Quarterly Financial Information (Unaudited) (Details) $ / shares in Units, MBoe in Millions, Bcfe in Millions, $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2016 USD ($) Bcfe MBoe $ / shares	Sep. 30, 2016 USD ($) Bcfe MBoe $ / shares	Jun. 30, 2016 USD ($) Bcfe MBoe $ / shares	Mar. 31, 2016 USD ($) Bcfe MBoe $ / shares	Dec. 31, 2015 USD ($) Bcfe MBoe $ / shares	Sep. 30, 2015 USD ($) Bcfe MBoe $ / shares	Jun. 30, 2015 USD ($) Bcfe MBoe $ / shares	Mar. 31, 2015 USD ($) Bcfe MBoe $ / shares	Dec. 31, 2016 USD ($) Bcfe MBoe $ / shares	Dec. 31, 2015 USD ($) Bcfe MBoe $ / shares	Dec. 31, 2014 USD ($) $ / shares
Quarterly Financial Information Disclosure [Line Items]
Revenues		$ 399.7	$ 382.4	$ 333.7	$ 261.3	$ 468.3	$ 507.6	$ 574.6	$ 468.1	$ 1,377.1	$ 2,018.6	$ 3,293.2
Operating income (loss)		(36.9)	(93.6)	(92.9)	(1,379.2)	(144.6)	(87.7)	(16.7)	(128.6)	(1,602.6)	(377.6)	(847.3)
Income (loss) from continuing operations										(1,245.0)	(149.4)	(409.5)
Net income from discontinued operations, net of income tax										0.0	0.0	1,193.9
Net income (loss)		(133.3)	(50.9)	(197.0)	(863.8)	(38.6)	21.1	(76.3)	(55.6)	(1,245.0)	(149.4)	$ 784.4
Net gain (loss) from asset sales and impairment		(6.1)	0.3	(1.6)	(1,181.9)	(22.4)	(2.1)	24.0	(50.5)	(1,189.3)	(51.0)
Nonrecurring items in operating income (loss)	[1]	$ 0.0	$ 25.0	$ 0.0	$ 7.7	$ 0.0	$ 0.0	$ 11.2	$ 0.0	$ 32.7	$ 11.2
Basic total | $ / shares		$ (0.56)	$ (0.21)	$ (0.90)	$ (4.55)	$ (0.22)	$ 0.12	$ (0.43)	$ (0.32)	$ (5.62)	$ (0.85)	$ 4.36
Diluted total | $ / shares		$ (0.56)	$ (0.21)	$ (0.90)	$ (4.55)	$ (0.22)	$ 0.12	$ (0.43)	$ (0.32)	$ (5.62)	$ (0.85)	4.36
Total equivalent production (Mboe) | MBoe		13,675.7	14,445.7	13,882.4	13,766.4	13,986.6	14,462.4	13,484.3	12,528.8	55,780.2	54,462.1
Total equivalent production (Bcfe) | Bcfe		82.1	86.6	83.3	82.7	84.0	86.7	80.9	75.2	334.7	326.8
Continuing Operations [Member]
Quarterly Financial Information Disclosure [Line Items]
Basic total | $ / shares										$ (5.62)	$ (0.85)	(2.28)
Diluted total | $ / shares										(5.62)	(0.85)	(2.28)
Discontinued Operations [Member]
Quarterly Financial Information Disclosure [Line Items]
Basic total | $ / shares										0.00	0.00	6.64
Diluted total | $ / shares										$ 0.00	$ 0.00	$ 6.64

[1] Reflects legal expenses and loss contingencies incurred during the year ended December 31, 2016, and a non-cash pension curtailment incurred during the year ended December 31, 2015.