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Balance Sheet Components (Details) | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($) | Dec. 31, 2017 USD ($)
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items] |  |  |  |  |  |
Asset Retirement Obligation, Revision of Estimate |  | $ 23,000,000 |  |  |  |
Letters of Credit Outstanding, Amount | $ 0 |  |  | $ 0 |  |
Finite-Lived Intangible Assets, Accumulated Amortization | 333,000,000 |  |  | 333,000,000 |  | $ 296,000,000
Amortization of Intangible Assets | 16,000,000 |  | $ 11,000,000 | 43,000,000 | $ 34,000,000 |
Syndicated credit facility scheduled to expire in 2019 | $ 500,000,000 |  |  | $ 500,000,000 |  |
Debt Instrument, Covenant Description |  |  |  | 0.65 |  |
Ratio of Indebtedness to Net Capital | 0.512 |  |  | 0.512 |  |
Short-term debt | $ 0 |  |  | $ 0 |  |
Commercial Paper | 0 |  |  | 0 |  |
Line of Credit Facility, Remaining Borrowing Capacity | 500,000,000 |  |  | 500,000,000 |  |
Line of Credit Facility, Current Borrowing Capacity | 220,000,000 |  |  | 220,000,000 |  |
Letters of credit issued | 59,000,000 |  |  | 59,000,000 |  |
Authorized Short-Term Debt | 900,000,000 |  |  | 900,000,000 |  |
Long-term Debt, Current Maturities | 300,000,000 |  |  | 300,000,000 |  | $ 0
Long-term Pollution Control Bond | 24,000,000 |  |  | $ 24,000,000 |  |
Extinguishment of Debt, Amount | $ 24,000,000 |  |  |  |  |

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