Fair Value of Financial Assets and Liabilities Fair Value of Financial Assets and Liabilities, Changes in Level 3 Commodity Derivatives (Details) - Commodity Contract - USD ($) |
3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
Sep. 30, 2017 |
Sep. 30, 2016 |
Sep. 30, 2017 |
Sep. 30, 2016 |
|
| Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward] | ||||
| Balance at beginning of period | $ 28,665,000 | $ 1,070,000 | $ 1,955,000 | $ 5,060,000 |
| Purchases | 43,000 | 274,000 | 39,376,000 | 5,426,000 |
| Settlements | (9,939,000) | (7,822,000) | (40,437,000) | (22,438,000) |
| Net gains recognized as regulatory assets and liabilities | 1,669,000 | 6,614,000 | 19,544,000 | 12,088,000 |
| Balance at end of period | 20,438,000 | 136,000 | 20,438,000 | 136,000 |
| Transfers into Level 3 | 0 | 0 | 0 | 0 |
| Transfers out of Level 3 | $ 0 | $ 0 | $ 0 | $ 0 |