Revenue (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
REVENUES
Oil and condensate, gas and NGL sales	$ 544.0	$ 380.9	$ 1,474.1	$ 1,139.1
Other revenue	3.8	3.6	11.8	10.3
Purchased oil and gas sales	13.0	5.6	36.2	44.5
Total Revenues	560.8	390.1	1,522.1	1,193.9
OPERATING EXPENSES
Purchased oil and gas expense	13.3	6.9	38.6	45.4
Lease operating expense	64.6	76.2	203.6	215.4
Transportation and processing costs	28.0	60.2	93.2	202.6
Gathering and other expense	4.6	1.7	10.8	5.0
General and administrative	48.3	43.4	164.2	108.3
Production and property taxes	37.4	28.5	103.9	86.1
Depreciation, depletion and amortization	234.9	176.9	673.6	560.2
Exploration expenses	0.0	21.3	0.1	21.7
Impairment	0.0	28.3	404.4	28.4
Total Operating Expenses	431.1	443.4	1,692.4	1,273.1
Net gain (loss) from asset sales, inclusive of restructuring costs	27.1	185.4	26.7	205.2
OPERATING INCOME (LOSS)	156.8	132.1	(143.6)	126.0
Realized and unrealized gains (losses) on derivative contracts (Note 7)	(108.0)	(104.3)	(240.3)	163.3
Interest and other income (expense)	(0.3)	0.1	(4.1)	2.5
Interest expense	(38.7)	(34.4)	(111.9)	(103.1)
INCOME (LOSS) BEFORE INCOME TAXES	9.8	(6.5)	(499.9)	188.7
Income tax (provision) benefit	(2.5)	3.2	117.6	(69.7)
Net income (loss)	$ 7.3	$ (3.3)	$ (382.3)	$ 119.0
Earnings (loss) per common share
Basic	$ 0.03	$ (0.01)	$ (1.60)	$ 0.49
Diluted	$ 0.03	$ (0.01)	$ (1.60)	$ 0.49
Weighted-average common shares outstanding
Used in basic calculation	236.9	240.7	238.3	240.5
Used in diluted calculation	237.0	240.7	238.3	240.5
As reported [Member]
REVENUES
Oil and condensate, gas and NGL sales	$ 544.0	$ 380.9	$ 1,474.1	$ 1,139.1
Other revenue	3.8		11.8
Purchased oil and gas sales	13.0		36.2
Total Revenues	560.8		1,522.1
OPERATING EXPENSES
Purchased oil and gas expense	13.3		38.6
Lease operating expense	64.6		203.6
Transportation and processing costs	28.0		93.2
Gathering and other expense	4.6		10.8
General and administrative	48.3		164.2
Production and property taxes	37.4		103.9
Depreciation, depletion and amortization	234.9		673.6
Exploration expenses	0.0		0.1
Impairment	0.0		404.4
Total Operating Expenses	431.1		1,692.4
Net gain (loss) from asset sales, inclusive of restructuring costs	27.1		26.7
OPERATING INCOME (LOSS)	156.8		(143.6)
Realized and unrealized gains (losses) on derivative contracts (Note 7)	(108.0)		(240.3)
Interest and other income (expense)	(0.3)		(4.1)
Interest expense	(38.7)		(111.9)
INCOME (LOSS) BEFORE INCOME TAXES	9.8		(499.9)
Income tax (provision) benefit	(2.5)		117.6
Net income (loss)	$ 7.3		$ (382.3)
Earnings (loss) per common share
Basic	$ 0.03		$ (1.60)
Diluted	$ 0.03		$ (1.60)
Weighted-average common shares outstanding
Used in basic calculation	236.9		238.3
Used in diluted calculation	237.0		238.3
Adjustments for New Accounting Pronouncement [Member]
REVENUES
Oil and condensate, gas and NGL sales	$ 15.8		$ 40.9
Other revenue	0.0		0.0
Purchased oil and gas sales	0.0		0.0
Total Revenues	15.8		40.9
OPERATING EXPENSES
Purchased oil and gas expense	0.0		0.0
Lease operating expense	0.0		0.0
Transportation and processing costs	15.8		40.9
Gathering and other expense	0.0		0.0
General and administrative	0.0		0.0
Production and property taxes	0.0		0.0
Depreciation, depletion and amortization	0.0		0.0
Exploration expenses	0.0		0.0
Impairment	0.0		0.0
Total Operating Expenses	15.8		40.9
Net gain (loss) from asset sales, inclusive of restructuring costs	0.0		0.0
OPERATING INCOME (LOSS)	0.0		0.0
Realized and unrealized gains (losses) on derivative contracts (Note 7)	0.0		0.0
Interest and other income (expense)	0.0		0.0
Interest expense	0.0		0.0
INCOME (LOSS) BEFORE INCOME TAXES	0.0		0.0
Income tax (provision) benefit			0.0
Net income (loss)	$ 0.0		$ 0.0
Earnings (loss) per common share
Basic	$ 0.00		$ 0.00
Diluted	$ 0.00		$ 0.00
Weighted-average common shares outstanding
Used in basic calculation	0.0		0.0
Used in diluted calculation	0.0		0.0
Income Statement, as adjusted [Member]
REVENUES
Oil and condensate, gas and NGL sales	$ 559.8		$ 1,515.0
Other revenue	3.8		11.8
Purchased oil and gas sales	13.0		36.2
Total Revenues	576.6		1,563.0
OPERATING EXPENSES
Purchased oil and gas expense	13.3		38.6
Lease operating expense	64.6		203.6
Transportation and processing costs	43.8		134.1
Gathering and other expense	4.6		10.8
General and administrative	48.3		164.2
Production and property taxes	37.4		103.9
Depreciation, depletion and amortization	234.9		673.6
Exploration expenses	0.0		0.1
Impairment	0.0		404.4
Total Operating Expenses	446.9		1,733.3
Net gain (loss) from asset sales, inclusive of restructuring costs	27.1		26.7
OPERATING INCOME (LOSS)	156.8		(143.6)
Realized and unrealized gains (losses) on derivative contracts (Note 7)	(108.0)		(240.3)
Interest and other income (expense)	(0.3)		(4.1)
Interest expense	(38.7)		(111.9)
INCOME (LOSS) BEFORE INCOME TAXES	9.8		(499.9)
Income tax (provision) benefit	(2.5)		117.6
Net income (loss)	$ 7.3		$ (382.3)
Earnings (loss) per common share
Basic	$ 0.03		$ (1.60)
Diluted	$ 0.03		$ (1.60)
Weighted-average common shares outstanding
Used in basic calculation	236.9		238.3
Used in diluted calculation	237.0		238.3