COMMON EQUITY (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Changes to issued common stock
Common stock issued, balance at the beginning of the period	78,287,906
Shares issued - Stock Investment Plan	114,365
Shares issued - Stock-based compensation	317,677
Shares issued - Rabbi trust Shares	90,000
Common stock issued, balance at the end of the period	78,809,948	78,287,906
Issuance of common stock under stock investment plan and stock-based compensation plans	$ 22.3
Reconciliation of shares outstanding
Common stock, shares issued	78,809,948	78,287,906
Deferred compensation rabbi trust (in shares)	456,852	385,439
Total common shares outstanding	78,353,096	77,902,467
Average cost of shares in the rabbi trust (in dollars per share)	$ 48.23	$ 46.03