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COMMON STOCK EQUITY - CHANGE IN COMMON STOCK EQUITY (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Oct. 01, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]      
Balance as of beginning of period (shares) 88,292,956    
Balance as of beginning of period $ 1,418,978 $ 1,236,643  
Net income 86,248 123,699  
Other comprehensive income 234 (5,204)  
Common stock issued:      
Incentive plan $ 2,134 $ 15,404  
Dividend reinvestment plan (in shares) 82,000 90,000  
Dividend reinvestment plan [1] $ 3,692 $ 3,799  
Waiver discount (in shares) 168,000 554,000  
Waiver discount $ 7,964 $ 22,690  
Cash dividend declared (25,938) (23,831)  
Treasury stock and other $ 1,504 25,430  
Balance as of end of period (shares) 88,680,174    
Balance as of end of period $ 1,497,050 $ 1,347,770  
Cash dividend declared per share (usd per share) $ 0.2925 $ 0.2725  
Common Stock      
Increase (Decrease) in Stockholders' Equity [Roll Forward]      
Balance as of beginning of period (shares) 88,293,000 86,556,000  
Balance as of beginning of period $ 226,196 $ 222,258  
Common stock issued:      
Incentive plan (in shares) 137,000 525,000  
Incentive plan $ 343 $ 1,453  
Dividend reinvestment plan (in shares) [1] 82,000 90,000  
Waiver discount (in shares) 168,000 554,000  
Waiver discount $ 0 $ 1,384  
Treasury stock and other (shares) 0 (250,000)  
Balance as of end of period (shares) 88,680,000 87,475,000  
Balance as of end of period $ 226,539 $ 225,095  
Premium on Common Stock      
Increase (Decrease) in Stockholders' Equity [Roll Forward]      
Balance as of beginning of period 274,748 219,696  
Common stock issued:      
Incentive plan 1,791 13,951  
Dividend reinvestment plan [1] 454 245  
Waiver discount 1,293 21,306  
Treasury stock and other 0 56  
Balance as of end of period 278,286 255,142  
Accumulated Other Comprehensive (Loss) Income      
Increase (Decrease) in Stockholders' Equity [Roll Forward]      
Balance as of beginning of period (12,610) (3,256)  
Other comprehensive income 234 (5,204)  
Common stock issued:      
Adoption of ASU     $ (3,446)
Balance as of end of period (15,822) (8,460)  
Treasury Stock And Other      
Increase (Decrease) in Stockholders' Equity [Roll Forward]      
Balance as of beginning of period (76,473) (70,039)  
Common stock issued:      
Dividend reinvestment plan [1] 3,238 3,554  
Waiver discount 6,671    
Treasury stock and other 1,504 25,374  
Balance as of end of period (65,060) (91,859)  
Retained Earnings      
Increase (Decrease) in Stockholders' Equity [Roll Forward]      
Balance as of beginning of period 1,007,117 867,984  
Net income 86,248 123,699  
Common stock issued:      
Cash dividend declared (25,938) (23,831)  
Balance as of end of period $ 1,073,107 $ 967,852  
Accounting Standards Update 2016-01      
Common stock issued:      
Adoption of ASU [2]     0
Accounting Standards Update 2016-01 | Accumulated Other Comprehensive (Loss) Income      
Common stock issued:      
Adoption of ASU [2]     (3,446)
Accounting Standards Update 2016-01 | Retained Earnings      
Common stock issued:      
Adoption of ASU [2]     3,446
Accounting Standards Update 2017-05      
Common stock issued:      
Adoption of ASU [2]     4,970
Accounting Standards Update 2014-09/ASC 606      
Common stock issued:      
Adoption of ASU [2]     (2,736)
Accounting Standards Update 2014-09/ASC 606 | Retained Earnings      
Common stock issued:      
Adoption of ASU [2]     $ (2,736)
[1] Shares sold through the DRP are issued from treasury stock at average cost, which may differ from the actual market price paid.
[2] See Note 2. Summary of Significant Accounting Policies - Recently Adopted Updates to the Accounting Standards Codification section for more details.