Quarterly Financial Information (Unaudited) (Details) $ / shares in Units, $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2018 USD ($) MMBoe $ / shares	Sep. 30, 2018 USD ($) MMBoe $ / shares	Jun. 30, 2018 USD ($) MMBoe $ / shares	Mar. 31, 2018 USD ($) MMBoe $ / shares	Dec. 31, 2017 USD ($) MBoe $ / shares	Sep. 30, 2017 USD ($) MBoe $ / shares	Jun. 30, 2017 USD ($) MBoe $ / shares	Mar. 31, 2017 USD ($) MBoe $ / shares	Dec. 31, 2018 USD ($) MMBoe $ / shares	Dec. 31, 2017 USD ($) MBoe $ / shares	Dec. 31, 2016 USD ($) $ / shares
Quarterly Financial Information Disclosure [Line Items]
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 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)
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)
Net gain (loss) from asset sales, inclusive of restructuring costs and impairment		$ (1,158.2)	$ 27.1	$ (407.6)	$ 2.8	(42.2)	157.1	19.8	(0.1)
Nonrecurring items in operating income (loss)(1)	[1]					$ 0.0	$ 8.2	$ 0.0	$ 0.0
Basic | $ / shares		$ (2.66)	$ 0.03	$ (1.42)	$ (0.22)	$ 0.62	$ (0.01)	$ 0.19	$ 0.32	$ (4.25)	$ 1.12	$ (5.62)
Diluted | $ / shares		$ (2.66)	$ 0.03	$ (1.42)	$ (0.22)	$ 0.62	$ (0.01)	$ 0.19	$ 0.32	$ (4.25)	$ 1.12	$ (5.62)
Total equivalent production (Mboe)		11,627,200	14,400,000	14,106,100	11,724,600	12,069,900	14,124,100	13,860,600	13,090,300
Total [Member]
Quarterly Financial Information Disclosure [Line Items]
Revenues										$ 1,932.6	$ 1,622.9
Operating income (loss)										(1,260.4)	101.5
Net income (loss)										(1,011.6)	269.3
Net gain (loss) from asset sales, inclusive of restructuring costs and impairment										$ (1,535.9)	134.6
Nonrecurring items in operating income (loss)(1)	[1]										$ 8.2
Basic | $ / shares										$ (4.25)	$ 1.12
Diluted | $ / shares										$ (4.25)	$ 1.12
Total equivalent production (Mboe)										51,857,900	53,144,900

[1] Reflects legal expenses and loss contingencies incurred during the years ended December 31, 2017.