CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating revenue:
Electric	$ 699,686	$ 668,984
Natural gas	330,284	400,065
Other	8,038	8,183
Total operating revenue	1,038,008	1,077,232
Energy costs:
Purchased electricity	154,206	179,582
Electric generation fuel	42,423	51,310
Residential exchange	(23,943)	(23,447)
Purchased natural gas	127,615	152,801
Unrealized (gain) loss on derivative instruments, net	(996)	19,288
Utility operations and maintenance	160,524	152,030
Non-utility expense and other	12,830	5,195
Depreciation and amortization	184,512	115,253
Conservation amortization	36,864	34,761
Taxes other than income taxes	111,188	118,732
Total operating expenses	805,223	805,505
Operating income (loss)	232,785	271,727
Other income (expense):
Other income	13,455	5,959
Other expense	(2,098)	(1,215)
Non-hedged interest rate swap (expense) income	0	28
Interest charges:
AFUDC	2,884	2,176
Interest expense	(88,326)	(88,582)
Income (loss) before income taxes	158,700	190,093
Income tax (benefit) expense	11,803	62,543
Net income (loss)	146,897	127,550
Subsidiaries [Member]
Operating revenue:
Electric	699,686	668,984
Natural gas	330,284	400,065
Other	8,038	8,183
Total operating revenue	1,038,008	1,077,232
Energy costs:
Purchased electricity	154,206	179,582
Electric generation fuel	42,423	51,310
Residential exchange	(23,943)	(23,447)
Purchased natural gas	127,615	152,801
Unrealized (gain) loss on derivative instruments, net	(996)	19,288
Utility operations and maintenance	160,524	152,030
Non-utility expense and other	9,781	8,491
Depreciation and amortization	184,490	115,253
Conservation amortization	36,864	34,761
Taxes other than income taxes	111,188	118,732
Total operating expenses	802,152	808,801
Operating income (loss)	235,856	268,431
Other income (expense):
Other income	7,641	5,959
Other expense	(2,098)	(1,215)
Interest charges:
AFUDC	2,884	2,176
Interest expense	(59,555)	(60,461)
Income (loss) before income taxes	184,728	214,890
Income tax (benefit) expense	21,691	71,798
Net income (loss)	$ 163,037	$ 143,092