RETIREMENT BENEFITS AND ASSETS HELD IN TRUST Schedule of Net Defined Benefit Cost Components (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Retirement Plan Benefits
Defined Benefit Plan Disclosure
Service cost	$ 1,315	$ 1,040	$ 3,946	$ 3,120
Interest cost	698	647	2,094	1,942
Expected return on plan assets	(717)	(569)	(2,151)	(1,708)
Amortization of prior service cost	(10)	(11)	(31)	(32)
Amortization of unrecognized loss	679	868	2,036	2,603
Net cost	1,965	1,975	5,894	5,925
Other Postretirement Benefits
Defined Benefit Plan Disclosure
Service cost	1,443	1,358	4,330	4,075
Interest cost	390	388	1,171	1,164
Expected return on plan assets	(353)	(254)	(1,061)	(762)
Amortization of prior service cost	0	31	0	93
Amortization of unrecognized loss	55	134	165	401
Net cost	$ 1,535	$ 1,657	$ 4,605	$ 4,971