INCOME TAXES Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands		Dec. 31, 2015	Dec. 31, 2014
Deferred Tax Assets and Liabilities
Property, plant and equipment		$ (678,567)	$ (560,960)
Goodwill		(147,894)	(133,138)
Pension and postretirement liabilities		18,508	14,196
State income tax NOLs (net of federal benefit)		20,375	20,004
Share-based compensation		13,661	12,211
Other — net		(5,775)	(7,404)
Deferred Tax Liabilities, Net		735,426	642,051
Gross deferred income tax liabilities		(888,727)	(810,141)
Gross deferred income tax assets		153,301	168,090
ADIT Deferral
Deferred Tax Assets and Liabilities
Deferred tax liabilities, regulatory assets	[1]	(15,300)	(15,164)
Net Revenue Accruals and Deferrals, Including Accrued Interest
Deferred Tax Assets and Liabilities
Deferred tax assets, regulatory assets and liabilities	[1]	961	22,047
Refund Liability
Deferred Tax Assets and Liabilities
Deferred tax assets, regulatory assets and liabilities	[1]	70,234	18,878
METC | Regulatory Deferrals
Deferred Tax Assets and Liabilities
Deferred tax liabilities, regulatory assets	[1]	$ (11,629)	$ (12,721)

[1] Described in Note 5.