FAIR VALUE - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Assets
Risk Management Asset	$ 8.2		$ 8.2		$ 2.4
Liabilities
Risk Management Liability	2.7		2.7		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	0.2		0.2		0.1
Fair value measurements on a recurring basis | Level 2
Assets
Risk Management Asset	0		0		0
Liabilities
Risk Management Liability	0		0		0
Fair value measurements on a recurring basis | Level 3
Assets
Risk Management Asset	7.4		7.4		1.7
Liabilities
Risk Management Liability	2.5		2.5		3.0
Fair value measurements on a recurring basis | Total
Assets
Risk Management Asset	8.2		8.2		2.4
Liabilities
Risk Management Liability	2.7		2.7		3.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 1
Assets
Risk Management Asset	0.6		0.6		0.6
Liabilities
Risk Management Liability	0.2		0.2		0.1
Fair value measurements on a recurring basis | Natural gas contracts | Level 2
Assets
Risk Management Asset	0		0		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	0.6		0.6		0.6
Liabilities
Risk Management Liability	0.2		0.2		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	4.7		4.7		1.5
Liabilities
Risk Management Liability	0.7		0.7		0.3
Fair value measurements on a recurring basis | FTRs | Total
Assets
Risk Management Asset	4.7		4.7		1.5
Liabilities
Risk Management Liability	0.7		0.7		0.3
Fair value measurements on a recurring basis | Petroleum product contracts | Level 1
Assets
Risk Management Asset	0.2		0.2		0.1
Fair value measurements on a recurring basis | Petroleum product contracts | Level 2
Assets
Risk Management Asset	0		0		0
Fair value measurements on a recurring basis | Petroleum product contracts | Level 3
Assets
Risk Management Asset	0		0		0
Fair value measurements on a recurring basis | Petroleum product contracts | Total
Assets
Risk Management Asset	0.2		0.2		0.1
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	2.7		2.7		0.2
Liabilities
Risk Management Liability	1.8		1.8		2.7
Fair value measurements on a recurring basis | Coal contracts | Total
Assets
Risk Management Asset	2.7		2.7		0.2
Liabilities
Risk Management Liability	$ 1.8		$ 1.8		$ 2.7