FAIR VALUE - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Assets
Risk Management Asset	$ 7.2		$ 7.2		$ 2.4
Liabilities
Risk Management Liability	3.5		3.5		3.1
Transfers Between the Levels of the Fair Value Hierarchy
Fair Value Transfers, Amount	0	0	0	0
Fair value measurements on a recurring basis | Level 1
Assets
Risk Management Asset	0.8		0.8		0.7
Liabilities
Risk Management Liability	0.7		0.7		0.1
Fair value measurements on a recurring basis | Level 2
Assets
Risk Management Asset	0.6		0.6		0
Liabilities
Risk Management Liability	0		0		0
Fair value measurements on a recurring basis | Level 3
Assets
Risk Management Asset	5.8		5.8		1.7
Liabilities
Risk Management Liability	2.8		2.8		3.0
Fair value measurements on a recurring basis | Total
Assets
Risk Management Asset	7.2		7.2		2.4
Liabilities
Risk Management Liability	3.5		3.5		3.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 1
Assets
Risk Management Asset	0.8		0.8		0.6
Liabilities
Risk Management Liability	0.4		0.4		0.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 2
Assets
Risk Management Asset	0.6		0.6		0
Liabilities
Risk Management Liability	0		0		0
Fair value measurements on a recurring basis | Natural gas contracts | Level 3
Assets
Risk Management Asset	0		0		0
Liabilities
Risk Management Liability	0		0		0
Fair value measurements on a recurring basis | Natural gas contracts | Total
Assets
Risk Management Asset	1.4		1.4		0.6
Liabilities
Risk Management Liability	0.4		0.4		0.1
Fair value measurements on a recurring basis | FTRs | Level 1
Assets
Risk Management Asset	0		0		0
Liabilities
Risk Management Liability	0		0		0
Fair value measurements on a recurring basis | FTRs | Level 2
Assets
Risk Management Asset	0		0		0
Liabilities
Risk Management Liability	0		0		0
Fair value measurements on a recurring basis | FTRs | Level 3
Assets
Risk Management Asset	3.4		3.4		1.5
Liabilities
Risk Management Liability	0.4		0.4		0.3
Fair value measurements on a recurring basis | FTRs | Total
Assets
Risk Management Asset	3.4		3.4		1.5
Liabilities
Risk Management Liability	0.4		0.4		0.3
Fair value measurements on a recurring basis | Petroleum product contracts | Level 1
Assets
Risk Management Asset					0.1
Liabilities
Risk Management Liability	0.3		0.3
Fair value measurements on a recurring basis | Petroleum product contracts | Level 2
Assets
Risk Management Asset					0
Liabilities
Risk Management Liability	0		0
Fair value measurements on a recurring basis | Petroleum product contracts | Level 3
Assets
Risk Management Asset					0
Liabilities
Risk Management Liability	0		0
Fair value measurements on a recurring basis | Petroleum product contracts | Total
Assets
Risk Management Asset					0.1
Liabilities
Risk Management Liability	0.3		0.3
Fair value measurements on a recurring basis | Coal contracts | Level 1
Assets
Risk Management Asset	0		0		0
Liabilities
Risk Management Liability	0		0		0
Fair value measurements on a recurring basis | Coal contracts | Level 2
Assets
Risk Management Asset	0		0		0
Liabilities
Risk Management Liability	0		0		0
Fair value measurements on a recurring basis | Coal contracts | Level 3
Assets
Risk Management Asset	2.4		2.4		0.2
Liabilities
Risk Management Liability	2.4		2.4		2.7
Fair value measurements on a recurring basis | Coal contracts | Total
Assets
Risk Management Asset	2.4		2.4		0.2
Liabilities
Risk Management Liability	$ 2.4		$ 2.4		$ 2.7