Retirement Benefits - Estimated Future Benefit Payments (Details) $ in Millions	Dec. 31, 2016 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2015	$ 23
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	23
Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2015	1,360
2016	1,170
2017	1,191
2018	1,223
2019	1,275
2020 through 2024	6,791
Total estimated future benefit payments through 2026	13,010
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	1,360
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2015	244
2016	250
2017	256
2018	263
2019	272
2020 through 2024	1,456
Total estimated future benefit payments through 2026	2,741
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	$ 244