Retirement Benefits Retirement Benefits - Estimated Future Benefit Payments (Details) $ in Millions	Dec. 31, 2018 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2018	$ 25
Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2018	1,196
2019	1,221
2020	1,258
2021	1,284
2022	1,302
2022 through 2026	6,770
Total estimated future benefit payments through 2028	13,031
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
2018	255
2019	263
2020	269
2021	274
2022	282
2022 through 2026	1,483
Total estimated future benefit payments through 2028	$ 2,826