DERIVATIVE INSTRUMENTS (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 40,544	$ 36,539
Derivative Liability, Fair Value, Gross Asset	$ 64,056	$ 42,693
Basis and Index Related Purchase and Sales Contracts [Member]
Derivative [Line Items]
Outstanding derivative contracts notional amount	36.2
Natural Gas (in MMdts) [Member] | Derivative Transaction Type, Purchase [Member]
Derivative [Line Items]
Outstanding derivative contracts notional amount	17.9
Natural Gas (in MMdts) [Member] | Derivative Transaction Type, Sale [Member]
Derivative [Line Items]
Outstanding derivative contracts notional amount	20.2
Electricity (in mwh) [Member] | Derivative Transaction Type, Purchase [Member]
Derivative [Line Items]
Outstanding derivative contracts notional amount	1,000,000.0
Electricity (in mwh) [Member] | Derivative Transaction Type, Sale [Member]
Derivative [Line Items]
Outstanding derivative contracts notional amount	1,000,000.0