Other Comprehensive Income (Loss): Reclassification Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Interest expense	$ 35,974	$ 35,438
Fuel, purchased power and cost of natural gas sold	248,779	247,639
Operations and maintenance	123,913	116,096
Income before income taxes	124,625	113,175
Income tax benefit (expense)	(17,263)	25,802
Net income	107,362	136,634
Reclassification Out Of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Net income	(277)	(1,488)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges | Reclassification Out Of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	(159)	(1,334)
Income tax benefit (expense)	35	297
Net income	(124)	(1,037)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges | Reclassification Out Of Accumulated Other Comprehensive Income | Interest rate swaps
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Interest expense	(713)	(713)
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges | Reclassification Out Of Accumulated Other Comprehensive Income | Commodity Contract
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Fuel, purchased power and cost of natural gas sold	554	(621)
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 [Line Items]
Operations and maintenance	19	45
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) | Reclassification Out Of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Operations and maintenance	(220)	(622)
Accumulated Defined Benefit Plans Adjustment | Reclassification Out Of Accumulated Other Comprehensive Income
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income before income taxes	(201)	(577)
Income tax benefit (expense)	48	126
Net income	$ (153)	$ (451)