COMMON EQUITY - EARNINGS PER SHARE (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator:
Net income from continuing operations	$ 278.1	$ 267.5	$ 238.9
Discontinued operations, net of tax	1.8	87.3	45.4
Preferred stock dividends of subsidiary	(3.1)	(3.1)	(3.1)
Noncontrolling interest in subsidiaries	0.1	0.1	0.2
Net income attributed to common shareholders	276.9	351.8	281.4
Effect of dilutive securities - Deferred compensation	0	(0.1)	0
Net income attributed to common shareholders, diluted	$ 276.9	$ 351.7	$ 281.4
Denominator:
Average shares of common stock - basic	80.2	79.5	78.6
Effect of dilutive securities
Effect of dilutive securities - Stock-based compensation (in shares)	0.5	0.4	0.5
Effect of dilutive securities - Deferred compensation (in shares)	0	0.2	0.2
Average shares of common stock - diluted	80.7	80.1	79.3
Earnings per common share
Basic (in dollars per share)	$ 3.45	$ 4.43	$ 3.58
Diluted (in dollars per share)	$ 3.43	$ 4.39	$ 3.55
Stock-based compensation
Disclosure of antidilutive shares
Weighted-average outstanding securities that had an anti-dilutive effect	0.2	0.3	0.7
Deferred compensation
Disclosure of antidilutive shares
Weighted-average outstanding securities that had an anti-dilutive effect	0.3	0.1	0