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Comprehensive Income (Loss) (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                      
Interest expense                 $ 140,756 $ 139,447 $ 86,226
Revenue $ 455,298 $ 335,611 $ 341,829 $ 547,528 $ 455,390 $ 324,147 $ 317,795 $ 441,584 1,680,266 1,538,916 1,261,322
Fuel, purchased power and cost of natural gas sold                 563,288 499,132 456,887
Income before income taxes                 281,742 205,894 220,205
Income tax benefit (expense)                 (73,367) (59,101) (78,657)
Net income (loss)                 191,276 82,631 $ (32,111)
Reclassification out of Accumulated Other Comprehensive Income                      
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                      
Net income (loss)                 (3,372) 3,180  
Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income                      
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                      
Income before income taxes                 (2,271) 7,106  
Net income (loss)                 (1,396) 4,404  
Accumulated Defined Benefit Plans Adjustment | Reclassification out of Accumulated Other Comprehensive Income                      
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                      
Income before income taxes                 (1,460) (1,757)  
Net income (loss)                 (1,976) (1,224)  
Interest Expense | Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income | Interest Rate Contract                      
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                      
Interest expense                 (2,941) (3,899)  
Fuel, purchased power and cost of natural gas sold | Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income | Commodity Contract                      
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                      
Fuel, purchased power and cost of natural gas sold                 (243) (14)  
Operating Expense | Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) | Reclassification out of Accumulated Other Comprehensive Income                      
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                      
Operations and maintenance                 168 194  
Operating Expense | Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) | Reclassification out of Accumulated Other Comprehensive Income                      
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                      
Operations and maintenance                 (1,599) (1,881)  
Discontinued Operations | Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income | Commodity Contract                      
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                      
Revenue                 913 11,019  
Discontinued Operations | Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) | Reclassification out of Accumulated Other Comprehensive Income                      
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                      
Operations and maintenance                 29 27  
Discontinued Operations | Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) | Reclassification out of Accumulated Other Comprehensive Income                      
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                      
Operations and maintenance                 (58) (97)  
Income Tax Benefit (Expense) | Cash Flow Hedging | Reclassification out of Accumulated Other Comprehensive Income                      
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                      
Income tax benefit (expense)                 875 (2,702)  
Income Tax Benefit (Expense) | Accumulated Defined Benefit Plans Adjustment | Reclassification out of Accumulated Other Comprehensive Income                      
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]                      
Income tax benefit (expense)                 $ (516) $ 533