CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
REVENUES
Oil and condensate, gas and NGL sales									$ 1,871.3	$ 1,545.3	$ 1,269.7
Other revenues									12.5	15.0	6.2
Purchased oil and gas sales									48.8	62.6	101.2
Total Revenues	$ 410.5	$ 560.8	$ 532.4	$ 428.9	$ 429.0	$ 390.1	$ 383.7	$ 420.1	1,932.6	1,622.9	1,377.1
OPERATING EXPENSES
Purchased oil and gas expense									51.0	64.3	105.5
Lease operating expense									263.1	294.8	224.7
Transportation and processing costs									117.6	245.3	289.2
Gathering and other expense									15.5	7.3	5.0
General and administrative									221.7	153.5	196.5
Production and property taxes									130.8	114.3	94.8
Depreciation, depletion and amortization									857.1	754.5	871.1
Exploration expenses									0.3	22.0	1.7
Impairment									1,560.9	78.9	1,194.3
Total Operating Expenses									3,218.0	1,734.9	2,982.8
Net gain (loss) from asset sales, inclusive of restructuring costs									25.0	213.5	5.0
OPERATING INCOME (LOSS)	(1,116.8)	156.8	(321.8)	21.4	(24.5)	132.1	(0.9)	(5.2)	(1,260.4)	101.5	(1,600.7)
Realized and unrealized gains (losses) on derivative contracts (Note 7)									90.4	24.5	(233.0)
Interest and other income (expense)									(9.6)	1.6	23.7
Loss from early extinguishment of debt									0.0	(32.7)	0.0
Interest expense									(149.4)	(137.8)	(143.2)
INCOME (LOSS) BEFORE INCOME TAXES									(1,329.0)	(42.9)	(1,953.2)
Income tax (provision) benefit									317.4	312.2	708.2
NET INCOME (LOSS)	$ (629.3)	$ 7.3	$ (336.0)	$ (53.6)	$ 150.3	$ (3.3)	$ 45.4	$ 76.9	$ (1,011.6)	$ 269.3	$ (1,245.0)
Earnings (loss) per common share
Basic	$ (2.66)	$ 0.03	$ (1.42)	$ (0.22)	$ 0.62	$ (0.01)	$ 0.19	$ 0.32	$ (4.25)	$ 1.12	$ (5.62)
Diluted	$ (2.66)	$ 0.03	$ (1.42)	$ (0.22)	$ 0.62	$ (0.01)	$ 0.19	$ 0.32	$ (4.25)	$ 1.12	$ (5.62)
Weighted-average common shares outstanding
Used in basic calculation									237.9	240.6	221.7
Used in diluted calculation									237.9	240.6	221.7