INCOME TAXES (Details) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2019	Jan. 31, 2019
Deferred tax expense, remeasured deferred taxes			$ 2,200,000
Decrease in net deferred tax liability due to enacted tax rate changes			518,700,000
Tax Cuts and Jobs Act, Decrease in Deferred Tax Liability due to Transition Tax			520,900,000
Regulatory Liability, Noncurrent		$ 503,518,000	520,866,000
Reduction to revenues due to the change in tax rates collected in our rates			36,600,000
Deferred Tax Assets, Operating Loss Carryforwards, State and Local			13,600,000
Current income tax provision (benefit)
Federal		24,537,000	0	$ 0
State		5,008,000	289,000	750,000
Total current income tax provision (benefit)		29,545,000	289,000	750,000
Deferred income tax provision
Federal		8,375,000	42,413,000	83,138,000
State		4,932,000	10,829,000	9,255,000
Total deferred income tax provision		13,307,000	53,242,000	92,393,000
Income Tax Reconciliation [Abstract]
Income before income taxes		229,601,000	225,765,000	256,138,000
Provision for federal income taxes		48,215,000	47,411,000	89,648,000
State income taxes, net of federal benefit		9,758,000	8,783,000	6,503,000
Effective Income Tax Rate Reconciliation, nonregulated deferred tax rate decrease		0	74,000	2,162,000
Amortization of EDIT regulatory liability		(12,828,000)	0	0
Effective Income Tax Rate Reconciliation, tax benefit of employee share based compensation		(2,116,000)	(2,770,000)	(5,162,000)
Other, net		(177,000)	33,000	(8,000)
Income tax provision		42,852,000	53,531,000	$ 93,143,000
Deferred tax assets
Deferred tax assets, employee benefits and other accrued liabilities		32,036,000	48,243,000
Deferred tax asset, regulatory adjustments		124,680,000	129,201,000
Net operating loss		752,000	2,778,000
Deferred Tax Assets, Leasing Arrangements		8,599,000	0
Other		2,772,000	34,000
Total deferred tax assets		168,839,000	180,256,000
Deferred tax liabilities
Excess of tax over book depreciation		742,860,000	717,903,000
Purchased-gas cost adjustment		3,556,000	8,981,000
Other regulatory assets and liabilities, net		96,456,000	105,798,000
Deferred Tax Liabilities, Leasing Arrangements		8,599,000	0
Total deferred tax liabilities		851,471,000	832,682,000
Net deferred tax liabilities		$ 682,632,000	652,426,000
Oklahoma Natural Gas tax reform regulatory liability						$ 15,800,000
Kansas Gas Service tax reform regulatory liability					$ 16,600,000
Texas Gas Service one-time refunds related to tax reform	$ 6,600,000
Minimum [Member]
Operating Loss Carryforwards, Expiration Date		Dec. 31, 2025
Maximum [Member]
Operating Loss Carryforwards, Expiration Date		Dec. 31, 2027
Federal income tax rate changes [Member]
Regulatory Liability, Noncurrent		$ 503,518,000	520,866,000
Regulatory Liabilities		$ 513,815,000	$ 551,800,000