SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Deferred Charges (Details) - USD ($) $ in Thousands	1 Months Ended
	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Other deferred charges
Regulatory liabilities		$ 122,169	$ 137,369
Preliminary surveys and investigations		21,261	20,660
Advances to operating agents of jointly owned facilities		3,917	13,161
Interest rate swap			8,576
Operating lease right-of-use assets		7,622
Other		9,872	4,056
Total other deferred charges		$ 42,672	46,453
Interest rate swap - unrealized gain
Other deferred charges
Regulatory liabilities			$ 8,576
Gain recognized on termination of interest rate swap agreement	$ 0