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Fair Value Measurements - Summary of Gain (Loss) Derivative Instruments (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fair value adjustments, net $ 4,377 $ 715 $ 8,201 $ 2,907
Equity Securities [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Unrealized Gain (Loss) on Securities 3,877 286 7,515 (2,898)
Realized gain (loss) on equity securities 485 (3) 859 5,199
Zinc Options [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Net derivative gain (loss) 0 226 0 588
Interest rate swap        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fair value adjustments, net $ 15 $ 206 $ (173) $ 18