Mineral Properties - Rollforward (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($)
Balance at beginning of period	$ 43,212
Amortization	(657)
Balance at end of period	42,555
Lost Creek Project
Balance at beginning of period	10,184
Amortization	(657)
Balance at end of period	9,527
Pathfinder properties
Balance at beginning of period	19,850
Balance at end of period	19,850
Other U.S. Properties [Member]
Balance at beginning of period	13,178
Balance at end of period	$ 13,178