CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Operating revenue:
Electric	$ 501,510	$ 529,807	$ 1,201,196	$ 1,198,792
Natural gas	160,196	180,105	490,480	580,169
Other	10,146	9,855	18,184	18,038
Total operating revenue	671,852	719,767	1,709,860	1,796,999
Energy costs:
Purchased electricity	129,114	129,799	283,320	309,381
Electric generation fuel	29,750	34,163	72,173	85,473
Residential exchange	(16,091)	(15,121)	(40,035)	(38,568)
Purchased natural gas	53,872	63,183	181,487	215,984
Unrealized (gain) loss on derivative instruments, net	(6,911)	3,834	(7,907)	23,121
Utility operations and maintenance	140,131	145,555	300,655	297,618
Non-utility expense and other	8,419	6,144	21,249	11,339
Depreciation and amortization	152,105	119,457	336,617	234,710
Conservation amortization	24,025	25,691	60,888	60,453
Taxes other than income taxes	73,347	77,032	184,535	195,731
Total operating expenses	587,761	589,737	1,392,982	1,395,242
Operating income (loss)	84,091	130,030	316,878	401,757
Other income (expense):
Other income	8,116	6,263	21,573	12,223
Other expense	(2,330)	(2,042)	(4,428)	(3,257)
Non-hedged interest rate swap (expense) income	0	0	0	28
Interest charges:
AFUDC	3,318	2,555	6,201	4,730
Interest expense	(86,084)	(88,409)	(174,410)	(176,991)
Income (loss) before income taxes	7,111	48,397	165,814	238,490
Income tax (benefit) expense	3,469	13,122	15,272	75,665
Net income (loss)	3,642	35,275	150,542	162,825
Subsidiaries [Member]
Operating revenue:
Electric	501,510	529,807	1,201,196	1,198,792
Natural gas	160,196	180,105	490,480	580,169
Other	10,146	9,855	18,184	18,038
Total operating revenue	671,852	719,767	1,709,860	1,796,999
Energy costs:
Purchased electricity	129,114	129,799	283,320	309,381
Electric generation fuel	29,750	34,163	72,173	85,473
Residential exchange	(16,091)	(15,121)	(40,035)	(38,568)
Purchased natural gas	53,872	63,183	181,487	215,984
Unrealized (gain) loss on derivative instruments, net	(6,911)	3,834	(7,907)	23,121
Utility operations and maintenance	140,131	145,555	300,655	297,618
Non-utility expense and other	10,834	9,374	20,614	17,865
Depreciation and amortization	152,080	119,457	336,570	234,710
Conservation amortization	24,025	25,691	60,888	60,453
Taxes other than income taxes	73,347	77,032	184,535	195,731
Total operating expenses	590,151	592,967	1,392,300	1,401,768
Operating income (loss)	81,701	126,800	317,560	395,231
Other income (expense):
Other income	8,113	6,126	15,756	12,086
Other expense	(2,330)	(2,042)	(4,428)	(3,257)
Interest charges:
AFUDC	3,318	2,555	6,201	4,730
Interest expense	(57,020)	(59,991)	(116,575)	(120,453)
Income (loss) before income taxes	33,782	73,448	218,514	288,337
Income tax (benefit) expense	7,004	22,794	28,696	94,591
Net income (loss)	$ 26,778	$ 50,654	$ 189,818	$ 193,746