PENSION PLANS AND OTHER POST-RETIREMENT BENEFITS - Benefit Obligation Rollforward (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accumulated benefit obligation	$ 129.6	$ 253.0
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	293.4	323.5
Energen portion of liability divested	(144.7)	0
Liability loss due to estimated allocation	16.9	0
Service cost	5.1	14.2	10.5
Interest cost	4.1	11.2	10.8
Actuarial (gain) loss	7.8	(28.3)
Curtailment gain	0	(4.2)
Retiree drug subsidy program	0	0
Benefits paid	(33.8)	(23.0)
Benefit obligation, end of year	148.8	293.4	323.5
Postretirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of year	63.6	85.8
Energen portion of liability divested	(13.3)	0
Liability loss due to estimated allocation	7.7	0
Service cost	0.4	1.7	1.9
Interest cost	1.9	3.5	4.2
Actuarial (gain) loss	4.3	(21.7)
Curtailment gain	0	(1.3)
Retiree drug subsidy program	0.3	0.3
Benefits paid	(4.0)	(4.7)
Benefit obligation, end of year	$ 60.9	$ 63.6	$ 85.8