Revenue (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
REVENUES
Oil and condensate, gas and NGL sales	$ 409.8	$ 385.2
Other revenue	5.0	4.0
Purchased oil and gas sales	14.1	30.9
Total Revenues	428.9	420.1
OPERATING EXPENSES
Purchased oil and gas expense	15.5	29.4
Lease operating expense	72.5	69.2
Transportation and processing costs	34.0	70.2
Gathering and other expense	2.8	1.5
General and administrative	60.1	33.6
Production and property taxes	28.9	29.1
Depreciation, depletion and amortization	196.5	191.8
Exploration expenses	0.0	0.4
Impairment	0.7	0.1
Total Operating Expenses	411.0	425.3
Net gain (loss) from asset sales	3.5	0.0
OPERATING INCOME (LOSS)	21.4	(5.2)
Realized and unrealized gains (losses) on derivative contracts (Note 7)	(53.2)	160.9
Interest and other income (expense)	(0.7)	0.6
Interest expense	(35.0)	(33.8)
INCOME (LOSS) BEFORE INCOME TAXES	(67.5)	122.5
Income tax (provision) benefit	13.9	(45.6)
Net income (loss)	$ (53.6)	$ 76.9
Earnings (loss) per common share
Basic	$ 0	$ 0.32
Diluted	$ 0	$ 0.32
Weighted-average common shares outstanding
Used in basic calculation	240.9	240.2
Used in diluted calculation	240.9	240.3
Dividends per common share	$ 0	$ 0.00
As reported [Member]
REVENUES
Oil and condensate, gas and NGL sales	$ 409.8	$ 385.2
Other revenue	5.0
Purchased oil and gas sales	14.1
Total Revenues	428.9
OPERATING EXPENSES
Purchased oil and gas expense	15.5
Lease operating expense	72.5
Transportation and processing costs	34.0
Gathering and other expense	2.8
General and administrative	60.1
Production and property taxes	28.9
Depreciation, depletion and amortization	196.5
Exploration expenses	0.0
Impairment	0.7
Total Operating Expenses	411.0
Net gain (loss) from asset sales	3.5
OPERATING INCOME (LOSS)	21.4
Realized and unrealized gains (losses) on derivative contracts (Note 7)	(53.2)
Interest and other income (expense)	(0.7)
Interest expense	(35.0)
INCOME (LOSS) BEFORE INCOME TAXES	(67.5)
Income tax (provision) benefit	13.9
Net income (loss)	$ (53.6)
Earnings (loss) per common share
Basic	$ (0.22)
Diluted	$ (0.22)
Weighted-average common shares outstanding
Used in basic calculation	240.9
Used in diluted calculation	240.9
Dividends per common share	$ 0.00
Adjustments for New Accounting Pronouncement [Member]
REVENUES
Oil and condensate, gas and NGL sales	$ 12.7
Other revenue	0.0
Purchased oil and gas sales	0.0
Total Revenues	12.7
OPERATING EXPENSES
Purchased oil and gas expense	0.0
Lease operating expense	0.0
Transportation and processing costs	12.7
Gathering and other expense	0.0
General and administrative	0.0
Production and property taxes	0.0
Depreciation, depletion and amortization	0.0
Exploration expenses	0.0
Impairment	0.0
Total Operating Expenses	12.7
Net gain (loss) from asset sales	0.0
OPERATING INCOME (LOSS)	0.0
Realized and unrealized gains (losses) on derivative contracts (Note 7)	0.0
Interest and other income (expense)	0.0
Interest expense	0.0
INCOME (LOSS) BEFORE INCOME TAXES	0.0
Income tax (provision) benefit	0.0
Net income (loss)	$ 0.0
Earnings (loss) per common share
Basic	$ 0.00
Diluted	$ 0.00
Weighted-average common shares outstanding
Used in basic calculation	0.0
Used in diluted calculation	0.0
Dividends per common share	$ 0.00
Income Statement, as adjusted [Member]
REVENUES
Oil and condensate, gas and NGL sales	$ 422.5
Other revenue	5.0
Purchased oil and gas sales	14.1
Total Revenues	441.6
OPERATING EXPENSES
Purchased oil and gas expense	15.5
Lease operating expense	72.5
Transportation and processing costs	46.7
Gathering and other expense	2.8
General and administrative	60.1
Production and property taxes	28.9
Depreciation, depletion and amortization	196.5
Exploration expenses	0.0
Impairment	0.7
Total Operating Expenses	423.7
Net gain (loss) from asset sales	3.5
OPERATING INCOME (LOSS)	21.4
Realized and unrealized gains (losses) on derivative contracts (Note 7)	(53.2)
Interest and other income (expense)	(0.7)
Interest expense	(35.0)
INCOME (LOSS) BEFORE INCOME TAXES	(67.5)
Income tax (provision) benefit	13.9
Net income (loss)	$ (53.6)
Earnings (loss) per common share
Basic	$ (0.22)
Diluted	$ (0.22)
Weighted-average common shares outstanding
Used in basic calculation	240.9
Used in diluted calculation	240.9
Dividends per common share	$ 0.00