Supplemental Gas and Oil Information (Unaudited) Standardized Measure Of Future Net Cash Flows (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]
Future cash inflows	$ 9,657.0	$ 14,447.6	$ 26,482.6
Future production costs	(4,728.9)	(6,070.6)	(9,539.9)
Future development costs	(1,671.0)	(2,275.2)	(4,441.5)
Future income tax expenses	(294.8)	(845.8)	(2,553.6)
Future net cash flows	2,962.3	5,256.0	9,947.6
10% annual discount for estimated timing of net cash flows	(1,427.0)	(2,579.7)	(4,991.9)
Standardized measure of discounted future net cash flows	$ 1,535.3	$ 2,676.3	$ 4,955.7	$ 3,097.3