QUARTERLY FINANCIAL INFORMATION (UNAUDITED) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 397.4us-gaap_Revenues	$ 370.4us-gaap_Revenues	$ 358.8us-gaap_Revenues	$ 555.7us-gaap_Revenues	$ 406.2us-gaap_Revenues	$ 371.9us-gaap_Revenues	$ 367.8us-gaap_Revenues	$ 433.4us-gaap_Revenues	$ 1,682.3us-gaap_Revenues	$ 1,579.3us-gaap_Revenues	$ 1,499.2us-gaap_Revenues
Operating income	56.4us-gaap_OperatingIncomeLoss	77.9us-gaap_OperatingIncomeLoss	36.5us-gaap_OperatingIncomeLoss	87.6us-gaap_OperatingIncomeLoss	50.5us-gaap_OperatingIncomeLoss	65.2us-gaap_OperatingIncomeLoss	46.9us-gaap_OperatingIncomeLoss	77.4us-gaap_OperatingIncomeLoss	258.4us-gaap_OperatingIncomeLoss	240.0us-gaap_OperatingIncomeLoss	224.2us-gaap_OperatingIncomeLoss
Net income attributed to common shareholder	$ 28.0	$ 42.2	$ 17.1	$ 50.3	$ 27.3	$ 37.0	$ 25.9	$ 44.6	$ 137.6	$ 134.8	$ 131.7

Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 260

-SubTopic 10

-Section 45

-Paragraph 11

-URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e1377-109256

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- Definition

The net result for the period of deducting operating expenses from operating revenues.

+ References

No definition available.

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- Definition

Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 5

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