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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenues [Abstract]
Revenues	$ 307.5	$ 560.8	$ 884.3	$ 1,522.1
Operating Expenses [Abstract]
General and administrative	29.6	48.3	124.4	164.2
Production and property taxes	20.0	37.4	67.6	103.9
Depreciation, depletion and amortization	144.2	234.9	395.5	673.6
Exploration expenses	0.0	0.0	0.0	0.1
Impairment	0.0	0.0	5.0	404.4
Total Operating Expenses	253.3	431.1	778.2	1,692.4
Net gain (loss) from asset sales, inclusive of restructuring costs	(2.1)	27.1	2.5	26.7
OPERATING INCOME (LOSS)	52.1	156.8	108.6	(143.6)
Realized and unrealized gains (losses) on derivative contracts (Note 7)	87.4	(108.0)	(55.8)	(240.3)
Interest and other income (expense)	0.9	(0.3)	4.6	(4.1)
Interest expense	(32.8)	(38.7)	(100.0)	(111.9)
INCOME (LOSS) BEFORE INCOME TAXES	107.6	9.8	(42.6)	(499.9)
Income tax (provision) benefit	(26.6)	(2.5)	55.7	117.6
Net income (loss)	$ 81.0	$ 7.3	$ 13.1	$ (382.3)
Earnings Per Share [Abstract]
Basic	$ 0.34	$ 0.03	$ 0.06	$ (1.60)
Diluted	$ 0.34	$ 0.03	$ 0.06	$ (1.60)
Weighted Average Number of Shares Outstanding, Diluted [Abstract]
Used in basic calculation	237.9	236.9	237.7	238.3
Used in diluted calculation	237.9	237.0	237.7	238.3
Oil and Gas, Exploration and Production [Member]
Revenues [Abstract]
Revenues	$ 305.6	$ 544.0	$ 875.8	$ 1,474.1
Product and Service, Other [Member]
Revenues [Abstract]
Revenues	1.8	3.8	7.1	11.8
Oil and Gas, Purchased [Member]
Revenues [Abstract]
Revenues	0.1	13.0	1.4	36.2
Operating Expenses [Abstract]
Operating Costs and Expenses	0.1	13.3	1.5	38.6
Lease Operating Expense [Member]
Operating Expenses [Abstract]
Operating Costs and Expenses	38.3	64.6	135.5	203.6
Natural Gas, Gathering, Transportation, Marketing and Processing [Member]
Operating Expenses [Abstract]
Operating Costs and Expenses	18.0	28.0	38.8	93.2
Gathering and other expense [Member]
Operating Expenses [Abstract]
Operating Costs and Expenses	$ 3.1	$ 4.6	$ 9.9	$ 10.8