QUARTERLY FINANCIAL INFORMATION (UNAUDITED) (Tables)

12 Months Ended

Dec. 31, 2014

Quarterly Financial Information Disclosure [Abstract]

Schedule of quarterly financial information (unaudited)

(Millions)	First Quarter		Second Quarter		Third Quarter		Fourth Quarter		Total
2014
Operating revenues	$	555.7	$	358.8	$	370.4	$	397.4	$	1,682.3
Operating income	87.6		36.5		77.9		56.4		258.4
Net income attributed to common shareholder	50.3		17.1		42.2		28.0		$	137.6
2013
Operating revenues	$	433.4	$	367.8	$	371.9	$	406.2	$	1,579.3
Operating income	77.4		46.9		65.2		50.5		240.0
Net income attributed to common shareholder	44.6		25.9		37.0		27.3		134.8