Summary of Significant Accounting Policies Operations and Maintenance (Details) - USD ($) $ in Millions |
12 Months Ended | ||
---|---|---|---|
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
|
Operating Expense [Member] | |||
Results of Operations for Oil and Gas Producing Activities, by Geographic Area [Line Items] | |||
Results of Operations, Expense from Oil and Gas Producing Activities | $ 363 | $ 342 | $ 349 |