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FAIR VALUE MEASUREMENTS - ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Assets
Derivative asset	$ 5.4		$ 12.4
Liabilities
Derivative liability	5.1		11.6
Fair value measurements on a recurring basis
Assets
Derivative asset	5.4		12.4
Investments held in rabbi trust	103.6		120.7
Liabilities
Derivative liability	5.1		11.6
Net Unrealized Gain (Loss) on Investments
Net unrealized gains (losses) included in earnings related to investments held in rabbi trust	(3.1)	$ 5.2
Fair value measurements on a recurring basis \| Level 1
Assets
Derivative asset	2.3		3.0
Investments held in rabbi trust	103.6		120.7
Liabilities
Derivative liability	3.4		7.0
Fair value measurements on a recurring basis \| Level 2
Assets
Derivative asset	1.6		5.0
Investments held in rabbi trust	0.0		0.0
Liabilities
Derivative liability	1.7		4.6
Fair value measurements on a recurring basis \| Level 3
Assets
Derivative asset	1.5		4.4
Investments held in rabbi trust	0.0		0.0
Liabilities
Derivative liability	0.0		0.0
Fair value measurements on a recurring basis \| Natural gas contracts
Assets
Derivative asset	2.2		5.7
Liabilities
Derivative liability	4.8		10.8
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 1
Assets
Derivative asset	1.7		1.8
Liabilities
Derivative liability	3.4		7.0
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 2
Assets
Derivative asset	0.5		3.9
Liabilities
Derivative liability	1.4		3.8
Fair value measurements on a recurring basis \| Natural gas contracts \| Level 3
Assets
Derivative asset	0.0		0.0
Liabilities
Derivative liability	0.0		0.0
Fair value measurements on a recurring basis \| Petroleum products contracts
Assets
Derivative asset	0.6		1.2
Fair value measurements on a recurring basis \| Petroleum products contracts \| Level 1
Assets
Derivative asset	0.6		1.2
Fair value measurements on a recurring basis \| Petroleum products contracts \| Level 2
Assets
Derivative asset	0.0		0.0
Fair value measurements on a recurring basis \| Petroleum products contracts \| Level 3
Assets
Derivative asset	0.0		0.0
Fair value measurements on a recurring basis \| FTRs
Assets
Derivative asset	1.5		4.4
Fair value measurements on a recurring basis \| FTRs \| Level 1
Assets
Derivative asset	0.0		0.0
Fair value measurements on a recurring basis \| FTRs \| Level 2
Assets
Derivative asset	0.0		0.0
Fair value measurements on a recurring basis \| FTRs \| Level 3
Assets
Derivative asset	1.5		4.4
Fair value measurements on a recurring basis \| Coal contracts
Assets
Derivative asset	1.1		1.1
Liabilities
Derivative liability	0.3		0.8
Fair value measurements on a recurring basis \| Coal contracts \| Level 1
Assets
Derivative asset	0.0		0.0
Liabilities
Derivative liability	0.0		0.0
Fair value measurements on a recurring basis \| Coal contracts \| Level 2
Assets
Derivative asset	1.1		1.1
Liabilities
Derivative liability	0.3		0.8
Fair value measurements on a recurring basis \| Coal contracts \| Level 3
Assets
Derivative asset	0.0		0.0
Liabilities
Derivative liability	$ 0.0		$ 0.0