Revenue (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
REVENUES
Oil and condensate, gas and NGL sales	$ 275.6	$ 409.8
Other revenue	3.7	5.0
Purchased oil and gas sales	1.3	14.1
Total Revenues	280.6	428.9
OPERATING EXPENSES
Purchased oil and gas expense	1.4	15.5
Lease operating expense	51.5	72.5
Transportation and processing costs	10.9	34.0
Gathering and other expense	3.8	2.8
General and administrative	63.3	60.1
Production and property taxes	24.0	28.9
Depreciation, depletion and amortization	123.3	196.5
Impairment	5.0	0.7
Total Operating Expenses	283.2	411.0
Net gain (loss) from asset sales, inclusive of restructuring costs	(13.2)	3.5
OPERATING INCOME (LOSS)	(15.8)	21.4
Realized and unrealized gains (losses) on derivative contracts (Note 7)	(181.7)	(53.2)
Interest and other income (expense)	2.8	(0.7)
Interest expense	(34.0)	(35.0)
INCOME (LOSS) BEFORE INCOME TAXES	(228.7)	(67.5)
Income tax (provision) benefit	112.0	13.9
Net income (loss)	$ (116.7)	$ (53.6)
Earnings (loss) per common share
Basic	$ (0.49)	$ (0.22)
Diluted	$ (0.49)	$ (0.22)
Weighted-average common shares outstanding
Used in basic calculation	237.1	240.9
Used in diluted calculation	237.1	240.9
As reported [Member]
REVENUES
Oil and condensate, gas and NGL sales	$ 275.6	$ 409.8