CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
REVENUES
Revenues	$ 1,206.2	$ 1,932.6	$ 1,622.9
OPERATING EXPENSES
General and administrative	155.8	221.7	153.5
Production and property taxes	95.9	130.8	114.3
Depreciation, depletion and amortization	540.0	857.1	754.5
Exploration expenses	0.1	0.3	22.0
Impairment	5.0	1,560.9	78.9
Total Operating Expenses	1,052.6	3,218.0	1,734.9
Net gain (loss) from asset sales, inclusive of restructuring costs	3.9	25.0	213.5
OPERATING INCOME (LOSS)	157.5	(1,260.4)	101.5
Realized and unrealized gains (losses) on derivative contracts (Note 7)	(173.4)	90.4	24.5
Interest and other income (expense)	4.7	(9.6)	1.6
Loss from early extinguishment of debt	(1.0)	0.0	(32.7)
Interest expense	(128.1)	(149.4)	(137.8)
INCOME (LOSS) BEFORE INCOME TAXES	(140.3)	(1,329.0)	(42.9)
Income tax (provision) benefit	43.0	317.4	312.2
NET INCOME (LOSS)	$ (97.3)	$ (1,011.6)	$ 269.3
Earnings (loss) per common share
Basic	$ (0.41)	$ (4.25)	$ 1.12
Diluted	$ (0.41)	$ (4.25)	$ 1.12
Weighted-average common shares outstanding
Used in basic calculation	237.7	237.9	240.6
Used in diluted calculation	237.7	237.9	240.6
Oil and Gas, Exploration and Production [Member]
REVENUES
Revenues	$ 1,187.4	$ 1,871.3	$ 1,545.3
Product and Service, Other [Member]
REVENUES
Revenues	7.9	12.5	15.0
Oil and Gas, Purchased [Member]
REVENUES
Revenues	10.9	48.8	62.6
OPERATING EXPENSES
Other Cost and Expense, Operating	11.0	51.0	64.3
Lease Operating Expense [Member]
OPERATING EXPENSES
Other Cost and Expense, Operating	182.9	263.1	294.8
Natural Gas, Gathering, Transportation, Marketing and Processing [Member]
OPERATING EXPENSES
Other Cost and Expense, Operating	48.7	117.6	245.3
Gathering and other expense [Member]
OPERATING EXPENSES
Other Cost and Expense, Operating	$ 13.2	$ 15.5	$ 7.3