RETIREMENT BENEFITS AND ASSETS HELD IN TRUST Assets Measured at Fair Value Subject to Three-Tier Hierarchy (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure
Fair value, level transfers	$ 0	$ 0
Retirement Plan
Defined Benefit Plan Disclosure
Fair value, level transfers	0	0
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure
Fair value, level transfers	0	0
Fair value amount	41,668,000	32,397,000	$ 24,004,000
Recurring Basis | Retirement Plan | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value amount	58,127,000	52,649,000
Recurring Basis | Retirement Plan | Fair Value, Inputs, Level 1 | U.S. equity securities
Defined Benefit Plan Disclosure
Fair value amount	23,427,000	23,770,000
Recurring Basis | Retirement Plan | Fair Value, Inputs, Level 1 | International equity securities
Defined Benefit Plan Disclosure
Fair value amount	5,409,000	5,096,000
Recurring Basis | Retirement Plan | Fair Value, Inputs, Level 1 | Fixed income securities
Defined Benefit Plan Disclosure
Fair value amount	29,291,000	23,783,000
Recurring Basis | Retirement Plan | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value amount		3,741,000
Recurring Basis | Retirement Plan | Fair Value, Inputs, Level 2 | Guaranteed deposit fund
Defined Benefit Plan Disclosure
Fair value amount		3,741,000
Recurring Basis | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure
Fair value amount	41,668,000	31,744,000
Recurring Basis | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1 | Cash and cash equivalents
Defined Benefit Plan Disclosure
Fair value amount	30,000	5,099,000
Recurring Basis | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1 | U.S. equity securities
Defined Benefit Plan Disclosure
Fair value amount	19,981,000	13,070,000
Recurring Basis | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1 | International equity securities
Defined Benefit Plan Disclosure
Fair value amount	863,000	785,000
Recurring Basis | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 1 | Fixed income securities
Defined Benefit Plan Disclosure
Fair value amount	$ 20,794,000	12,790,000
Recurring Basis | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure
Fair value amount		653,000
Recurring Basis | Other Postretirement Benefits Plan | Fair Value, Inputs, Level 2 | Guaranteed deposit fund
Defined Benefit Plan Disclosure
Fair value amount		$ 653,000