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Operations and Significant Accounting Policies - Concentration of Credit Risk (Details) - Customer Concentration Risk [Member] - Large Power Customers [Member] $ in Millions | 12 Months Ended
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Dec. 31, 2018 USD ($) Customers | Dec. 31, 2017 USD ($) Customers | Dec. 31, 2016 Customers
Concentration Risk [Line Items] |  |  |
Number of Large Power Customers | Customers | 8 | 9 | 9
Receivables from Large Power Customers | $ | $ 11.7 | $ 13.8 |
Largest Customer [Member] | Consolidated Operating Revenue [Member] |  |  |
Concentration Risk [Line Items] |  |  |
Percent of Consolidated Operating Revenue | 10.00% | 10.00% | 8.00%

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