CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Operating revenue:
Electric	$ 537,543	$ 495,321	$ 1,736,335	$ 1,622,664
Natural gas	111,516	114,458	691,685	601,309
Other	11,318	8,499	29,356	25,170
Total operating revenue	660,377	618,278	2,457,376	2,249,143
Energy costs:
Purchased electricity	115,881	94,849	425,263	356,296
Electric generation fuel	66,584	70,503	152,057	165,627
Residential exchange	(14,246)	(15,577)	(52,814)	(49,093)
Purchased natural gas	32,224	34,041	248,208	205,418
Unrealized (gain) loss on derivative instruments, net	(23)	6,327	23,098	(57,218)
Utility operations and maintenance	141,003	138,265	438,622	422,273
Non-utility expense and other	7,319	4,708	18,658	15,520
Depreciation and amortization	120,829	110,022	355,538	328,809
Conservation amortization	25,395	21,800	85,847	77,551
Taxes other than income taxes	66,367	65,268	262,099	235,431
Total operating expenses	561,333	530,206	1,956,576	1,700,614
Operating income (loss)	99,044	88,072	500,800	548,529
Other income (deductions):
Other income	7,151	6,130	19,375	19,187
Other expense	(2,878)	(5,025)	(6,134)	(8,488)
Non-hedged interest rate swap (expense) income	0	563	28	(651)
Interest charges:
AFUDC	3,123	2,702	7,853	7,663
Interest expense	(88,780)	(89,297)	(265,771)	(266,786)
Income (loss) before income taxes	17,660	3,145	256,151	299,454
Income tax (benefit) expense	4,824	810	80,489	91,380
Net income (loss)	12,836	2,335	175,662	208,074
Subsidiaries [Member]
Operating revenue:
Electric	537,543	495,321	1,736,335	1,622,664
Natural gas	111,516	114,458	691,685	601,309
Other	11,318	8,815	29,356	25,487
Total operating revenue	660,377	618,594	2,457,376	2,249,460
Energy costs:
Purchased electricity	115,881	94,849	425,263	356,296
Electric generation fuel	66,584	70,503	152,057	165,627
Residential exchange	(14,246)	(15,577)	(52,814)	(49,093)
Purchased natural gas	32,224	34,041	248,208	205,418
Unrealized (gain) loss on derivative instruments, net	(23)	6,327	23,098	(57,218)
Utility operations and maintenance	141,003	138,265	438,622	422,273
Non-utility expense and other	9,994	8,620	27,857	26,474
Depreciation and amortization	120,829	110,022	355,538	328,809
Conservation amortization	25,395	21,800	85,847	77,551
Taxes other than income taxes	66,367	65,268	262,099	235,431
Total operating expenses	564,008	534,118	1,965,775	1,711,568
Operating income (loss)	96,369	84,476	491,601	537,892
Other income (deductions):
Other income	6,778	6,131	18,861	19,184
Other expense	(2,878)	(5,025)	(6,134)	(8,488)
Interest charges:
AFUDC	3,123	2,702	7,853	7,663
Interest expense	(59,868)	(60,914)	(180,320)	(182,336)
Income (loss) before income taxes	43,524	27,370	331,861	373,915
Income tax (benefit) expense	14,424	8,393	109,015	117,533
Net income (loss)	$ 29,100	$ 18,977	$ 222,846	$ 256,382