Fair Value Measurement - Statement Of Income Classification (Details) - USD ($) $ in Thousands |
3 Months Ended | 12 Months Ended | |
---|---|---|---|
Mar. 31, 2019 |
Mar. 31, 2018 |
Dec. 31, 2018 |
|
Fair Value Statement Of Income Classification [Line Items] | |||
Derivative gain (loss) | $ 900 | $ (100) | |
Sales | |||
Fair Value Statement Of Income Classification [Line Items] | |||
Derivative gain (loss) | 1,837 | $ 571 | |
Other Operating Income (Expense) | |||
Fair Value Statement Of Income Classification [Line Items] | |||
Gold and silver rounds/bullion gain | $ 15 | $ 16 |