Employee Benefit Plans (Schedule of Estimated Future Employer Contributions) (Details) $ in Millions	Dec. 31, 2017 USD ($)
Qualified Plan
2017	$ 120
2018	123
2019	125
2020	127
2021	130
2022 - 2026	665
Non-Qualified Plan
2017	11
2018	12
2019	13
2020	13
2021	13
2022 - 2026	66
Postretirement Benefit Plan
2017	6	[1]
2018	5	[1]
2019	5	[1]
2020	5	[1]
2021	5	[1]
2022 - 2026	$ 19	[1]

[1] Estimated benefit payments in the postretirement benefit plan are net of estimated Medicare subsidies.