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Nature of Regulation - ITC (Details) - ITC $ in Millions | 1 Months Ended | 5 Months Ended | 12 Months Ended | 16 Months Ended
--- | --- | --- | --- | ---
Feb. 28, 2017 USD ($) | Sep. 30, 2016 company | Dec. 31, 2016 USD ($) | Dec. 31, 2015 | Dec. 31, 2013 complaint | May 30, 2016 | Feb. 28, 2015
ROE refund liability |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |
Complaints (number) | complaint |  |  |  |  | 2 |  |
Estimated potential refund |  |  | $ 258 |  |  |  |
ROE refund liability | Minimum |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |
Estimated potential refund |  |  | 221 |  |  |  |
ROE refund liability | Maximum |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |
Estimated potential refund |  |  | $ 258 |  |  |  |
Subsequent event | ROE refund liability, initial complaint |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |
Refunds, including interest | $ 119 |  |  |  |  |  |
FERC |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |
Approved cost-based formula (years) |  |  | 1 year |  |  |  |
Approved cost-based formula, annual true-up (period) |  |  | 2 years |  |  |  |
Capital structure of common equity (percent) |  |  | 60.00% | 60.00% |  |  |
ROE (percent) |  | 12.38% |  |  |  |  | 10.32%
Recommended ROE (percent) |  |  |  |  |  | 9.70% |
Number of utilities | company |  | 3 |  |  |  |  |
FERC | Maximum |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |
ROE (percent) |  | 13.88% |  |  |  |  | 11.35%
Recommended ROE (percent) |  |  |  |  |  | 10.68% |
FERC | ROE refund liability |  |  |  |  |  |  |
Public Utilities, General Disclosures [Line Items] |  |  |  |  |  |  |
Complaints (number) | complaint |  |  |  |  | 2 |  |

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