Quarterly Financial Information (Unaudited) (Details) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($) MMBoe $ / shares	Sep. 30, 2019 USD ($) MMBoe $ / shares	Jun. 30, 2019 USD ($) MMBoe $ / shares	Mar. 31, 2019 USD ($) MMBoe $ / shares	Dec. 31, 2018 USD ($) MBoe $ / shares	Sep. 30, 2018 USD ($) MBoe $ / shares	Jun. 30, 2018 USD ($) MBoe $ / shares	Mar. 31, 2018 USD ($) MBoe $ / shares	Dec. 31, 2019 USD ($) MMBoe $ / shares	Dec. 31, 2018 USD ($) MBoe $ / shares	Dec. 31, 2017 USD ($) $ / shares
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 321.9	$ 307.5	$ 296.2	$ 280.6	$ 410.5	$ 560.8	$ 532.4	$ 428.9	$ 1,206.2	$ 1,932.6
Operating income (loss)	48.9	52.1	72.3	(15.8)	(1,116.8)	156.8	(321.8)	21.4	157.5	(1,260.4)	$ 101.5
Net income (loss)	(110.4)	81.0	48.8	(116.7)	(629.3)	7.3	(336.0)	(53.6)	(97.3)	(1,011.6)	$ 269.3
Net gain (loss) from asset sales, inclusive of restructuring costs and impairment	$ 1.4	$ (2.1)	$ 17.8	$ (18.2)	$ (1,158.2)	$ 27.1	$ (407.6)	$ 2.8	$ (1.1)	$ (1,535.9)
Basic | $ / shares	$ (0.46)	$ 0.34	$ 0.20	$ (0.49)	$ (2.66)	$ 0.03	$ (1.42)	$ (0.22)	$ (0.41)	$ (4.25)	$ 1.12
Total equivalent production (Mboe)	8,465,300	8,404,000.0	7,534,700	7,806,300	11,627,200	14,400,000.0	14,106,100	11,724,600	32,210,300	51,857,900
Diluted | $ / shares	$ (0.46)	$ 0.34	$ 0.20	$ (0.49)	$ (2.66)	$ 0.03	$ (1.42)	$ (0.22)	$ (0.41)	$ (4.25)	$ 1.12