CONSOLIDATED STATEMENTS OF INCOME - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Electric	$ 2,497,041,000	$ 2,455,919,000	$ 2,420,663,000
Natural gas	875,371,000	850,748,000	997,759,000
Other	28,718,000	39,829,000	41,854,000
Total operating revenue	3,401,130,000	3,346,496,000	3,460,276,000
Purchased electricity	652,560,000	638,775,000	590,030,000
Electric generation fuel	282,864,000	204,174,000	206,275,000
Residential exchange	(79,187,000)	(77,454,000)	(75,933,000)
Purchased natural gas	290,976,000	296,699,000	360,009,000
Unrealized (gain) loss on derivative instruments, net	3,574,000	(41,662,000)	30,790,000
Utility operations and maintenance	596,676,000	602,638,000	592,277,000
Non-utility expense and other	47,907,000	54,519,000	53,864,000
Depreciation and amortization	656,323,000	666,432,000	481,969,000
Conservation amortization	96,571,000	111,714,000	121,216,000
Taxes other than income taxes	333,858,000	336,603,000	360,673,000
Total operating expenses	2,882,122,000	2,792,438,000	2,721,170,000
Operating income (loss)	519,008,000	554,058,000	739,106,000
Other income	59,905,000	52,957,000	49,283,000
Other Nonoperating Expense	(9,053,000)	(11,201,000)	(14,076,000)
AFUDC	14,559,000	13,695,000	10,826,000
Interest expense	(356,638,000)	(343,795,000)	(354,802,000)
Income (loss) before income taxes	227,781,000	265,714,000	430,337,000
Income tax (benefit) expense	17,073,000	30,092,000	255,143,000
Net Income (Loss) Attributable to Parent	210,708,000	235,622,000	175,194,000
Subsidiaries [Member]
Electric	2,497,041,000	2,455,919,000	2,420,663,000
Natural gas	875,371,000	850,748,000	997,759,000
Other	28,718,000	39,829,000	41,854,000
Total operating revenue	3,401,130,000	3,346,496,000	3,460,276,000
Purchased electricity	652,560,000	638,775,000	590,030,000
Electric generation fuel	282,864,000	204,174,000	206,275,000
Residential exchange	(79,187,000)	(77,454,000)	(75,933,000)
Purchased natural gas	290,976,000	296,699,000	360,009,000
Unrealized (gain) loss on derivative instruments, net	(3,574,000)	41,662,000	(30,790,000)
Utility operations and maintenance	596,676,000	602,638,000	592,277,000
Non-utility expense and other	44,403,000	51,549,000	52,389,000
Depreciation and amortization	656,220,000	666,324,000	481,955,000
Conservation amortization	96,571,000	111,714,000	121,216,000
Taxes other than income taxes	333,858,000	336,603,000	360,673,000
Total operating expenses	2,878,515,000	2,789,360,000	2,719,681,000
Operating income (loss)	522,615,000	557,136,000	740,595,000
Other income	47,766,000	39,847,000	34,867,000
Other Nonoperating Expense	(9,053,000)	(11,201,000)	(14,104,000)
AFUDC	14,559,000	13,695,000	10,826,000
Interest expense	(243,815,000)	(231,615,000)	(240,144,000)
Income (loss) before income taxes	332,072,000	367,862,000	532,040,000
Income tax (benefit) expense	39,148,000	50,700,000	211,986,000
Net Income (Loss) Attributable to Parent	$ 292,924,000	$ 317,162,000	$ 320,054,000