CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenues [Abstract]
Revenues	$ 225.8	$ 280.6
Operating Expenses [Abstract]
General and administrative	15.9	63.3
Production and property taxes	18.7	24.0
Depreciation, depletion and amortization	142.2	123.3
Impairment	0.0	5.0
Total Operating Expenses	236.7	283.2
Net gain (loss) from asset sales, inclusive of restructuring costs	3.7	(13.2)
OPERATING INCOME (LOSS)	(7.2)	(15.8)
Realized and unrealized gains (losses) on derivative contracts	449.9	(181.7)
Interest and other income (expense)	(2.6)	2.8
Gain from early extinguishment of debt	25.2	0.0
Interest expense	(31.6)	(34.0)
INCOME (LOSS) BEFORE INCOME TAXES	433.7	(228.7)
Income tax (provision) benefit	(66.3)	112.0
Net income (loss)	$ 367.4	$ (116.7)
Earnings Per Share [Abstract]
Basic	$ 1.54	$ (0.49)
Diluted	$ 1.54	$ (0.49)
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Used in basic calculation	239.1	237.1
Used in diluted calculation	239.1	237.1
Oil and Gas, Exploration and Production [Member]
Revenues [Abstract]
Revenues	$ 221.8	$ 275.6
Product and Service, Other [Member]
Revenues [Abstract]
Revenues	0.4	3.7
Oil and Gas, Purchased [Member]
Revenues [Abstract]
Revenues	3.6	1.3
Operating Expenses [Abstract]
Operating Costs and Expenses	3.5	1.4
Lease Operating Expense [Member]
Operating Expenses [Abstract]
Operating Costs and Expenses	40.2	51.5
Natural Gas, Gathering, Transportation, Marketing and Processing [Member]
Operating Expenses [Abstract]
Operating Costs and Expenses	13.5	10.9
Gathering and other expense [Member]
Operating Expenses [Abstract]
Operating Costs and Expenses	$ 2.7	$ 3.8