POSTRETIREMENT BENEFITS (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
POSTRETIREMENT BENEFITS [Abstract]
Percentage of net benefit cost capitalized as cost of constructing gas distribution facilities (in hundredths)			25.00%
Pension benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2.4	$ 2.4	$ 7.4	$ 7.3
Interest cost	4.1	4.0	12.3	12.0
Expected return on plan assets	(7.7)	(7.2)	(23.3)	(21.7)
Recognized net actuarial loss (gain)	2.5	3.0	7.3	8.9
Amortization of prior service cost	0.1	0.1	0.3	0.3
Net benefit costs (credit)	1.4	2.3	4.0	6.8
Health care and other benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.6	0.6	1.8	1.7
Interest cost	3.1	3.0	9.2	8.9
Expected return on plan assets	0	0	0	0
Recognized net actuarial loss (gain)	1.5	1.0	4.5	3.1
Amortization of prior service cost	0	0	0.1	0.1
Net benefit costs (credit)	$ 5.2	$ 4.6	$ 15.6	$ 13.8