RETIREMENT BENEFITS AND ASSETS HELD IN TRUST Schedule of Net Defined Benefit Cost Components (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Defined Benefit Plans
Defined Benefit Plan Disclosure
Service cost	$ 1,604	$ 1,624
Interest cost	872	924
Expected return on plan assets	(932)	(960)
Amortization of prior service credit	(4)	(10)
Amortization of unrecognized loss	876	1,061
Net cost	2,416	2,639
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure
Service cost	1,855	2,122
Interest cost	630	619
Expected return on plan assets	(530)	(463)
Amortization of unrecognized loss	0	125
Net cost	$ 1,955	$ 2,403