Quarterly Financial Information (Unaudited) (Details) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020 USD ($) MMBoe $ / shares	Sep. 30, 2020 USD ($) MMBoe $ / shares	Jun. 30, 2020 USD ($) MMBoe $ / shares	Mar. 31, 2020 USD ($) MMBoe $ / shares	Dec. 31, 2019 USD ($) MBoe $ / shares	Sep. 30, 2019 USD ($) MBoe $ / shares	Jun. 30, 2019 USD ($) MBoe $ / shares	Mar. 31, 2019 USD ($) MBoe $ / shares	Dec. 31, 2020 USD ($) MMBoe $ / shares	Dec. 31, 2019 USD ($) MBoe $ / shares	Dec. 31, 2018 USD ($) $ / shares
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 200.2	$ 177.8	$ 120.6	$ 225.8	$ 321.9	$ 307.5	$ 296.2	$ 280.6	$ 724.4	$ 1,206.2
Operating income (loss)	(61.9)	(42.1)	(112.5)	(7.2)	48.9	52.1	72.3	(15.8)	(223.7)	157.5	$ (1,260.4)
Net income (loss)	(130.6)	(49.2)	(184.4)	367.4	(110.4)	81.0	48.8	(116.7)	3.2	(97.3)	$ (1,011.6)
Net gain (loss) from asset sales, inclusive of restructuring costs and impairment	$ (11.3)	$ 0.1	$ 0.0	$ 3.7	$ 1.4	$ (2.1)	$ 17.8	$ (18.2)	$ (7.5)	$ (1.1)
Basic | $ / shares	$ (0.54)	$ (0.20)	$ (0.76)	$ 1.54	$ (0.46)	$ 0.34	$ 0.20	$ (0.49)	$ 0.01	$ (0.41)	$ (4.25)
Total equivalent production (Mboe)	7,364,100	7,057,000.0	7,972,900	7,930,900	8,465,300	8,404,000.0	7,534,700	7,806,300	30,324,900	32,210,300
Diluted | $ / shares	$ (0.54)	$ (0.20)	$ (0.76)	$ 1.54	$ (0.46)	$ 0.34	$ 0.20	$ (0.49)	$ 0.01	$ (0.41)	$ (4.25)