CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenues [Abstract]
Revenues	$ 177.8	$ 307.5	$ 524.2	$ 884.3
Operating Expenses [Abstract]
General and administrative	20.9	29.6	63.1	124.4
Production and property taxes	14.0	20.0	42.3	67.6
Depreciation, depletion and amortization	133.0	144.2	424.6	395.5
Impairment	0.0	0.0	0.0	5.0
Total Operating Expenses	220.0	253.3	689.8	778.2
Net gain (loss) from asset sales, inclusive of restructuring costs	0.1	(2.1)	3.8	2.5
OPERATING INCOME (LOSS)	(42.1)	52.1	(161.8)	108.6
Realized and unrealized gains (losses) on derivative contracts	(34.2)	87.4	317.0	(55.8)
Interest and other income (expense)	7.7	0.9	7.7	4.6
Gain from early extinguishment of debt	(7.4)	0.0	18.2	0.0
Interest expense	(28.4)	(32.8)	(89.8)	(100.0)
INCOME (LOSS) BEFORE INCOME TAXES	(104.4)	107.6	91.3	(42.6)
Income tax (provision) benefit	55.2	(26.6)	42.5	55.7
Net income (loss)	$ (49.2)	$ 81.0	$ 133.8	$ 13.1
Earnings Per Share [Abstract]
Basic	$ (0.20)	$ 0.34	$ 0.55	$ 0.06
Diluted	$ (0.20)	$ 0.34	$ 0.55	$ 0.06
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Used in basic calculation	242.3	237.9	241.2	237.7
Used in diluted calculation	242.3	237.9	241.2	237.7
Oil and Gas, Exploration and Production [Member]
Revenues [Abstract]
Revenues	$ 175.8	$ 305.6	$ 515.9	$ 875.8
Product and Service, Other [Member]
Revenues [Abstract]
Revenues	1.4	1.8	1.6	7.1
Oil and Gas, Purchased [Member]
Revenues [Abstract]
Revenues	0.6	0.1	6.7	1.4
Operating Expenses [Abstract]
Operating Costs and Expenses	0.8	0.1	8.2	1.5
Lease Operating Expense [Member]
Operating Expenses [Abstract]
Operating Costs and Expenses	35.5	38.3	104.5	135.5
Natural Gas, Gathering, Transportation, Marketing and Processing [Member]
Operating Expenses [Abstract]
Operating Costs and Expenses	12.4	18.0	38.2	38.8
Gathering and other expense [Member]
Operating Expenses [Abstract]
Operating Costs and Expenses	$ 3.4	$ 3.1	$ 8.9	$ 9.9