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Fair Value Measurements (Tables) | 12 Months Ended
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Dec. 31, 2018
Fair Value Disclosures [Abstract] |
Fair value of assets and liabilities that are measured at fair value on a recurring basis | The following table presents the fair value at December 31, 2018 of our assets and liabilities that are measured at fair value on a recurring basis (dollars in thousands):
| Quoted Prices in Active Markets for Identical Assets (Level 1) |  | Significant Other Observable Inputs (Level 2) |  | Significant Unobservable Inputs (Level 3) |  | Other |  |  |  | Balance at December 31, 2018
Assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash equivalents | $ | 1,200 |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  |  |  | $ | 1,200 |
Risk management activities — derivative instruments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commodity contracts | — |  |  | 3,140 |  |  | 2 |  |  | (2,029 | ) |  | (a) |  | 1,113 |
Nuclear decommissioning trust: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity securities | 5,203 |  |  | — |  |  | — |  |  | 2,148 |  |  | (b) |  | 7,351 |
U.S. commingled equity funds | — |  |  | — |  |  | — |  |  | 396,805 |  |  | (c) |  | 396,805 |
U.S. Treasury debt | 148,173 |  |  | — |  |  | — |  |  | — |  |  |  |  | 148,173 |
Corporate debt | — |  |  | 96,656 |  |  | — |  |  | — |  |  |  |  | 96,656 |
Mortgage-backed debt securities | — |  |  | 113,115 |  |  | — |  |  | — |  |  |  |  | 113,115 |
Municipal bonds | — |  |  | 79,073 |  |  | — |  |  | — |  |  |  |  | 79,073 |
Other fixed income | — |  |  | 9,961 |  |  | — |  |  | — |  |  |  |  | 9,961 |
Subtotal nuclear decommissioning trust | 153,376 |  |  | 298,805 |  |  | — |  |  | 398,953 |  |  |  |  | 851,134 |
Other special use funds: |  |  |  |  |  |  |  |  |  |  |
Equity securities | 45,130 |  |  | — |  |  | — |  |  | 593 |  |  | (b) |  | 45,723 |
U.S. Treasury debt | 173,310 |  |  | — |  |  | — |  |  | — |  |  |  |  | 173,310 |
Municipal bonds | — |  |  | 17,068 |  |  | — |  |  | — |  |  |  |  | 17,068 |
Subtotal other special use funds | 218,440 |  |  | 17,068 |  |  | — |  |  | 593 |  |  |  |  | 236,101 |
Total Assets | $ | 373,016 |  |  | $ | 319,013 |  |  | $ | 2 |  |  | $ | 397,517 |  |  |  |  | $ | 1,089,548 |
Liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk management activities — derivative instruments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commodity contracts | $ | — |  |  | $ | (52,696 | ) |  | $ | (8,216 | ) |  | $ | 875 |  |  | (a) |  | $ | (60,037 | )
(a) | Represents counterparty netting, margin, and collateral. See Note 16.
(b) | Represents net pending securities sales and purchases.
(c) | Valued using NAV as a practical expedient and, therefore, are not classified in the fair value hierarchy.
The following table presents the fair value at December 31, 2017 of our assets and liabilities that are measured at fair value on a recurring basis (dollars in thousands):
| Quoted Prices in Active Markets for Identical Assets (Level 1) |  | Significant Other Observable Inputs (Level 2) |  | Significant Unobservable Inputs (a) (Level 3) |  | Other |  |  |  | Balance at December 31, 2017
Assets |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash equivalents | $ | 10,630 |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  |  |  | $ | 10,630 |
Risk management activities — derivative instruments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commodity contracts | — |  |  | 5,683 |  |  | 1,036 |  |  | (4,737 | ) |  | (b) |  | 1,982 |
Nuclear decommissioning trust: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash and cash equivalents | 7,224 |  |  | — |  |  | — |  |  | 109 |  |  | (d) |  | 7,333 |
U.S. commingled equity funds | — |  |  | — |  |  | — |  |  | 417,390 |  |  | (e) |  | 417,390 |
U.S. Treasury debt | 127,662 |  |  | — |  |  | — |  |  | — |  |  |  |  | 127,662 |
Corporate debt | — |  |  | 114,007 |  |  | — |  |  | — |  |  |  |  | 114,007 |
Mortgage-backed debt securities | — |  |  | 111,874 |  |  | — |  |  | — |  |  |  |  | 111,874 |
Municipal bonds | — |  |  | 79,049 |  |  | — |  |  | — |  |  |  |  | 79,049 |
Other fixed income | — |  |  | 13,685 |  |  | — |  |  | — |  |  |  |  | 13,685 |
Subtotal nuclear decommissioning trust | 134,886 |  |  | 318,615 |  |  | — |  |  | 417,499 |  |  |  |  | 871,000 |
Other special use funds (c): | 455 |  |  | 31,562 |  |  | — |  |  | 525 |  |  |  |  | 32,542 |
Total Assets | $ | 145,971 |  |  | $ | 355,860 |  |  | $ | 1,036 |  |  | $ | 413,287 |  |  |  |  | $ | 916,154 |
Liabilities |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Risk management activities — derivative instruments: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commodity contracts | $ | — |  |  | $ | (78,646 | ) |  | $ | (19,292 | ) |  | $ | 1,516 |  |  | (b) |  | $ | (96,422 | )
(a) | Primarily consists of long-dated electricity contracts.
(b) | Represents counterparty netting, margin, and collateral. See Note 16.
(c) | Primarily consists of fixed income municipal bonds. Presented as coal reclamation escrow in 2017.
(d) | Represents nuclear decommissioning trust net pending securities sales and purchases.
(e) Valued using NAV as a practical expedient and, therefore, are not classified in the fair value hierarchy.
Information regarding the entity's internally developed significant unobservable inputs used to value its level 3 instruments | The following tables provide information regarding our significant unobservable inputs used to value our risk management derivative Level 3 instruments at December 31, 2018 and December 31, 2017:
| December 31, 2018 Fair Value (thousands) |  | Valuation Technique |  | Significant Unobservable Input |  | Range |  | Weighted-Average
Commodity Contracts | Assets |  | Liabilities |
Electricity: |  |  |  |  |  |  |  |  |  |  |  |  |  |
Forward Contracts (a) | $ | — |  |  | $ | 2,456 |  |  | Discounted cash flows |  | Electricity forward price (per MWh) |  | $17.88 - $37.03 |  | $ | 26.10 |
Natural Gas: |  |  |  |  |  |  |  |  |  |  |  |  |  |
Forward Contracts (a) | 2 |  |  | 5,760 |  |  | Discounted cash flows |  | Natural gas forward price (per MMBtu) |  | $1.79 - $2.92 |  | $ | 2.48 |
Total | $ | 2 |  |  | $ | 8,216 |  |  |  |  |  |  |  |  |  |
(a) | Includes swaps and physical and financial contracts.
| December 31, 2017 Fair Value (thousands) |  | Valuation Technique |  | Significant Unobservable Input |  | Range |  | Weighted-Average
Commodity Contracts | Assets |  | Liabilities |
Electricity: |  |  |  |  |  |  |  |  |  |  |  |  |  |
Forward Contracts (a) | $ | 21 |  |  | $ | 15,485 |  |  | Discounted cash flows |  | Electricity forward price (per MWh) |  | $18.51 - $38.75 |  | $ | 27.89 |
Natural Gas: |  |  |  |  |  |  |  |  |  |  |  |  |  |
Forward Contracts (a) | 1,015 |  |  | 3,807 |  |  | Discounted cash flows |  | Natural gas forward price (per MMBtu) |  | $2.33 - $3.11 |  | $ | 2.71 |
Total | $ | 1,036 |  |  | $ | 19,292 |  |  |  |  |  |  |  |  |  |
(a) | Includes swaps and physical and financial contracts.
Changes in fair value for assets and liabilities that are measured at fair value on a recurring basis using Level 3 inputs | The following table shows the changes in fair value for our risk management activities' assets and liabilities that are measured at fair value on a recurring basis using Level 3 inputs for the years ended December 31, 2018 and 2017 (dollars in thousands):
|  | Year Ended December 31,
Commodity Contracts |  | 2018 |  | 2017
Net derivative balance at beginning of period |  | $ | (18,256 | ) |  | $ | (47,406 | )
Total net gains (losses) realized/unrealized: |  |  |  |  |  |
Included in earnings |  | — |  |  | — |
Included in OCI |  | — |  |  | 3 |
Deferred as a regulatory asset or liability |  | (1,130 | ) |  | (13,643 | )
Settlements |  | (787 | ) |  | 5,834 |
Transfers into Level 3 from Level 2 |  | (12,830 | ) |  | (10,026 | )
Transfers from Level 3 into Level 2 |  | 24,789 |  |  | 46,982 |
Net derivative balance at end of period |  | $ | (8,214 | ) |  | $ | (18,256 | )
Net unrealized gains included in earnings related to instruments still held at end of period |  | $ | — |  |  | $ | — |

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- Definition Tabular disclosure of the fair value measurement of assets and liabilities using significant unobservable inputs (Level 3), a reconciliation of the beginning and ending balances. + References No definition available. + Details
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- Definition Tabular disclosure of input and valuation technique used to measure fair value and change in valuation approach and technique for each separate class of asset and liability measured on recurring and nonrecurring basis. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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- Definition Tabular disclosure of assets and liabilities, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19190-110258 + Details
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