DEBT - Credit Facilities and Senior Notes (Details) shares in Millions, $ in Millions	1 Months Ended				3 Months Ended						6 Months Ended		12 Months Ended
	Feb. 28, 2017 USD ($)	Aug. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Oct. 31, 2014 USD ($)	Jun. 30, 2017 USD ($) item	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Jun. 30, 2017 USD ($) item	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) shares	May 01, 2017 USD ($)
Debt
Outstanding borrowings					$ 5,169		$ 5,268				$ 5,169		$ 5,268
Repayment of revolving credit facility											733	$ 701
Repayment of term loan											66	61
Pre-tax gain on repurchase of debt									$ 44		4	$ 133
2014 First Out Credit Facilities (Secured First Lien)
Debt
Maximum principal amount of unsecured senior notes that may repurchase											$ 140
Minimum percent of asset sale proceeds must be in cash											75.00%
Percentage of proceeds from non-borrowing base asset sales used to repurchase debt if leverage ratio is least than 4:00 to 1:00											50.00%
Approval percentage measured by total exposure of lenders for approval of decreases in borrowing base											67.00%
Percent of commitment required to decrease or affirm the borrowing base					67.00%						67.00%
Number of consecutive redeterminations | item					3						3
Financial performance covenants
Minimum monthly liquidity					$ 250						$ 250
Available borrowing capacity											$ 437
Percent of net cash proceeds from asset sales which must be used (other than permitted development joint ventures and other transactions) to repay loans outstanding under the 2014 First-Out Credit Facilities											100.00%
Maximum percent of net cash proceeds from non-borrowing base asset sales that may be used to repurchase notes											50.00%
Percentage of proceeds from non-borrowing base asset sales used to repurchase debt if leverage ratio is less than 4:00 to 1:00											40.00%
Maximum additional non-Credit Facility indebtedness											$ 50
Amount of cash on hand above which amounts owed under the Revolving Credit Facility must be repaid											$ 150
2014 First Out Credit Facilities (Secured First Lien) | Federal fund rate
Debt
Interest rate added to variable rate basis (as a percent)											0.50%
2014 First Out Credit Facilities (Secured First Lien) | One-Month LIBOR
Debt
Interest rate added to variable rate basis (as a percent)											1.00%
2014 First Out Credit Facilities (Secured First Lien) | Quarters ending through June 30, 2017
Financial performance covenants
First-Lien First-Out Leverage Ratio, maximum					3.50						3.50
Minimum interest expense coverage ratio					1.20						1.20
2014 First Out Credit Facilities (Secured First Lien) | Quarters ending through September 30, 2017
Financial performance covenants
First-Lien First-Out Leverage Ratio, maximum					3.25						3.25
Minimum interest expense coverage ratio					1.20						1.20
2014 First Out Credit Facilities (Secured First Lien) | Quarters ending through December 31, 2017
Financial performance covenants
First-Lien First-Out Leverage Ratio, maximum					3.25						3.25
Minimum interest expense coverage ratio					1.20						1.20
2014 First Out Credit Facilities (Secured First Lien) | Quarters ending after first quarter of 2018
Financial performance covenants
First-Lien First-Out Leverage Ratio, maximum					2.25						2.25
Minimum interest expense coverage ratio					2.00						2.00
2014 First Out Credit Facilities (Secured First Lien) | 5% Notes Due 2020
Debt
Threshold amount of remaining outstanding balance of debt above which prepayment on the 2014 First Out Credit Agreement accelerates					$ 100						$ 100
Financial performance covenants
Period before maturity of the specified notes											182 days
2016 Second Out Credit Agreement (Secured First Lien)
Debt
Outstanding borrowings					1,000		1,000				$ 1,000		1,000
Original issue discount		$ 10
Percentage of premium to par value of coupon for make-whole premium		50.00%
Financial performance covenants
Aggregate principal amount issued		$ 1,000			1,000		$ 1,000				1,000		$ 1,000
2016 Second Out Credit Agreement (Secured First Lien) | Any June 30 and December 31
Financial performance covenants
First-lien asset coverage ratio		1.20
2016 Second Out Credit Agreement (Secured First Lien) | 5% senior unsecured notes due January 15, 2020 (the 2020 Notes) and 5.50% senior unsecured notes due September 15, 2021 (the 2021 Notes)
Debt
Outstanding amount that triggers accelerated payment					$ 100						$ 100
Financial performance covenants
Period before maturity of the specified notes											91 days
Senior Notes (Secured Second Lien) | 8% Notes Due 2022
Debt
Interest rate (as a percent)					8.00%		8.00%				8.00%		8.00%
Outstanding borrowings					$ 2,250		$ 2,250				$ 2,250		$ 2,250
Financial performance covenants
Aggregate principal amount issued			$ 2,250
Senior Notes (Secured Second Lien) | 8% Notes Due 2022 | Prior to December 15, 2017, from proceeds of certain equity offerings
Financial performance covenants
Percentage of principal amount at which notes can be redeemed											108.00%
Percentage of principal amount that may be redeemed											35.00%
Redemption period start											Dec. 01, 2015
Redemption period end											Dec. 14, 2017
Senior Notes (Secured Second Lien) | 8% Notes Due 2022 | Prior to December 15, 2018
Financial performance covenants
Percentage of principal amount at which notes can be redeemed											100.00%
Redemption period start											Dec. 15, 2017
Redemption period end											Dec. 14, 2018
Senior Notes (Secured Second Lien) | 8% Notes Due 2022 | On or after December 15, 2018
Financial performance covenants
Percentage of principal amount at which notes can be redeemed											104.00%
Redemption period start											Dec. 15, 2018
Redemption period end											Dec. 31, 2018
Senior Notes (Secured Second Lien) | 8% Notes Due 2022 | 2019
Financial performance covenants
Percentage of principal amount at which notes can be redeemed											102.00%
Redemption period start											Jan. 01, 2019
Redemption period end											Dec. 31, 2019
Senior Notes (Secured Second Lien) | 8% Notes Due 2022 | Thereafter
Financial performance covenants
Percentage of principal amount at which notes can be redeemed											100.00%
Redemption period start											Jan. 01, 2020
Redemption period end											Dec. 15, 2022
Senior Unsecured Notes
Debt
Principal amount of debt reduction through payment or repurchase			2,800				1,500						1,500
Repurchase value of the principal amounts							$ 750						$ 750
Pre-tax gain on repurchase of debt			560
Financial performance covenants
Aggregate principal amount issued				$ 5,000
Net proceeds from private placement				4,950
Number of shares issued for converted debt | shares													3.4
Original debt exchanged													$ 100
Percentage of principal amount at which notes can be redeemed in case of change control											101.00%
Percentage of principal amount at which notes can be redeemed											100.00%
Senior Unsecured Notes | 5% Notes Due 2020
Debt
Interest rate (as a percent)					5.00%		5.00%				5.00%		5.00%
Outstanding borrowings					$ 165		$ 193				$ 165		$ 193
Principal amount of debt reduction through payment or repurchase			33			$ 28
Repurchase value of the principal amounts			$ 13			24
Financial performance covenants
Aggregate principal amount issued				1,000
Senior Unsecured Notes | 5.5% Notes Due 2021
Debt
Interest rate (as a percent)					5.50%		5.50%				5.50%		5.50%
Outstanding borrowings					$ 135		$ 135				$ 135		$ 135
Financial performance covenants
Aggregate principal amount issued				$ 1,750
Senior Unsecured Notes | 6% Notes Due 2024
Debt
Interest rate (as a percent)				6.00%	6.00%		6.00%				6.00%		6.00%
Outstanding borrowings					$ 193		$ 193				$ 193		$ 193
Financial performance covenants
Aggregate principal amount issued				$ 2,250
Minimum | 2014 First Out Credit Facilities (Secured First Lien)
Debt
Approval percentage measured by total exposure of lenders for approval of increases in borrowing base					80.00%						80.00%
Financial performance covenants
Total leverage ratio											3.00
Minimum | 2014 First Out Credit Facilities (Secured First Lien) | LIBOR loans
Debt
Interest rate added to variable rate basis (as a percent)											2.50%
Minimum | 2014 First Out Credit Facilities (Secured First Lien) | Alternate Base Rate loans
Debt
Interest rate added to variable rate basis (as a percent)											1.50%
Minimum | 2014 First Out Credit Facilities (Secured First Lien) | Any June 30 and December 31
Financial performance covenants
First-lien asset coverage ratio					1.20						1.20
Maximum | 2014 First Out Credit Facilities (Secured First Lien) | LIBOR loans
Debt
Interest rate added to variable rate basis (as a percent)											3.50%
Maximum | 2014 First Out Credit Facilities (Secured First Lien) | Alternate Base Rate loans
Debt
Interest rate added to variable rate basis (as a percent)											2.50%
Term loan facility | 2014 First Out Credit Facilities (Secured First Lien)
Debt
Maximum borrowing capacity					$ 584						$ 584
Outstanding borrowings					584		650				584		650
Quarterly installment payment of Term Loan					25	$ 25	25	$ 25	$ 25	$ 25
Prepayment from proceeds of non-core asset sale	$ 16
Prepayment of term loan		$ 250
Term loan facility | 2016 Second Out Credit Agreement (Secured First Lien) | LIBOR loans
Debt
Interest rate added to variable rate basis (as a percent)		10.375%
Floor rate (as a percent)		1.00%
Term loan facility | 2016 Second Out Credit Agreement (Secured First Lien) | Alternate Base Rate loans
Debt
Interest rate added to variable rate basis (as a percent)		9.375%
Revolving credit facility | 2014 First Out Credit Facilities (Secured First Lien)
Debt
Maximum borrowing capacity					1,400						1,400
Increase to maximum borrowing capacity with lender approval											245
Additional commitments from lenders					5						5
Outstanding borrowings					842		$ 847				$ 842		$ 847
Commitment fees on unused portion of the Revolving Credit Facility											0.50%
Repayment of revolving credit facility		$ 740
Revolving credit facility | Letter of Credit
Debt
Maximum borrowing capacity					400						$ 400
Term loan and Revolving credit facility | 2014 First Out Credit Facilities (Secured First Lien)
Debt
Maximum borrowing capacity					$ 2,000						$ 2,000
Borrowing base														$ 2,300