SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Deferred Charges (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Other deferred charges
Preliminary surveys and investigations	$ 107,146	$ 131,693
Advances to operating agents of jointly owned facilities	11,537	20,567
Other	3,852	3,940
Total other deferred charges	122,535	$ 156,200
Holcomb Station Project
Other deferred charges
Preliminary surveys and investigations	86,700
Eastern Plains Transmission Project
Other deferred charges
Preliminary surveys and investigations	$ 28,500