Revenue Recognition (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Disaggregation of Revenue [Line Items]
Third-party lessee payment lag for oil and natural gas delivery			90 days
Change in prior period estimate	$ 2.3	$ 0.0	$ 0.0	$ 0.0
Reimbursable costs revenue	2.2	1.0	3.6	1.1
Production performance obligations satisfied in prior period, amount recognized	0.0	1.4	0.0	1.4
Revenue recognized in contract liability	0.2	$ 0.2	0.4	$ 0.4
Contract assets recognized	0.0		0.0
Thacker Pass
Disaggregation of Revenue [Line Items]
Advance paid	3.5		3.5
Cost reimbursement, commercial mining milestone, fee	$ 4.7		$ 4.7
Minimum
Disaggregation of Revenue [Line Items]
Primary term of contract			3 years
Maximum
Disaggregation of Revenue [Line Items]
Primary term of contract			5 years
Maximum | Thacker Pass
Disaggregation of Revenue [Line Items]
Capital expenditures			$ 50.0