RETIREMENT BENEFITS AND ASSETS HELD IN TRUST Schedule of Net Defined Benefit Cost Components (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Defined Benefit Plans
Defined Benefit Plan Disclosure
Service cost	$ 1,604	$ 1,624	$ 3,208	$ 3,248
Interest cost	872	924	1,744	1,848
Expected return on plan assets	(932)	(959)	(1,864)	(1,919)
Amortization of prior service credit	(4)	(11)	(8)	(21)
Amortization of unrecognized loss	876	1,060	1,752	2,121
Net cost	2,416	2,638	4,832	5,277
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure
Service cost	1,855	2,121	3,710	4,243
Interest cost	630	619	1,260	1,238
Expected return on plan assets	(531)	(463)	(1,061)	(926)
Amortization of unrecognized loss	0	125	0	250
Net cost	$ 1,954	$ 2,402	$ 3,909	$ 4,805