DERIVATIVE INSTRUMENTS - Narrative (Details) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2005 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) MMcfe	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Derivative [Line Items]
Unrealized gains (losses) included in its BGSS related to open financial contracts | $		$ 0.8		$ 0.8		$ 4.7
Initial cost of Treasury Locks | $	$ 1.4
Amortization period of Treasury Locks	30 years
Unamortized balance of Treasury Locks | $		0.9		0.9		$ 0.9
Net realized gains (losses), derivative instruments, energy-related contracts | $		$ 0.9	$ (1.6)	$ (3.5)	$ (6.4)
Basis and Index Related Purchase and Sales Contracts
Derivative [Line Items]
Notional amount (in mmcfe)				4,400
Basis and Index Related Purchase Contracts
Derivative [Line Items]
Notional amount (in mmcfe)				4,600
Basis and Index Related Sales Contracts
Derivative [Line Items]
Notional amount (in mmcfe)				9,000
Derivative Transaction Type, Purchase
Derivative [Line Items]
Notional amount (in mmcfe)				8,700
Derivative Transaction Type, Sale
Derivative [Line Items]
Notional amount (in mmcfe)				600