Employee Benefit Plans (Expected Benefit Payments for Company's Pension and Postretirement Plans as well as Expected Medicare Part D Subsidy Receipts Related to Postretirement Plan) (Details) $ in Millions	Dec. 31, 2017 USD ($)
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
2018	$ 789
2019	849
2020	803
2021	827
2022	866
2023-2027	4,534
Total	8,668
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
2018	149
2019	152
2020	154
2021	155
2022	155
2023-2027	766
Total	1,531
Other Postretirement Benefits– Medicare Part D Subsidy Receipts
Defined Benefit Plan Disclosure [Line Items]
2018	10
2019	10
2020	10
2021	11
2022	11
2023-2027	59
Total	$ 111