Employee Benefit Plans - Unrecognized Amounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	$ 16
Defined Benefit Plan, Reconciliation of Amounts Not Yet Recognized As Components of Net Periodic Benefit Cost [Roll Forward]
Defined Benefit Plan, Amortization of Gains (Losses), Net	33
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	16
Defined Benefit Plan, Amortization of Regulatory Deferrals, Net	(2)
Defined Benefit Plan, Amounts That Will Be Amortized from Regulatory Assets (Liabilities) and Accumulated Other Comprehensive Income Loss In Next Fiscal Year	15
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Net Gains (Losses), Before Tax			$ 14	$ 88
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Net Prior Service Cost (Credit), Before Tax			(37)	(52)
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	43	$ 37	(16)	43
Beginning balance	43	37
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	73	(6)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	15
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	14	(12)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	(59)	6
Ending balance	(16)	43
Defined Benefit Plan, Reconciliation of Amounts Not Yet Recognized As Components of Net Periodic Benefit Cost [Roll Forward]
Defined Benefit Plan, Amortization of Gains (Losses), Net	1
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	15
Defined Benefit Plan, Amortization of Regulatory Deferrals, Net	1
Defined Benefit Plan, Amounts That Will Be Amortized from Regulatory Assets (Liabilities) and Accumulated Other Comprehensive Income Loss In Next Fiscal Year	(13)
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	55	49	10	55
Beginning balance	55	49
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	(52)	(5)
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	7	(11)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	(45)	6
Ending balance	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	(12)	(12)	(26)	(12)
Beginning balance	(12)	(12)
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	21	(1)
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	(7)	(1)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	(14)	0
Ending balance	(26)	(12)
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			510	590
Defined Benefit Plan, Reconciliation of Amounts Not Yet Recognized As Components of Net Periodic Benefit Cost [Roll Forward]
Defined Benefit Plan, Amortization of Gains (Losses), Net	60
Pension Plan [Member] | UNITED KINGDOM | Accumulated Other Comprehensive Income (Loss) [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	590	592	510	590
Beginning balance	590	592
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	(50)	(148)
Defined Benefit Plan, Accumulated Benefit Obligation, (Increase) Decrease for Settlement and Curtailment	(17)	0
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	63	(44)
Foreign currency exchange rate changes	50	(106)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	(80)	(2)
Ending balance	510	590
Pension Plan [Member] | Domestic Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Net Gains (Losses), Before Tax			649	775
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Net Prior Service Cost (Credit), Before Tax			(3)	(7)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Regulatory Deferrals, Before Tax			(4)	(7)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	761	741	642	761
Beginning balance	761	741
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	94	(65)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	1	1
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	25	46
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	(119)	20
Ending balance	642	761
Defined Benefit Plan, Reconciliation of Amounts Not Yet Recognized As Components of Net Periodic Benefit Cost [Roll Forward]
Defined Benefit Plan, Amortization of Gains (Losses), Net	32
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	1	1
Defined Benefit Plan, Amortization of Regulatory Deferrals, Net	(3)
Defined Benefit Plan, Amounts That Will Be Amortized from Regulatory Assets (Liabilities) and Accumulated Other Comprehensive Income Loss In Next Fiscal Year	28
Pension Plan [Member] | Domestic 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	761	729	665	761
Beginning balance	761	729
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	(68)	76
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net		1
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	(28)	45
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	(96)	32
Ending balance	665	761
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		1
Pension Plan [Member] | Domestic 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	(13)	(1)	(43)	(13)
Beginning balance	(13)	(1)
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	29	(11)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net		0
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	1	(1)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	(30)	(12)
Ending balance	(43)	(13)
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
Pension Plan [Member] | Domestic 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	13	13	$ 20	$ 13
Beginning balance	13	13
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	3	0
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net		0
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	4	0
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	7	0
Ending balance	$ 20	13
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