SHORT-TERM DEBT AND LINES OF CREDIT CAPACITY (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Short-term borrowings
Total short-term credit capacity	$ 250.0	$ 250.0
Short-term borrowings outstanding	145.1	25.6
Available capacity under existing agreements	104.9	224.4
Debt to capitalization ratio required to be maintained (as a percent)	65.00%
Revolving credit facility maturing on May 17, 2014
Short-term borrowings
Total short-term credit capacity	0	135.0
Revolving Credit Facility, Maturing May 7, 2015
Short-term borrowings
Total short-term credit capacity	135.0	0
Revolving credit facility maturing on June 13, 2017
Short-term borrowings
Total short-term credit capacity	115.0	115.0
Commercial paper
Short-term borrowings
Short-term borrowings outstanding	$ 145.1	$ 25.6	$ 95.4