SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Deferred Credits and Other Liabilities (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
	Jan. 12, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Deferred credits and other liabilities
Transmission easements			$ 21,337	$ 20,562
Unearned revenue			3,735	4,000
Customer deposits			2,898	3,338
Other			25,426	22,743
Total other deferred credits and other liabilities			53,396	66,164
Transmission Services Agreement | TEP
Deferred credits and other liabilities
Refund received on transmission services agreements				15,500
Payment for transmission services settlement	$ 7,750
Reduction in transmission expense		$ 7,750
TEP transmission refund				$ 15,521
Transmission Right of Way Easements
Deferred credits and other liabilities
Total due for easement right of way			$ 33,400