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