ACQUISITION AND DISPOSITION OF PROPERTIES - Additional Information (Details) $ in Millions		12 Months Ended
	Mar. 31, 2015 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 MBoe
Business Acquisition [Line Items]
Payments to acquire unproved leasehold properties		$ 273.3
Disposal Group, Not Discontinued Operations | Permian and San Juan Basin
Business Acquisition [Line Items]
Disposal group, consideration			$ 552.0
Cash received from sale of oil properties			532.2
Purchase price adjustments		(0.6)	19.0
Transaction costs			5.0
Pre-tax gain on disposal			246.3
Disposal Group, Not Discontinued Operations | San Juan Basin properties
Business Acquisition [Line Items]
Disposal group, consideration	$ 395.0
Cash received from sale of oil properties	384.0
Purchase price adjustments	11.0
Transaction costs	2.8
Pre-tax gain on disposal	$ 27.0
Proved reserves (in mboe) | MBoe					69,038
Delaware Basin
Business Acquisition [Line Items]
Payments to acquire unproved leasehold properties		217.4
Midland Basin
Business Acquisition [Line Items]
Payments to acquire unproved leasehold properties		36.9
Delaware Basin - Mineral Purchases
Business Acquisition [Line Items]
Payments to acquire unproved leasehold properties		$ 19.0
Permian Basin
Business Acquisition [Line Items]
Payments to acquire unproved leasehold properties			143.7	$ 85.7
Lea County, New Mexico
Business Acquisition [Line Items]
Payments to acquire unproved leasehold properties			$ 77.0