Property and Equipment, Net (Summary of Average Realized Price of Crude Oil) (Details)		3 Months Ended				12 Months Ended
	Mar. 31, 2021 $ / Boe	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) $ / Boe	Dec. 31, 2019 USD ($) $ / Boe	Dec. 31, 2018 USD ($) $ / Boe	Dec. 31, 2017 $ / Boe
Property, Plant and Equipment [Line Items]
Impairment of evaluated oil and gas properties | $			$ 585,800,000	$ 685,000,000.0	$ 1,300,000,000	$ 2,547,241,000	$ 0	$ 0
Average 12-month price, net of differentials						37.44	53.90	58.40	49.48
Percent increase (decrease) in 12-Month Average Realized Price						(31.00%)	(8.00%)	18.00%
Forecast
Property, Plant and Equipment [Line Items]
Impairment of evaluated oil and gas properties | $		$ 100,000,000.0
Average 12-month price, net of differentials	40.23
Commodity - Oil
Property, Plant and Equipment [Line Items]
Average 12-month price, net of differentials						37.44	53.90	58.40