Operations and Significant Accounting Policies - Concentration of Credit Risk (Details) - Customer Concentration Risk [Member] - Large Power Customers [Member] $ in Millions |
12 Months Ended | ||
|---|---|---|---|
|
Dec. 31, 2019
USD ($)
Customers
|
Dec. 31, 2018
USD ($)
Customers
|
Dec. 31, 2017
Customers
|
|
| Concentration Risk [Line Items] | |||
| Number of Large Power Customers | Customers | 8 | 9 | 9 |
| Receivables from Large Power Customers | $ | $ 7.8 | $ 11.7 | |
| Largest Customer [Member] | Consolidated Operating Revenue [Member] | |||
| Concentration Risk [Line Items] | |||
| Percent of Consolidated Operating Revenue | 12.00% | 10.00% | 10.00% |