CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating revenue:
Electric	$ 534,569	$ 537,543	$ 1,735,765	$ 1,736,335
Natural gas	106,826	111,516	597,306	691,685
Other	10,069	11,318	28,253	29,356
Total operating revenue	651,464	660,377	2,361,324	2,457,376
Energy costs:
Purchased electricity	148,536	115,881	431,856	425,263
Electric generation fuel	71,004	66,584	143,177	152,057
Residential exchange	(15,401)	(14,246)	(55,436)	(52,814)
Purchased natural gas	30,192	32,224	211,679	248,208
Unrealized (gain) loss on derivative instruments, net	(14,046)	(23)	(21,953)	23,098
Utility operations and maintenance	139,361	141,003	440,016	438,622
Non-utility expense and other	19,338	7,319	40,587	18,658
Depreciation and amortization	149,760	120,829	486,377	355,538
Conservation amortization	21,601	25,395	82,489	85,847
Taxes other than income taxes	63,822	66,367	248,357	262,099
Total operating expenses	614,167	561,333	2,007,149	1,956,576
Operating income (loss)	37,297	99,044	354,175	500,800
Other income (expense):
Other income	24,806	7,151	46,378	19,375
Other expense	(3,250)	(2,878)	(7,678)	(6,134)
Non-hedged interest rate swap (expense) income	0	0	0	28
Interest charges:
AFUDC	3,911	3,123	10,112	7,853
Interest expense	(87,578)	(88,780)	(261,988)	(265,771)
Income (loss) before income taxes	(24,814)	17,660	140,999	256,151
Income tax (benefit) expense	(2,844)	4,824	12,428	80,489
Net income (loss)	(21,970)	12,836	128,571	175,662
Subsidiaries [Member]
Operating revenue:
Electric	534,569	537,543	1,735,765	1,736,335
Natural gas	106,826	111,516	597,306	691,685
Other	10,069	11,318	28,253	29,356
Total operating revenue	651,464	660,377	2,361,324	2,457,376
Energy costs:
Purchased electricity	148,536	115,881	431,856	425,263
Electric generation fuel	71,004	66,584	143,177	152,057
Residential exchange	(15,401)	(14,246)	(55,436)	(52,814)
Purchased natural gas	30,192	32,224	211,679	248,208
Unrealized (gain) loss on derivative instruments, net	(14,046)	(23)	(21,953)	23,098
Utility operations and maintenance	139,361	141,003	440,016	438,622
Non-utility expense and other	10,518	9,994	31,132	27,857
Depreciation and amortization	149,730	120,829	486,300	355,538
Conservation amortization	21,601	25,395	82,489	85,847
Taxes other than income taxes	63,822	66,367	248,357	262,099
Total operating expenses	605,317	564,008	1,997,617	1,965,775
Operating income (loss)	46,147	96,369	363,707	491,601
Other income (expense):
Other income	13,596	6,778	29,352	18,861
Other expense	(3,250)	(2,878)	(7,678)	(6,134)
Interest charges:
AFUDC	3,911	3,123	10,112	7,853
Interest expense	(58,278)	(59,868)	(174,853)	(180,320)
Income (loss) before income taxes	2,126	43,524	220,640	331,861
Income tax (benefit) expense	(1,765)	14,424	26,931	109,015
Net income (loss)	$ 3,891	$ 29,100	$ 193,709	$ 222,846