RETIREMENT BENEFITS AND ASSETS HELD IN TRUST Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Retirement Benefits Disclosure
Unrealized (loss) gain on available-for-sale securities	$ (176)	$ (32)	$ 71
Retirement Plan
Retirement Benefits Disclosure
Employer contributions	4,100	3,800	6,900
Retirement Plan | Expectation
Retirement Benefits Disclosure
Expected contribution	2,800
Supplemental Benefit Plans
Retirement Benefits Disclosure
Employer contributions	9,400	5,100	600
Other non-current assets	35,600	26,500
Unrealized (loss) gain on available-for-sale securities	200	100
Defined Benefit Plans
Retirement Benefits Disclosure
Employer contributions	4,102	3,822
Other non-current assets	6,408	4,683
Other Postretirement Benefits Plan
Retirement Benefits Disclosure
Employer contributions	9,122	6,286	1,500
Other Postretirement Benefits Plan | Expectation
Retirement Benefits Disclosure
Expected contribution	9,200
Defined Contribution Plan
Retirement Benefits Disclosure
Plan cost	$ 4,600	$ 4,500	$ 4,500