)
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) 5 4
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement (2) (4)
Defined Benefit Plan, Benefit Obligation 55 52
Change in plan assets:    
Defined Benefit Plan, Plan Assets, Amount 43 1
Defined Benefit Plan, Plan Assets, Increase (Decrease) for Actual Return (Loss) 5 6
Defined Benefit Plan, Plan Assets, Contributions by Employer 1 1
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement (2) (4)
Defined Benefit Plan, Plan Assets, Amount 45 43
Defined Benefit Plan, Funded (Unfunded) Status of Plan 10 9
Amounts recognized in the consolidated balance sheets consist of:    
Assets for Plan Benefits, Defined Benefit Plan 0
Liability, Defined Benefit Plan, Current 0 0
Liability, Defined Benefit Plan, Noncurrent (10) (9)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position (10)  
Amounts recognized in accumulated other comprehensive loss (pre-tax basis) consist of:    
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Reclassification Adjustment, before Tax 0 0
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax 0 0
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax 0  
Defined Benefit Plan, Plan Assets, Benefits Paid (2) (1)
Unfunded Plans [Member]    
Change in benefit obligation:    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement 0 0
Change in plan assets:    
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement 0 0
Unfunded Plans [Member] | Pension Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Defined Benefit Plan, Service Cost 0 0
Change in benefit obligation:    
Defined Benefit Plan, Benefit Obligation 34 37
Defined Benefit Plan, Interest Cost 1 1
Defined Benefit Plan, Benefits Paid (Deprecated 2017-01-31) (5) (7)
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) 1 0
Defined Benefit Plan, Benefit Obligation 31 34
Change in plan assets:    
Defined Benefit Plan, Plan Assets, Amount 0 0
Defined Benefit Plan, Plan Assets, Increase (Decrease) for Actual Return (Loss) 0 0
Defined Benefit Plan, Plan Assets, Contributions by Employer 0 0
Defined Benefit Plan, Plan Assets, Amount 0 0
Defined Benefit Plan, Funded (Unfunded) Status of Plan 31 34
Amounts recognized in the consolidated balance sheets consist of:    
Assets for Plan Benefits, Defined Benefit Plan 0 0
Liability, Defined Benefit Plan, Current (4) (5)
Liability, Defined Benefit Plan, Noncurrent (27) (29)
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position (31) (34)
Amounts recognized in accumulated other comprehensive loss (pre-tax basis) consist of:    
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Reclassification Adjustment, before Tax 2 1
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax 0 0
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax 2 1
Defined Benefit Plan, Plan Assets, Benefits Paid 0 0
SemGroup Compensation Plans [Member] [Member]    
Change in benefit obligation:    
Defined Benefit Plan, Benefit Obligation 0  
Defined Benefit Plan, Benefit Obligation 0 0
SemGroup Compensation Plans [Member] [Member] | Other Postretirement Benefits    
Change in plan assets:    
Defined Benefit Plan, Plan Assets, Amount 0  
Defined Benefit Plan, Plan Assets, Amount 0 0
SemGroup Compensation Plans [Member] [Member] | Funded Plans [Member] | Pension Benefits    
Change in benefit obligation:    
Defined Benefit Plan, Benefit Obligation 50  
Defined Benefit Plan, Benefit Obligation 0 50
Change in plan assets:    
Defined Benefit Plan, Plan Assets, Amount 40  
Defined Benefit Plan, Plan Assets, Amount 0 40
SemGroup Compensation Plans [Member] [Member] | Unfunded Plans [Member]    
Change in benefit obligation:    
Defined Benefit Plan, Benefit Obligation 3  
Defined Benefit Plan, Benefit Obligation 0 3
SemGroup Compensation Plans [Member] [Member] | Unfunded Plans [Member] | Pension Benefits    
Change in plan assets:    
Defined Benefit Plan, Plan Assets, Amount 0  
Defined Benefit Plan, Plan Assets, Amount $ 0 $ 0