CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Operating revenue:
Electric	$ 668,984	$ 630,191
Natural gas	400,065	323,408
Other	8,183	9,098
Total operating revenue	1,077,232	962,697
Energy costs:
Purchased electricity	179,582	142,897
Electric generation fuel	51,310	54,193
Residential exchange	(23,447)	(20,140)
Purchased natural gas	152,801	123,103
Unrealized (gain) loss on derivative instruments, net	19,288	(16,822)
Utility operations and maintenance	152,030	145,990
Non-utility expense and other	5,195	5,634
Depreciation and amortization	115,253	107,515
Conservation amortization	34,761	33,211
Taxes other than income taxes	118,732	102,292
Total operating expenses	805,505	677,873
Operating income (loss)	271,727	284,824
Other income (deductions):
Other income	5,959	5,977
Other expense	(1,215)	(1,340)
Non-hedged interest rate swap (expense) income	28	(855)
Interest charges:
AFUDC	2,176	2,358
Interest expense	(88,582)	(88,813)
Income (loss) before income taxes	190,093	202,151
Income tax (benefit) expense	62,543	60,965
Net income (loss)	127,550	141,186
Subsidiaries [Member]
Operating revenue:
Electric	668,984	630,191
Natural gas	400,065	323,408
Other	8,183	9,098
Total operating revenue	1,077,232	962,697
Energy costs:
Purchased electricity	179,582	142,897
Electric generation fuel	51,310	54,193
Residential exchange	(23,447)	(20,140)
Purchased natural gas	152,801	123,103
Unrealized (gain) loss on derivative instruments, net	19,288	(16,822)
Utility operations and maintenance	152,030	145,990
Non-utility expense and other	8,491	9,033
Depreciation and amortization	115,253	107,515
Conservation amortization	34,761	33,211
Taxes other than income taxes	118,732	102,292
Total operating expenses	808,801	681,272
Operating income (loss)	268,431	281,425
Other income (deductions):
Other income	5,959	5,975
Other expense	(1,215)	(1,340)
Interest charges:
AFUDC	2,176	2,358
Interest expense	(60,461)	(60,775)
Income (loss) before income taxes	214,890	227,643
Income tax (benefit) expense	71,798	71,138
Net income (loss)	$ 143,092	$ 156,505