DERIVATIVE INSTRUMENTS (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 48,468	$ 36,539
Derivative Liability, Fair Value, Gross Asset	$ 72,633	$ 42,693
Basis and Index Related Purchase and Sales Contracts [Member]
Derivative [Line Items]
Outstanding derivative contracts notional amount	77.4
Natural Gas (in MMdts) [Member] | Derivative Transaction Type, Purchase [Member]
Derivative [Line Items]
Outstanding derivative contracts notional amount	17.8
Natural Gas (in MMdts) [Member] | Derivative Transaction Type, Sale [Member]
Derivative [Line Items]
Outstanding derivative contracts notional amount	24.2
Electricity (in mwh) [Member] | Derivative Transaction Type, Purchase [Member]
Derivative [Line Items]
Outstanding derivative contracts notional amount	1,300,000.0
Electricity (in mwh) [Member] | Derivative Transaction Type, Sale [Member]
Derivative [Line Items]
Outstanding derivative contracts notional amount	1,200,000.0