INCOME TAXES (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Regulatory Liability, Noncurrent	$ 547,563,000	$ 503,518,000
Current income tax provision (benefit)
Federal	20,129,000	24,537,000	$ 0
State	2,965,000	5,008,000	289,000
Total current income tax provision (benefit)	23,094,000	29,545,000	289,000
Deferred income tax provision
Federal	10,757,000	8,375,000	42,413,000
State	7,728,000	4,932,000	10,829,000
Total deferred income tax provision	18,485,000	13,307,000	53,242,000
Income Tax Reconciliation [Abstract]
Income before income taxes	$ 237,991,000	$ 229,601,000	$ 225,765,000
Federal Statutory Income Tax Rate	21.00%	21.00%	21.00%
Provision for federal income taxes	$ 49,978,000	$ 48,215,000	$ 47,411,000
State income taxes, net of federal benefit	10,693,000	9,758,000	8,783,000
Effective Income Tax Rate Reconciliation, nonregulated deferred tax rate decrease	0	0	74,000
Amortization of EDIT regulatory liability	(17,031,000)	(12,828,000)	0
Effective Income Tax Rate Reconciliation, tax benefit of employee share based compensation	(1,489,000)	(2,116,000)	(2,770,000)
Other, net	(572,000)	(177,000)	33,000
Income tax provision	41,579,000	42,852,000	$ 53,531,000
Deferred tax assets
Deferred tax assets, employee benefits and other accrued liabilities	28,127,000	32,036,000
Deferred tax asset, regulatory adjustments	121,738,000	124,680,000
Net operating loss	0	752,000
Deferred Tax Assets, Leasing Arrangements	9,319,000	8,599,000
Other	4,790,000	2,772,000
Total deferred tax assets	163,974,000	168,839,000
Deferred tax liabilities
Excess of tax over book depreciation	717,492,000	742,860,000
Purchased-gas cost adjustment	5,240,000	3,556,000
Other regulatory assets and liabilities, net	88,260,000	96,456,000
Deferred Tax Liabilities, Leasing Arrangements	9,788,000	8,599,000
Total deferred tax liabilities	820,780,000	851,471,000
Net deferred tax liabilities	656,806,000	682,632,000
Kansas Gas Service tax reform regulatory liability	81,500,000
Reduction to base rates due to Kansas Gas Service tax reform regulatory liability	4,900,000
Federal income tax rate changes [Member]
Regulatory Liability, Noncurrent	547,563,000	503,518,000
Regulatory Liabilities	$ 547,563,000	$ 513,815,000