INCOME TAXES (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Decrease in net deferred tax liability due to enacted tax rate changes	$ 518,700,000
Regulatory Liability, Noncurrent	520,866,000	$ 519,421,000
Deferred tax expense, remeasured deferred taxes	2,200,000
Impact to cash flows attributable to adjustments to net deferred tax liability for enacted tax rate changes	0.0
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	50,200,000
Current income tax provision (benefit)
Federal	0	0	$ (2,016,000)
State	289,000	750,000	471,000
Total current income tax provision (benefit)	289,000	750,000	(1,545,000)
Deferred income tax provision
Federal	42,413,000	83,138,000	76,247,000
State	10,829,000	9,255,000	10,541,000
Total deferred income tax provision	53,242,000	92,393,000	86,788,000
Income Tax Reconciliation [Abstract]
Income before income taxes	$ 225,765,000	$ 256,138,000	$ 225,338,000
Federal statutory income tax rate	21.00%	35.00%	35.00%
Provision for federal income taxes	$ 47,411,000	$ 89,648,000	$ 78,868,000
State income taxes, net of federal benefit	8,783,000	6,503,000	7,158,000
Effective Income Tax Rate Reconciliation, nonregulated deferred tax rate decrease	74,000	2,162,000	0
Effective Income Tax Rate Reconciliation, tax benefit of employee share based compensation	(2,770,000)	(5,162,000)	0
Other, net	33,000	(8,000)	(783,000)
Income tax provision	53,531,000	93,143,000	$ 85,243,000
Deferred tax assets
Deferred tax assets, employee benefits and other accrued liabilities	48,243,000	40,277,000
Deferred tax asset, regulatory adjustments	129,201,000	129,421,000
Net operating loss	2,778,000	24,712,000
Other	34,000	2,984,000
Total deferred tax assets	180,256,000	197,394,000
Deferred tax liabilities
Excess of tax over book depreciation	717,903,000	677,249,000
Purchased-gas cost adjustment	8,981,000	13,805,000
Other regulatory assets and liabilities, net	105,798,000	106,285,000
Total deferred tax liabilities	832,682,000	797,339,000
Net deferred tax liabilities	$ 652,426,000	599,945,000
NOL Limitation	80.00%
Oklahoma Natural Gas tax reform regulatory liability	$ 15,800,000
Kansas Gas Service tax reform regulatory liability	14,200,000
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, 2037
Federal income tax rate changes [Member]
Regulatory Liability, Noncurrent	$ 520,866,000	519,421,000
Regulatory Liabilities	$ 551,800,000	$ 519,421,000