Supplemental Gas and Oil Information (Unaudited) Results of Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Results of Operations for Oil and Gas Producing Activities, by Geographic Area [Line Items]
Income (loss) before income taxes	$ (140.3)	$ (1,329.0)	$ (42.9)
Exploration and Production [Member]
Results of Operations for Oil and Gas Producing Activities, by Geographic Area [Line Items]
Revenues	1,200.6	1,920.3	1,548.1
Production costs	361.9	507.3	675.4
Exploration expenses	0.1	0.3	22.0
Depreciation, depletion and amortization	528.5	836.4	735.1
Impairment	0.0	1,560.9	72.3
Total expenses	890.5	2,904.9	1,504.8
Income (loss) before income taxes	310.1	(984.6)	43.3
Income tax benefit (expense)	(69.5)	243.2	(16.0)
Results of operations from producing activities excluding allocated corporate overhead and interest expenses	$ 240.6	$ (741.4)	$ 27.3