Employee Benefit Plans - Unrecognized Amounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	$ 27
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	61
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	(27)
Defined Benefit Plan, Amortization of Regulatory Deferrals, Net	0
Defined Benefit Plan, Amounts That Will Be Amortized from Regulatory Assets (Liabilities) and Accumulated Other Comprehensive Income Loss In Next Fiscal Year	34
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			$ 768	$ 757
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Net Prior Service Cost (Credit), Before Tax			(25)	(31)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Regulatory Deferrals, Before Tax			(2)	(3)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	723	$ 441	741	723
Beginning balance	723	441
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	75	326
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	11
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	(53)	(44)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	18	282
Ending balance	741	723
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	58
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	(11)
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	46
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			97	108
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Net Prior Service Cost (Credit), Before Tax			(68)	(87)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Regulatory Deferrals, Before Tax			8	2
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	23	83	37	23
Beginning balance	23	83
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	0	(63)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	16
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	11	3
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	14	(60)
Ending balance	37	23
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	3
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	(16)
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	(12)
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			592	655
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	48
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	710	490	729	710
Beginning balance	710	490
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	76	258
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	(4)
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	(53)	(38)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	19	220
Ending balance	729	710
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	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	37	99	49	37
Beginning balance	37	99
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	(1)	(64)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	3
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	10	2
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	12	(62)
Ending balance	49	37
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	(6)	(58)	(1)	(6)
Beginning balance	(6)	(58)
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	5	52
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	5	52
Ending balance	(1)	(6)
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
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	(14)	(16)	(12)	(14)
Beginning balance	(14)	(16)
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	2	2
Ending balance	(12)	(14)
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	19	9	13	19
Beginning balance	19	9
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	(6)	16
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	(6)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	(6)	10
Ending balance	13	19
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] | UK Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Before Tax	655	751	$ 592	$ 655
Beginning balance	655	751
Defined Benefit Plan, Net Unamortized Gain (Loss) Arising During Period, Before Tax	38	(8)
Defined Benefit Plan, Net Amortization Recognized in Net Periodic Benefit Cost Before Tax	(62)	(51)
Foreign currency exchange rate changes	(39)	(37)
Defined Benefit Plan, Net Periodic Benefit Cost Not Yet Recognized, Arising During Period, Before Tax	(63)	(96)
Ending balance	592	$ 655
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	(4)
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	4
Defined Benefit Plan Amount Not Yet Recognized In Net Periodic Benefit Cost, Category [Member] | Domestic Other Postretirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Amortization of Prior Service Cost (Credit), Net	3
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)