CONSOLIDATED STATEMENTS OF INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Electric	$ 2,671,623	$ 2,319,416	$ 2,497,041
Natural gas	1,067,418	980,913	875,371
Other	66,620	26,121	28,718
Total operating revenue	3,805,661	3,326,450	3,401,130
Purchased electricity	784,565	593,719	652,560
Electric generation fuel	282,254	199,107	282,864
Residential exchange	(82,225)	(80,294)	(79,187)
Purchased natural gas	398,553	362,872	290,976
Unrealized (gain) loss on derivative instruments, net	(13,785)	26,807	3,574
Utility operations and maintenance	629,864	597,048	596,676
Non-utility expense and other	58,281	43,425	47,907
Depreciation and amortization	704,783	647,755	656,323
Conservation amortization	103,147	99,585	96,571
Taxes other than income taxes	362,527	328,602	333,858
Total operating expenses	3,227,964	2,818,626	2,882,122
Operating income (loss)	577,697	507,824	519,008
Other income	57,483	58,759	59,905
Other Nonoperating Expense	(14,467)	(23,207)	(9,053)
AFUDC	16,743	14,827	14,559
Interest expense	(352,092)	(373,822)	(356,638)
Income (loss) before income taxes	285,364	184,381	227,781
Income tax (benefit) expense	24,515	1,664	17,073
Net income (loss)	260,849	182,717	210,708
Subsidiaries [Member]
Electric	2,671,623	2,319,416	2,497,041
Natural gas	1,067,418	980,913	875,371
Other	66,620	26,121	28,718
Total operating revenue	3,805,661	3,326,450	3,401,130
Purchased electricity	784,565	593,719	652,560
Electric generation fuel	282,254	199,107	282,864
Residential exchange	(82,225)	(80,294)	(79,187)
Purchased natural gas	398,553	362,872	290,976
Unrealized (gain) loss on derivative instruments, net	(13,785)	26,807	3,574
Utility operations and maintenance	629,864	597,048	596,676
Non-utility expense and other	56,242	42,266	44,403
Depreciation and amortization	704,372	647,546	656,220
Conservation amortization	103,147	99,585	96,571
Taxes other than income taxes	362,527	328,602	333,858
Total operating expenses	3,225,514	2,817,258	2,878,515
Operating income (loss)	580,147	509,192	522,615
Other income	46,523	46,923	47,766
Other Nonoperating Expense	(14,467)	(23,207)	(9,053)
AFUDC	16,743	14,827	14,559
Interest expense	(248,624)	(247,213)	(243,815)
Income (loss) before income taxes	380,322	300,522	332,072
Income tax (benefit) expense	44,259	26,242	39,148
Net income (loss)	$ 336,063	$ 274,280	$ 292,924