Employee Benefit Plans - Unrecognized Amounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	$ 19
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	35
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	19
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	15
Domestic Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Net Gains (Losses), Before Tax			$ 775	$ 768
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Net Prior Service Cost (Credit), Before Tax			(7)	(25)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Regulatory Deferrals, Before Tax			(7)	(2)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	741	$ 723	761	741
Beginning balance	741	723
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	65	75
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	3	(4)
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	(46)	(53)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	20	18
Ending balance	761	741
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	3	(4)
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	28
Domestic Other Postretirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Net Gains (Losses), Before Tax			88	97
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Net Prior Service Cost (Credit), Before Tax			(52)	(68)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Regulatory Deferrals, Before Tax			7	8
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	37	23	43	37
Beginning balance	37	23
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	(6)	0
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	16	3
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	12	11
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	6	14
Ending balance	43	37
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	2
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	16	3
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)
UK Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Net Gains (Losses), Before Tax			590	592
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	65
Regulatory Asset, Pension and Other Postretirement Costs [Member] | Domestic Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	729	710	761	729
Beginning balance	729	710
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	76	76
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	1	(4)
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	(45)	(53)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	32	19
Ending balance	761	729
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	(4)
Regulatory Asset, Pension and Other Postretirement Costs [Member] | Domestic Other Postretirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	49	37	55	49
Beginning balance	49	37
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	(5)	(1)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net		3
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	11	10
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	6	12
Ending balance	55	49
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		3
Regulatory Liability, Pension and Other Postretirement Costs [Member] | Domestic Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	(1)	(6)	(13)	(1)
Beginning balance	(1)	(6)
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	(11)	5
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	0	0
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	(1)	0
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	(12)	5
Ending balance	(13)	(1)
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	0
Regulatory Liability, Pension and Other Postretirement Costs [Member] | Domestic Other Postretirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	(12)	(14)	(12)	(12)
Beginning balance	(12)	(14)
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	(1)	1
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	0	2
Ending balance	(12)	(12)
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
Accumulated Other Comprehensive Income (Loss) [Member] | Domestic Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	13	19	13	13
Beginning balance	13	19
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	0	(6)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	0	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	0	(6)
Ending balance	13	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	0
Accumulated Other Comprehensive Income (Loss) [Member] | UK Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	592	655	$ 590	$ 592
Beginning balance	592	655
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	148	38
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	(44)	(62)
Foreign currency exchange rate changes	(106)	(39)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	(2)	(63)
Ending balance	590	$ 592
Defined Benefit Plan Amount Not Yet Recognized In Net Periodic Benefit Cost, Category [Member] | Domestic Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	1
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