Employee Benefit Plans - Unrecognized Amounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, Prior Service Cost (Credit), before Tax				$ (8)	$ 0
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	$ 510	$ 510		480	510
Beginning balance	510
Ending balance	480	510
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				(22)	(37)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Regulatory Deferrals, Before Tax				7	7
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	(16)	43	$ 43	35	(16)
Beginning balance	(16)	43
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	(38)	(73)
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	13	(14)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	51	(59)
Ending balance	35	(16)	43
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	10	55	55	44	10
Beginning balance	10	55
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	23	(52)
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	11	(7)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	34	(45)
Ending balance	44	10	55
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	(26)	(12)	(12)	(10)	(26)
Beginning balance	(26)	(12)
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	(14)	(21)
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	(2)	(7)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	16	(14)
Ending balance	(10)	(26)	(12)
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	1			1
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	(1)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	1
Ending balance	1
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				472	510
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	(30)	(80)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, before Tax	(59)	(50)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax	(8)	0
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	510	590	590	480	510
Beginning balance	510	590
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	45	(63)
Foreign currency exchange rate changes	(30)	50
Ending balance	480	510	590
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	(22)	(17)
Pension Plan [Member] | Domestic Plan [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, Net Prior Service Cost (Credit), Before Tax				0	(3)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Regulatory Deferrals, Before Tax				(1)	(4)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	642	761	761	746	642
Beginning balance	642	761
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	(151)	94
Defined Benefit Plan, Accumulated Benefit Obligation, (Increase) Decrease for Settlement and Curtailment	(21)	0	0
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	28	25
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	104	(119)
Ending balance	746	642	761
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	(21)
Pension Plan [Member] | Domestic Plan [Member] | 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	665	761	761	730	665
Beginning balance	665	761
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	114	(68)
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	(28)	28
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	65	(96)
Ending balance	730	665	761
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	(21)
Pension Plan [Member] | Domestic Plan [Member] | 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	(43)	(13)	(13)	0	(43)
Beginning balance	(43)	(13)
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	(43)	(29)
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	0	(1)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	43	(30)
Ending balance	0	(43)	(13)
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	0
Pension Plan [Member] | Domestic Plan [Member] | 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	20	13	13	$ 16	$ 20
Beginning balance	20	13
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	(6)	(3)
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	0	4
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	(4)	7
Ending balance	16	$ 20	$ 13
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	$ 0