PENSION PLANS AND OTHER POST-RETIREMENT BENEFITS - Fair Value of Plan Assets Rollforward (Details) (USD $) In Millions, unless otherwise specified				9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2014 Pension Plans	Dec. 31, 2013 Pension Plans	Sep. 30, 2014 Postretirement Plans	Dec. 31, 2013 Postretirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of period	$ 0	$ 0	$ 14.5	$ 219.5	$ 209.3	$ 98.9	$ 87.1
Energen portion of assets divested				(90.9)	0	(22.0)	0
Asset gain due to estimated allocation				15.0	0	11.0	0
Actual return on plan assets				7.8	23.0	1.4	14.9
Employer contributions				1.6	10.2	0.3	1.6
Benefits paid				(33.8)	(23.0)	(4.0)	(4.7)
Fair value of plan assets at end of period	0	0	14.5	119.2	219.5	85.6	98.9
(Unfunded) funded status of plan				$ (29.6)	$ (73.8)	$ 24.7	$ 35.4

Defined Benefit Plan, Other Changes Plan Assets

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- Definition

The difference between fair value of plan assets at the end of the period and the fair value at the beginning of the period, adjusted for contributions and payments of benefits during the period, and after adjusting for taxes and other expenses, as applicable.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Actual Return on Plan Assets (Component of Net Periodic Postretirement Benefit Cost)

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-Publisher FASB

-Name Accounting Standards Codification

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- Definition

The amount of payments made for which participants are entitled under a pension plan, including pension benefits, death benefits, and benefits due on termination of employment. Also includes payments made under a postretirement benefit plan, including prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. This item represents a periodic decrease to the plan obligations and a decrease to plan assets.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Postretirement Benefits

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-Topic 715

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-Glossary Benefits

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- Definition

The increase in the fair value of plan assets from contributions made by the employer.

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The amount of decrease in the plan assets attributed to the sale, liquidation, spin-off or other divestiture of a corporate division or subsidiary.

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- Definition

Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.

+ References

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- Definition

The funded status is measured as the difference between the fair value of plan assets and the benefit obligation. Will normally be the same as the net Defined Benefit Plan, Amounts Recognized in Balance Sheet, Total.

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