Fair Value Measurements - Summary of Gain (Loss) Derivative Instruments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value adjustments, net	$ (5,296)	$ (2,462)	$ 3,824	$ 2,192
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized Gain (Loss) on Securities	(5,548)	(8,028)	3,637	(3,185)
Realized gain (loss) on equity securities	384	5,535	375	5,202
Zinc Options [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net derivative gain (loss)	0	219	0
Interest rate swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value adjustments, net	$ (132)	$ (188)	$ (188)	$ (188)