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, 2016	Dec. 31, 2015
Deferred credits and other liabilities
Transmission easements			$ 20,562	$ 21,416
Customer deposits			3,338	6,116
Unearned revenue			4,000	4,803
Other			22,743	25,088
Total other deferred credits and other liabilities			66,164	$ 57,423
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			$ 34,500