PENSIONS AND OTHER POSTRETIREMENT BENEFITS - Summary of Investments in the RIGP (Details) - RIGP - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension and other postretirement benefits
Fair value of plan assets	$ 5,544	$ 6,743	$ 6,003
Level 1
Pension and other postretirement benefits
Fair value of plan assets	3,224	3,767
Level 3
Pension and other postretirement benefits
Fair value of plan assets	2,320	2,976	$ 3,303
Mutual fund - fixed-income securities | Level 1
Pension and other postretirement benefits
Fair value of plan assets	2,759	3,107
Mutual fund - fixed-income securities | Level 3
Pension and other postretirement benefits
Fair value of plan assets	0	0
Mutual fund - money market | Level 1
Pension and other postretirement benefits
Fair value of plan assets	465	660
Mutual fund - money market | Level 3
Pension and other postretirement benefits
Fair value of plan assets	0	0
Coal lease | Level 1
Pension and other postretirement benefits
Fair value of plan assets	0	0
Coal lease | Level 3
Pension and other postretirement benefits
Fair value of plan assets	$ 2,320	$ 2,976