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Fair Value of Financial Assets and Liabilities, Derivative Assets and Liabilities at Fair Value (Details) - USD ($)
$ in Thousands
Mar. 31, 2016
Dec. 31, 2015
Derivatives, Fair Value [Line Items]    
Derivative Asset, Collateral, Obligation to Return Cash, Offset $ 0 $ 0
Derivative Liability, Collateral, Right to Reclaim Cash, Offset 4,400 4,300
Other Current Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 23,293 33,842
Other Noncurrent Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 55,612 51,083
Other Current Liabilities    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 27,553 29,839
Other Noncurrent Liabilities    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 167,299 168,311
Fair Value Measured on a Recurring Basis | Other Current Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 14,390 23,756
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 7,954 6,230
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 6,436 17,333
Fair Value Measured on a Recurring Basis | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset   193
Fair Value Measured on a Recurring Basis | Other Noncurrent Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 26,614 20,861
Fair Value Measured on a Recurring Basis | Other Noncurrent Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 26,605 20,861
Fair Value Measured on a Recurring Basis | Other Noncurrent Assets | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 9  
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Designated as Hedging Instrument | Vehicle Fuel And Other Commodity | Cash Flow Hedges    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 152 205
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 4,694 6,978
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 4,331 1,669
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 0 0
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 119 5,104
Fair Value Measured on a Recurring Basis | Other Current Liabilities | Other Derivative Instruments | Other Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 92  
Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities | Other Derivative Instruments    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 14,749 10,118
Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 14,743 10,118
Fair Value Measured on a Recurring Basis | Other Noncurrent Liabilities | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 6  
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 1,054 225
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 1,054 225
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Assets | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 0 0
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset   0
Fair Value Measured on a Recurring Basis | Level 1 | Other Noncurrent Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 250 0
Fair Value Measured on a Recurring Basis | Level 1 | Other Noncurrent Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 250 0
Fair Value Measured on a Recurring Basis | Level 1 | Other Noncurrent Assets | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 0  
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Liabilities | Designated as Hedging Instrument | Vehicle Fuel And Other Commodity | Cash Flow Hedges    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 0 0
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Liabilities | Other Derivative Instruments    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 1,334 152
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 1,334 152
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Liabilities | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 0 0
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Liabilities | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 0 0
Fair Value Measured on a Recurring Basis | Level 1 | Other Current Liabilities | Other Derivative Instruments | Other Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 0  
Fair Value Measured on a Recurring Basis | Level 1 | Other Noncurrent Liabilities | Other Derivative Instruments    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 215 0
Fair Value Measured on a Recurring Basis | Level 1 | Other Noncurrent Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 215 0
Fair Value Measured on a Recurring Basis | Level 1 | Other Noncurrent Liabilities | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 0  
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 17,417 11,116
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 17,417 10,620
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Assets | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 0 0
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset   496
Fair Value Measured on a Recurring Basis | Level 2 | Other Noncurrent Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 35,257 27,416
Fair Value Measured on a Recurring Basis | Level 2 | Other Noncurrent Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 35,248 27,416
Fair Value Measured on a Recurring Basis | Level 2 | Other Noncurrent Assets | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 9  
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Liabilities | Designated as Hedging Instrument | Vehicle Fuel And Other Commodity | Cash Flow Hedges    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 152 205
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Liabilities | Other Derivative Instruments    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 15,130 13,478
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 14,767 7,866
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Liabilities | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 0 0
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Liabilities | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 119 5,407
Fair Value Measured on a Recurring Basis | Level 2 | Other Current Liabilities | Other Derivative Instruments | Other Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 92  
Fair Value Measured on a Recurring Basis | Level 2 | Other Noncurrent Liabilities | Other Derivative Instruments    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 27,031 19,898
Fair Value Measured on a Recurring Basis | Level 2 | Other Noncurrent Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 27,025 19,898
Fair Value Measured on a Recurring Basis | Level 2 | Other Noncurrent Liabilities | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 6  
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 8,332 22,671
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 453 1,250
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Assets | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 7,879 21,421
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset   0
Fair Value Measured on a Recurring Basis | Level 3 | Other Noncurrent Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 0 0
Fair Value Measured on a Recurring Basis | Level 3 | Other Noncurrent Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 0 0
Fair Value Measured on a Recurring Basis | Level 3 | Other Noncurrent Assets | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 0  
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Liabilities | Designated as Hedging Instrument | Vehicle Fuel And Other Commodity | Cash Flow Hedges    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 0 0
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Liabilities | Other Derivative Instruments    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 1,478 4,643
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 35 555
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Liabilities | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 1,443 4,088
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Liabilities | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 0 0
Fair Value Measured on a Recurring Basis | Level 3 | Other Current Liabilities | Other Derivative Instruments | Other Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 0  
Fair Value Measured on a Recurring Basis | Level 3 | Other Noncurrent Liabilities | Other Derivative Instruments    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 0 0
Fair Value Measured on a Recurring Basis | Level 3 | Other Noncurrent Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 0 0
Fair Value Measured on a Recurring Basis | Level 3 | Other Noncurrent Liabilities | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 0  
Fair Value Measured on a Recurring Basis | Fair Value Total | Other Current Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 26,803 34,012
Fair Value Measured on a Recurring Basis | Fair Value Total | Other Current Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 18,924 12,095
Fair Value Measured on a Recurring Basis | Fair Value Total | Other Current Assets | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 7,879 21,421
Fair Value Measured on a Recurring Basis | Fair Value Total | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset   496
Fair Value Measured on a Recurring Basis | Fair Value Total | Other Noncurrent Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 35,507 27,416
Fair Value Measured on a Recurring Basis | Fair Value Total | Other Noncurrent Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 35,498 27,416
Fair Value Measured on a Recurring Basis | Fair Value Total | Other Noncurrent Assets | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 9  
Fair Value Measured on a Recurring Basis | Fair Value Total | Other Current Liabilities | Designated as Hedging Instrument | Vehicle Fuel And Other Commodity | Cash Flow Hedges    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 152 205
Fair Value Measured on a Recurring Basis | Fair Value Total | Other Current Liabilities | Other Derivative Instruments    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 17,942 18,273
Fair Value Measured on a Recurring Basis | Fair Value Total | Other Current Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 16,136 8,573
Fair Value Measured on a Recurring Basis | Fair Value Total | Other Current Liabilities | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 1,443 4,088
Fair Value Measured on a Recurring Basis | Fair Value Total | Other Current Liabilities | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 119 5,407
Fair Value Measured on a Recurring Basis | Fair Value Total | Other Current Liabilities | Other Derivative Instruments | Other Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 92  
Fair Value Measured on a Recurring Basis | Fair Value Total | Other Noncurrent Liabilities | Other Derivative Instruments    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 27,246 19,898
Fair Value Measured on a Recurring Basis | Fair Value Total | Other Noncurrent Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 27,240 19,898
Fair Value Measured on a Recurring Basis | Fair Value Total | Other Noncurrent Liabilities | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 6  
Fair Value Measured on a Recurring Basis | Netting | Other Current Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset (12,413) [1] (10,256) [2]
Fair Value Measured on a Recurring Basis | Netting | Other Current Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset (10,970) [1] (5,865) [2]
Fair Value Measured on a Recurring Basis | Netting | Other Current Assets | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset (1,443) [1] (4,088) [2]
Fair Value Measured on a Recurring Basis | Netting | Other Current Assets | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset [2]   (303)
Fair Value Measured on a Recurring Basis | Netting | Other Noncurrent Assets    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset (8,893) [1] (6,555) [2]
Fair Value Measured on a Recurring Basis | Netting | Other Noncurrent Assets | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset (8,893) [1] (6,555) [2]
Fair Value Measured on a Recurring Basis | Netting | Other Noncurrent Assets | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset [1] 0  
Fair Value Measured on a Recurring Basis | Netting | Other Current Liabilities | Designated as Hedging Instrument | Vehicle Fuel And Other Commodity | Cash Flow Hedges    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 0 [1] 0 [2]
Fair Value Measured on a Recurring Basis | Netting | Other Current Liabilities | Other Derivative Instruments    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability (13,248) [1] (11,295) [2]
Fair Value Measured on a Recurring Basis | Netting | Other Current Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability (11,805) [1] (6,904) [2]
Fair Value Measured on a Recurring Basis | Netting | Other Current Liabilities | Other Derivative Instruments | Electric Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability (1,443) [1] (4,088) [2]
Fair Value Measured on a Recurring Basis | Netting | Other Current Liabilities | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 0 [1] (303) [2]
Fair Value Measured on a Recurring Basis | Netting | Other Current Liabilities | Other Derivative Instruments | Other Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability [1] 0  
Fair Value Measured on a Recurring Basis | Netting | Other Noncurrent Liabilities | Other Derivative Instruments    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability (12,497) [1] (9,780) [2]
Fair Value Measured on a Recurring Basis | Netting | Other Noncurrent Liabilities | Other Derivative Instruments | Commodity Trading    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability (12,497) [1] (9,780) [2]
Fair Value Measured on a Recurring Basis | Netting | Other Noncurrent Liabilities | Other Derivative Instruments | Natural Gas Commodity    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability [1] 0  
Fair Value, Measurements, Nonrecurring | Other Current Assets | Purchased Power Agreements    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 8,903 [3] 10,086 [4]
Fair Value, Measurements, Nonrecurring | Other Noncurrent Assets | Purchased Power Agreements    
Derivatives, Fair Value [Line Items]    
Derivative Asset, Fair Value, Gross Asset 28,998 [3] 30,222 [4]
Fair Value, Measurements, Nonrecurring | Other Current Liabilities | Purchased Power Agreements    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability 22,859 [3] 22,861 [4]
Fair Value, Measurements, Nonrecurring | Other Noncurrent Liabilities | Purchased Power Agreements    
Derivatives, Fair Value [Line Items]    
Derivative Liability, Fair Value, Gross Liability $ 152,550 [3] $ 158,193 [4]
[1] Xcel Energy nets derivative instruments and related collateral in its consolidated balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at March 31, 2016. At March 31, 2016, derivative assets and liabilities include no obligations to return cash collateral and the rights to reclaim cash collateral of $4.4 million. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.
[2] Xcel Energy nets derivative instruments and related collateral in its consolidated balance sheet when supported by a legally enforceable master netting agreement, and all derivative instruments and related collateral amounts were subject to master netting agreements at Dec. 31, 2015. At Dec. 31, 2015, derivative assets and liabilities include no obligations to return cash collateral and rights to reclaim cash collateral of $4.3 million. The counterparty netting amounts presented exclude settlement receivables and payables and non-derivative amounts that may be subject to the same master netting agreements.
[3] In 2003, as a result of implementing new guidance on the normal purchase exception for derivative accounting, Xcel Energy began recording several long-term PPAs at fair value due to accounting requirements related to underlying price adjustments. As these purchases are recovered through normal regulatory recovery mechanisms in the respective jurisdictions, the changes in fair value for these contracts were offset by regulatory assets and liabilities. During 2006, Xcel Energy qualified these contracts under the normal purchase exception. Based on this qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts will be amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.
[4] In 2003, as a result of implementing new guidance on the normal purchase exception for derivative accounting, Xcel Energy began recording several long-term PPAs at fair value due to accounting requirements related to underlying price adjustments. As these purchases are recovered through normal regulatory recovery mechanisms in the respective jurisdictions, the changes in fair value for these contracts were offset by regulatory assets and liabilities. During 2006, Xcel Energy qualified these contracts under the normal purchase exception. Based on this qualification, the contracts are no longer adjusted to fair value and the previous carrying value of these contracts will be amortized over the remaining contract lives along with the offsetting regulatory assets and liabilities.