Property and Equipment, Net (Narrative) (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) $ / Boe	Jun. 30, 2018 USD ($) $ / Boe	Jun. 30, 2019 USD ($) $ / Boe	Jun. 30, 2018 USD ($) $ / Boe
Property, Plant and Equipment [Line Items]
Average depreciation, depletion and amortization, per Boe | $ / Boe	13.28	13.74	13.28	13.73
Internal costs capitalized, Oil and Gas producing activities	$ 3.8	$ 6.1	$ 12.9	$ 12.7
Capitalized interest	$ 8.6	$ 8.7	$ 17.6	$ 19.1