INCOME TAXES (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Deferred Tax Assets, Operating Loss Carryforwards, Domestic	$ 63,000
Operating Loss Carryforwards, Tax Benefit	11,000
Deferred Tax Assets, Operating Loss Carryforwards, State and Local	21,000
Income taxes receivable	1,397	$ 38,877
Current income tax provision (benefit)
Federal	(2,016)	7,135	$ 17,006
State	471	2,055	1,397
Total current income tax provision (benefit)	(1,545)	9,190	18,403
Deferred income tax provision
Federal	76,247	56,440	42,024
State	10,541	7,349	7,911
Total deferred income tax provision	86,788	63,789	49,935
Income Tax Reconciliation [Abstract]
Income before income taxes	$ 225,338	$ 192,009	$ 178,128
Federal statutory income tax rate	35.00%	35.00%	35.00%
Provision for federal income taxes	$ 78,868	$ 67,203	$ 62,345
State income taxes, net of federal benefit	7,158	6,114	6,051
Other, net	(783)	(338)	(58)
Income tax provision	85,243	72,979	$ 68,338
Deferred tax assets
Deferred tax assets, employee benefits and other accrued liabilities	123,333	110,148
Net operating loss	23,094	0
Other	5,716	7,848
Total deferred tax assets	152,143	117,996
Deferred tax liabilities
Excess of tax over book depreciation	990,682	897,667
Purchased-gas cost adjustment	13,822	3,999
Other regulatory assets and liabilities, net	186,207	168,115
Total deferred tax liabilities	1,190,711	1,069,781
Net deferred tax liabilities	$ 1,038,568	$ 951,785
Minimum [Member]
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2024
Maximum [Member]
Operating Loss Carryforwards, Expiration Date	Dec. 31, 2036