DEBT - Credit Facilities Amendments (Details) $ in Millions	Nov. 08, 2017 USD ($)
2016 Credit Agreement, Second Lien Notes and Senior Notes | Amendment to the 2014 Credit Facilities
Financial performance covenants
Percentage of proceeds from the power plant monetization that may be use to prepay debts	50.00%
2016 Credit Agreement, Second Lien Notes and Senior Notes | Amendment to the 2014 Credit Facilities | Up to $500 million
Financial performance covenants
Maximum percent of net cash proceeds from non-borrowing base asset sales that may be used to repurchase notes or prepay our 2016 Credit Agreement, Second Lien Notes and Senior Notes	75.00%
2016 Credit Agreement, Second Lien Notes and Senior Notes | Amendment to the 2014 Credit Facilities | $500 million and $1 billion
Financial performance covenants
Maximum percent of net cash proceeds from non-borrowing base asset sales that may be used to repurchase notes or prepay our 2016 Credit Agreement, Second Lien Notes and Senior Notes	50.00%
2016 Credit Agreement, Second Lien Notes and Senior Notes | Amendment to the 2014 Credit Facilities | In excess of $1 billion
Financial performance covenants
Maximum percent of net cash proceeds from non-borrowing base asset sales that may be used to repurchase notes or prepay our 2016 Credit Agreement, Second Lien Notes and Senior Notes	25.00%
2014 Credit Facilities (Secured First Lien), Amended | Amendment to the 2014 Credit Facilities
Financial performance covenants
Minimum new term loan proceeds required to be used to repay our 2014 Credit Facilities for amendment to become effective	$ 900.0
Minimum liquidity at closing of amendments to credit facility	$ 500.0
Minimum interest coverage ratio	1.20
2014 Credit Facilities (Secured First Lien), Amended | 5% Notes Due 2020 | Amendment to the 2014 Credit Facilities
Debt
Outstanding amount that triggers accelerated payment prior to June 30, 2021	$ 100.0
Financial performance covenants
Period before maturity of the specified notes	273 days
2014 Credit Facilities (Secured First Lien), Amended | 5.5% Notes Due 2021 | Amendment to the 2014 Credit Facilities
Debt
Outstanding amount that triggers accelerated payment prior to June 30, 2021	$ 100.0
Financial performance covenants
Period before maturity of the specified notes	273 days
Minimum | 2016 Credit Agreement, Second Lien Notes and Senior Notes | Amendment to the 2014 Credit Facilities | $500 million and $1 billion
Financial performance covenants
Proceeds from non-borrowing base asset sales that may be used to prepay our 2016 Credit Agreement, Second Lien Notes and Senior Notes	$ 500.0
Minimum | 2016 Credit Agreement, Second Lien Notes and Senior Notes | Amendment to the 2014 Credit Facilities | In excess of $1 billion
Financial performance covenants
Proceeds from non-borrowing base asset sales that may be used to prepay our 2016 Credit Agreement, Second Lien Notes and Senior Notes	$ 1,000.0
Minimum | 2014 Credit Facilities (Secured First Lien), Amended | Amendment to the 2014 Credit Facilities | Applicable margin on Alternate Base Rate loans
Debt
Interest rate added to variable rate basis (as a percent)	3.25%
Minimum | 2014 Credit Facilities (Secured First Lien), Amended | Amendment to the 2014 Credit Facilities | Applicable margin on LIBOR loans
Debt
Interest rate added to variable rate basis (as a percent)	2.25%
Maximum | 2016 Credit Agreement, Second Lien Notes and Senior Notes | Amendment to the 2014 Credit Facilities | Up to $500 million
Financial performance covenants
Proceeds from non-borrowing base asset sales that may be used to prepay our 2016 Credit Agreement, Second Lien Notes and Senior Notes	$ 500.0
Maximum | 2016 Credit Agreement, Second Lien Notes and Senior Notes | Amendment to the 2014 Credit Facilities | $500 million and $1 billion
Financial performance covenants
Proceeds from non-borrowing base asset sales that may be used to prepay our 2016 Credit Agreement, Second Lien Notes and Senior Notes	$ 1,000.0
Maximum | 2014 Credit Facilities (Secured First Lien), Amended | Amendment to the 2014 Credit Facilities | Applicable margin on Alternate Base Rate loans
Debt
Interest rate added to variable rate basis (as a percent)	4.00%
Maximum | 2014 Credit Facilities (Secured First Lien), Amended | Amendment to the 2014 Credit Facilities | Applicable margin on LIBOR loans
Debt
Interest rate added to variable rate basis (as a percent)	3.00%
Term Loan Facility | 2014 Credit Facilities (Secured First Lien), Amended | September 30, 2019 and thereafter
Financial performance covenants
Quarterly installment payments under amended agreement	$ 12.5
Revolving Credit Facility | 2014 Credit Facilities (Secured First Lien), Amended | Amendment to the 2014 Credit Facilities
Financial performance covenants
Minimum monthly liquidity	150.0
Commitment amount	$ 1,000.0
2014 Credit Facilities and New Term Loan | 2014 Credit Facilities (Secured First Lien), Amended | Amendment to the 2014 Credit Facilities | Through 2019
Financial performance covenants
Maximum leverage ratio	1.90
2014 Credit Facilities and New Term Loan | 2014 Credit Facilities (Secured First Lien), Amended | Amendment to the 2014 Credit Facilities | After 2019
Financial performance covenants
Maximum leverage ratio	1.50