IMMATERIAL RESTATEMENT OF PRIOR PERIOD FINANCIAL STATEMENTS (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Operating revenue	$ 167,417	$ 221,288	$ 174,938	$ 134,553	$ 164,296	$ 214,037	$ 173,438	$ 124,342	$ 698,196	$ 676,113	$ 609,370
Operating expenses:
Income taxes									18,589	35,279	29,235
Total operating expenses									587,656	569,030	526,734
Net operating income	$ 26,536	$ 47,329	$ 26,839	$ 9,836	$ 23,077	$ 43,902	$ 28,111	$ 11,993	110,540	107,083	82,636
Net income									$ 65,584	$ 72,940	$ 48,675
Earnings Per Share [Abstract]
Basic (in dollars per share)									$ 1.36	$ 1.52	$ 1.02
Diluted (in dollars per share)	$ 0.32	$ 0.75	$ 0.31	$ (0.02)	$ 0.31	$ 0.74	$ 0.42	$ 0.05	$ 1.36	$ 1.52	$ 1.01
Other assets:
Regulatory assets	$ 353,569				$ 405,482				$ 353,569	$ 405,482
Total other assets	416,279				468,872				416,279	468,872
TOTAL ASSETS	2,837,704				2,744,710				2,837,704	2,744,710
Capitalization:
Retained earnings	392,053				362,512				392,053	362,512
Total common stockholders' equity	730,157				699,221				730,157	699,221	$ 659,471	$ 642,155
Total capitalization	1,440,184				1,215,014				1,440,184	1,215,014
Deferred income taxes	213,033				194,617				213,033	194,617
Regulatory Liabilities	211,275				176,611				211,275	176,611
TOTAL CAPITALIZATION AND LIABILITIES	$ 2,837,704				2,744,710				2,837,704	2,744,710
Operating activities:
Net income									65,584	72,940	48,675
Other changes in noncurrent assets and liabilities									(12,644)	13,752	6,906
Net cash provided by operating activities									$ 179,019	147,842	160,444
As Adjusted
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Operating revenue										666,890
Operating expenses:
Income taxes										31,815
Total operating expenses										565,566
Net operating income										101,324
Net income										$ 67,181
Earnings Per Share [Abstract]
Basic (in dollars per share)										$ 1.40
Diluted (in dollars per share)										$ 1.40
Operating activities:
Net income										$ 67,181
Corrections
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Operating revenue										9,223
Operating expenses:
Income taxes										3,464
Total operating expenses										3,464
Net operating income										5,759
Net income										$ 5,759
Earnings Per Share [Abstract]
Basic (in dollars per share)										$ 0.12
Diluted (in dollars per share)										$ 0.12
Other assets:
Regulatory assets					4,335					$ 4,335
Total other assets					4,335					4,335
TOTAL ASSETS					4,335					4,335
Capitalization:
Retained earnings					5,759					5,759
Total common stockholders' equity					5,759					5,759
Total capitalization					5,759					5,759
Deferred income taxes					1,671					1,671
Regulatory Liabilities					(3,095)					(3,095)
TOTAL CAPITALIZATION AND LIABILITIES					4,335					4,335
Operating activities:
Net income										5,759
Other changes in noncurrent assets and liabilities										(5,759)
Net cash provided by operating activities										0
As Previously Reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Operating revenue		$ 218,983	$ 172,632	$ 132,247	161,991	$ 211,731	$ 171,132	$ 122,036		666,890
Operating expenses:
Income taxes										28,928	24,804
Total operating expenses										572,267	533,176
Net operating income		$ 45,548	$ 25,056	$ 8,053	$ 21,638	$ 42,462	$ 26,671	$ 10,553		94,623	$ 76,194
Net income										$ 67,181
Earnings Per Share [Abstract]
Basic (in dollars per share)										$ 1.40
Diluted (in dollars per share)		$ 0.72	$ 0.27	$ (0.05)	$ 0.29	$ 0.70	$ 0.39	$ 0.02		$ 1.40
Other assets:
Regulatory assets					$ 401,147					$ 401,147
Total other assets					464,537					464,537
TOTAL ASSETS					2,740,375					2,740,375
Capitalization:
Retained earnings					356,753					356,753
Total common stockholders' equity					693,462					693,462
Total capitalization					1,209,255					1,209,255
Deferred income taxes					192,946					192,946
Regulatory Liabilities					179,706					179,706
TOTAL CAPITALIZATION AND LIABILITIES					$ 2,740,375					2,740,375
Operating activities:
Net income										67,181
Other changes in noncurrent assets and liabilities										19,511
Net cash provided by operating activities										147,842
Adjustments
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Operating revenue										0
Operating expenses:
Income taxes										2,887
Total operating expenses										(6,701)
Net operating income										6,701
Net income										$ 0
Earnings Per Share [Abstract]
Basic (in dollars per share)										$ 0.00
Diluted (in dollars per share)										$ 0.00
Operating activities:
Net income										$ 0