CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Parent [Line Items]
Regulated Operating Revenue, Electric, Non-Nuclear	$ 2,455,919	$ 2,420,663	$ 2,238,492
Regulated Operating Revenue, Gas	850,748	997,759	890,510
Operating revenue:
Other	39,829	41,854	35,299
Total operating revenue	3,346,496	3,460,276	3,164,301
Energy costs:
Utilities Operating Expense, Purchased Power	638,775	590,030	531,596
Electric generation fuel	204,174	206,275	215,331
Residential exchange	(77,454)	(75,933)	(69,824)
Purchased natural gas	296,699	360,009	313,954
Unrealized (gain) loss on derivative instruments, net	(41,662)	30,790	(83,795)
Utility operations and maintenance	602,638	592,277	572,933
Non-utility expense and other	54,519	53,864	42,620
Depreciation and amortization	666,432	481,969	439,579
Conservation amortization	111,714	121,216	107,784
Taxes other than income taxes	336,603	360,673	328,649
Total operating expenses	2,792,438	2,721,170	2,398,827
Operating income (loss)	554,058	739,106	765,474
Other income (deductions):
Other income	52,957	49,283	45,449
Other expense	(11,201)	(14,104)	(10,923)
Non-hedged interest rate swap expense	0	28	(1,062)
Interest charges:
AFUDC	13,695	10,826	9,304
Interest expense	(343,795)	(354,802)	(355,139)
Income (loss) before income taxes	265,714	430,337	453,103
Income tax (benefit) expense	30,092	255,143	140,204
Net income (loss)	235,622	175,194	312,899
Subsidiaries [Member]
Parent [Line Items]
Regulated Operating Revenue, Electric, Non-Nuclear	2,455,919	2,420,663	2,238,492
Regulated Operating Revenue, Gas	850,748	997,759	890,510
Operating revenue:
Other	39,829	41,854	35,616
Total operating revenue	3,346,496	3,460,276	3,164,618
Energy costs:
Utilities Operating Expense, Purchased Power	638,775	590,030	531,596
Electric generation fuel	204,174	206,275	215,331
Residential exchange	(77,454)	(75,933)	(69,824)
Purchased natural gas	296,699	360,009	313,954
Unrealized (gain) loss on derivative instruments, net	(41,662)	30,790	(83,795)
Utility operations and maintenance	602,638	592,277	572,933
Non-utility expense and other	51,549	52,389	37,859
Depreciation and amortization	666,324	481,955	439,579
Conservation amortization	111,714	121,216	107,784
Taxes other than income taxes	336,603	360,673	328,649
Total operating expenses	2,789,360	2,719,681	2,394,066
Operating income (loss)	557,136	740,595	770,552
Other income (deductions):
Other income	39,847	34,867	29,978
Other expense	(11,201)	(14,104)	(10,923)
Interest charges:
AFUDC	13,695	10,826	9,304
Interest expense	(231,615)	(240,144)	(242,983)
Income (loss) before income taxes	367,862	532,040	555,928
Income tax (benefit) expense	50,700	211,986	175,347
Net income (loss)	$ 317,162	$ 320,054	$ 380,581