Employee Benefit Plans - Unrecognized Amounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, Prior Service Cost (Credit), before Tax				$ (6)	$ (8)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	$ 549	$ 480		549	480
Beginning balance	480
Ending balance	549	480
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Net Prior Service Cost (Credit), Before Tax				(14)	(22)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Regulatory Deferrals, Before Tax				6	7
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	(31)	35	$ (16)	(31)	35
Beginning balance	35	(16)
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	72	38	(1)
Defined Benefit Plan, Accumulated Benefit Obligation, (Increase) Decrease for Settlement and Curtailment	0	0	0
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	6	(13)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	(66)	51
Ending balance	(31)	35	(16)
Other Postretirement Benefits Plan [Member] | Regulatory Asset, Pension and Other Postretirement Costs [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	4	44	10	4	44
Beginning balance	44	10
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	(45)	23
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	5	(11)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	(40)	34
Ending balance	4	44	10
Other Postretirement Benefits Plan [Member] | Regulatory Liability, Pension and Other Postretirement Costs [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	(32)	(10)	(26)	(32)	(10)
Beginning balance	(10)	(26)
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	23	14
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	(1)	(2)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	(22)	16
Ending balance	(32)	(10)	(26)
Other Postretirement Benefits Plan [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	(3)	1		(3)	1
Beginning balance	1
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	4
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	(4)
Ending balance	(3)	1
Pension Plan [Member] | UNITED STATES
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Unamortized Prior Service Cost Arising During Period, Before Tax	(2)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Net Prior Service Cost (Credit), Before Tax				(2)	0
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Regulatory Deferrals, Before Tax				1	(1)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	652	746	642	652	746
Beginning balance	746	642
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	61	(151)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net		2
Defined Benefit Plan, Accumulated Benefit Obligation, (Increase) Decrease for Settlement and Curtailment	0	21	0
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	31	28
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	(94)	104
Ending balance	652	746	642
Defined Benefit Plan, Reconciliation of Amounts Not Yet Recognized As Components of Net Periodic Benefit Cost [Roll Forward]
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net		2
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax		(21)
Pension Plan [Member] | UNITED STATES | Regulatory Asset, Pension and Other Postretirement Costs [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Unamortized Prior Service Cost Arising During Period, Before Tax	0
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	661	730	665	661	730
Beginning balance	730	665
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	(38)	114
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net		0
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	(31)	28
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	(69)	65
Ending balance	661	730	665
Defined Benefit Plan, Reconciliation of Amounts Not Yet Recognized As Components of Net Periodic Benefit Cost [Roll Forward]
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net		0
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax		(21)
Pension Plan [Member] | UNITED STATES | Regulatory Liability, Pension and Other Postretirement Costs [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Unamortized Prior Service Cost Arising During Period, Before Tax	0
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	(33)	0	(43)	(33)	0
Beginning balance	0	(43)
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	33	43
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net		0
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	0	0
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	(33)	43
Ending balance	(33)	0	(43)
Defined Benefit Plan, Reconciliation of Amounts Not Yet Recognized As Components of Net Periodic Benefit Cost [Roll Forward]
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net		0
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax		0
Pension Plan [Member] | UNITED STATES | Accumulated Other Comprehensive Income (Loss) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Unamortized Prior Service Cost Arising During Period, Before Tax	2
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	24	16	20	24	16
Beginning balance	16	20
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	10	6
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net		2
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	0	0
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	8	(4)
Ending balance	24	16	20
Defined Benefit Plan, Reconciliation of Amounts Not Yet Recognized As Components of Net Periodic Benefit Cost [Roll Forward]
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net		2
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax		0
Pension Plan [Member] | UNITED KINGDOM
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Net Gains (Losses), Before Tax				543	472
Pension Plan [Member] | UNITED KINGDOM | Accumulated Other Comprehensive Income (Loss) [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive (Income) Loss, Defined Benefit Plan, before Reclassification Adjustment and Tax	69	(30)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax	(81)	59
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax	0	8
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	549	480	510	$ 549	$ 480
Beginning balance	480	510
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	46	(45)
Foreign currency exchange rate changes	60	(30)
Ending balance	549	480	$ 510
Defined Benefit Plan, Reconciliation of Amounts Not Yet Recognized As Components of Net Periodic Benefit Cost [Roll Forward]
Other Comprehensive Income (Loss), Defined Benefit Plan, Settlement and Curtailment Gain (Loss), before Tax	$ (26)	$ (22)