Revenue: Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue from contracts with customers									$ 1,726,392	$ 1,748,542
Revenue	$ 477,654	$ 325,548	$ 333,888	$ 597,810	$ 501,196	$ 321,979	$ 355,704	$ 575,389	1,734,900	1,754,268	$ 1,680,266
Services transferred at a point in time
Revenue from contracts with customers									27,180	33,609
Services transferred over time
Revenue from contracts with customers									1,699,212	1,714,933
Other revenues
Revenue									8,508	5,726
Intercompany Eliminations
Revenue from contracts with customers									(149,205)	(141,428)
Revenue									(150,769)	(142,974)	$ (477,940)
Intercompany Eliminations | Revenues | Scenario, Adjustment
Changes To Our Segment Performance Measure - Income Statement										3,500
Intercompany Eliminations | Services transferred at a point in time
Revenue from contracts with customers									(32,053)	(32,194)
Intercompany Eliminations | Services transferred over time
Revenue from contracts with customers									(117,152)	(109,234)
Intercompany Eliminations | Other revenues
Revenue									(1,564)	(1,546)
Electric Utilities
Revenue from contracts with customers									707,561	709,024
Revenue									712,752	711,451
Electric Utilities | Services transferred at a point in time
Revenue from contracts with customers									0	0
Electric Utilities | Services transferred over time
Revenue from contracts with customers									707,561	709,024
Electric Utilities | Other revenues
Revenue									5,191	2,427
Gas Utilities
Revenue from contracts with customers									1,009,646	1,024,352
Revenue									1,010,030	1,025,307
Gas Utilities | Services transferred at a point in time
Revenue from contracts with customers									0	0
Gas Utilities | Services transferred over time
Revenue from contracts with customers									1,009,646	1,024,352
Gas Utilities | Other revenues
Revenue									384	955
Power Generation
Revenue from contracts with customers									99,157	90,791
Revenue									101,258	92,451
Power Generation | Services transferred at a point in time
Revenue from contracts with customers									0	0
Power Generation | Services transferred over time
Revenue from contracts with customers									99,157	90,791
Power Generation | Other revenues
Revenue									2,101	1,660
Power Generation | Other revenues | Revenues | Scenario, Adjustment
Changes To Our Segment Performance Measure - Income Statement										35,000
Mining
Revenue from contracts with customers									59,233	65,803
Revenue									61,629	68,033
Mining | Services transferred at a point in time
Revenue from contracts with customers									59,233	65,803
Mining | Services transferred over time
Revenue from contracts with customers									0	0
Mining | Other revenues
Revenue									2,396	2,230
Retail
Revenue from contracts with customers									1,450,776	1,461,317
Retail | Intercompany Eliminations
Revenue from contracts with customers									(32,053)	(32,194)
Retail | Electric Utilities
Revenue from contracts with customers									605,756	594,329
Retail | Gas Utilities
Revenue from contracts with customers									817,840	833,379
Retail | Power Generation
Revenue from contracts with customers									0	0
Retail | Mining
Revenue from contracts with customers									59,233	65,803
Transportation
Revenue from contracts with customers									142,348	139,357
Transportation | Intercompany Eliminations
Revenue from contracts with customers									(1,042)	(1,348)
Transportation | Electric Utilities
Revenue from contracts with customers									0	0
Transportation | Gas Utilities
Revenue from contracts with customers									143,390	140,705
Transportation | Power Generation
Revenue from contracts with customers									0	0
Transportation | Mining
Revenue from contracts with customers									0	0
Wholesale
Revenue from contracts with customers									28,464	39,521
Wholesale | Intercompany Eliminations
Revenue from contracts with customers									(91,577)	(84,957)
Wholesale | Electric Utilities
Revenue from contracts with customers									20,884	33,687
Wholesale | Gas Utilities
Revenue from contracts with customers									0	0
Wholesale | Power Generation
Revenue from contracts with customers									99,157	90,791
Wholesale | Power Generation | Revenues | Scenario, Adjustment
Changes To Our Segment Performance Measure - Income Statement										38,000
Wholesale | Mining
Revenue from contracts with customers									0	0
Market - off-system sales
Revenue from contracts with customers									16,772	17,563
Market - off-system sales | Intercompany Eliminations
Revenue from contracts with customers									(7,736)	(8,102)
Market - off-system sales | Electric Utilities
Revenue from contracts with customers									23,817	24,799
Market - off-system sales | Gas Utilities
Revenue from contracts with customers									691	866
Market - off-system sales | Power Generation
Revenue from contracts with customers									0	0
Market - off-system sales | Mining
Revenue from contracts with customers									0	0
Transmission/Other
Revenue from contracts with customers									88,032	90,784
Transmission/Other | Intercompany Eliminations
Revenue from contracts with customers									(16,797)	(14,827)
Transmission/Other | Electric Utilities
Revenue from contracts with customers									57,104	56,209
Transmission/Other | Gas Utilities
Revenue from contracts with customers									47,725	49,402
Transmission/Other | Power Generation
Revenue from contracts with customers									0	0
Transmission/Other | Mining
Revenue from contracts with customers									$ 0	$ 0