RETIREMENT BENEFITS AND ASSETS HELD IN TRUST Defined Benefit Plans Disclosures (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Retirement Plan
Change in Plans' Assets:
Employer contributions	$ 4,100	$ 3,800	$ 6,900
Accumulated benefit obligation	(49,169)	(48,571)
Accumulated benefit obligation
Accumulated benefit obligation	(49,169)	(48,571)
Supplemental Benefit Plans
Change in Plans' Assets:
Employer contributions	9,400	5,100	600
Accumulated benefit obligation	(40,830)	(35,962)
Funded Status:
Other non-current assets	35,600	26,500
Accumulated benefit obligation
Accumulated benefit obligation	(40,830)	(35,962)
Defined Benefit Plans
Change in Benefit Obligation:
Beginning obligation	(95,740)	(73,468)
Service cost	(6,496)	(5,066)	(5,261)
Interest cost	(3,696)	(3,603)	(2,792)
Actuarial net gain (loss)	5,869	(14,937)
Ending obligation	(97,188)	(95,740)	(73,468)
Change in Plans' Assets:
Beginning plan assets at fair value	56,390	48,894
Actual return on plan assets	(129)	4,851
Employer contributions	4,102	3,822
Ending plan assets at fair value	58,127	56,390	48,894
Funded status, underfunded	(39,061)	(39,350)
Accumulated benefit obligation	(89,999)	(84,533)
Funded Status:
Accrued pension liabilities	(45,322)	(44,033)
Other non-current assets	6,408	4,683
Other current liabilities	(147)	0
Total	(39,061)	(39,350)
Accumulated benefit obligation
Accumulated benefit obligation	(89,999)	(84,533)
Unrecognized Amounts in Other Regulatory Assets:
Net actuarial loss	18,724	24,868
Prior service credit	66	25
Total	18,790	24,893
Other Postretirement Benefits Plan
Change in Benefit Obligation:
Beginning obligation	(57,927)	(42,706)
Service cost	(8,486)	(5,846)	(5,774)
Interest cost	(2,477)	(1,991)	(1,562)
Actuarial net gain (loss)	10,265	(7,695)
Ending obligation	(57,955)	(57,927)	(42,706)
Change in Plans' Assets:
Beginning plan assets at fair value	32,397	24,004
Actual return on plan assets	155	2,107
Employer contributions	9,122	6,286	1,500
Employer provided retiree premiums	662	311
Ending plan assets at fair value	41,668	32,397	$ 24,004
Funded status, underfunded	(16,287)	(25,530)
Funded Status:
Accrued pension liabilities	(16,287)	(25,530)
Total	(16,287)	(25,530)
Unrecognized Amounts in Other Regulatory Assets:
Net actuarial loss	191	9,258
Total	191	9,258
Change in Benefit Obligation | Defined Benefit Plans
Change in Benefit Obligation:
Benefits paid	(2,747)	(1,334)
Change in Plans' Assets:
Benefits paid	(2,747)	(1,334)
Other	(128)	0
Change in Benefit Obligation | Other Postretirement Benefits Plan
Change in Benefit Obligation:
Benefits paid	(662)	(311)
Change in Plans' Assets:
Benefits paid	(662)	(311)
Other	(8)	0
Change in Plan Assets | Defined Benefit Plans
Change in Benefit Obligation:
Benefits paid	(2,108)	(1,177)
Change in Plans' Assets:
Benefits paid	(2,108)	(1,177)
Other	(128)	0
Change in Plan Assets | Other Postretirement Benefits Plan
Change in Benefit Obligation:
Benefits paid	(662)	(311)
Change in Plans' Assets:
Benefits paid	(662)	(311)
Other	$ (6)	$ 0