Acquisitions - Northern Empire (Details) - Northern Empire Resources Corp. shares in Thousands	1 Months Ended
	Oct. 31, 2018 USD ($) shares	Oct. 31, 2017 USD ($) shares	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Common shares issued (value)	$ 63,887,000
Fair value of existing investment in Northern Empire	7,257,000
Transaction advisory fees and other acquisition costs			$ 2,449,000
Purchase price	$ 73,593,000
Assets:
Cash			5,309,000
Receivables and other assets			465,000
Restricted assets			4,300,000
Property, plant, and equipment			1,351,000
Mining properties, net			99,515,000
Deferred income taxes			587,000
Total assets acquired			111,527,000
Liabilities:
Accounts payable and accrued liabilities			10,895,000
Reclamation liabilities			2,659,000
Deferred income taxes			24,380,000
Total liabilities assumed			37,934,000
Net assets acquired			$ 73,593,000
Common Stock
Business Acquisition [Line Items]
Common stock conversion ratio	0.1850
Common shares issued (in shares) | shares	12,100	12,122,683
Price per common share issued (UDS per share)		$ 5,270