Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Assets:
Fair value of equity and debt securities	$ 15,086
Assets	15,752	$ 36,399
Liabilities:
Total liabilities		25,275
Other derivative instruments, net
Assets:
Fair value of other derivative instruments, net	666	753
Liabilities:
Fair value of derivative liability	0	275
Silvertip Mine
Liabilities:
Fair value of derivative liability		25,000
Equity and debt securities
Assets:
Fair value of equity and debt securities		35,646
Level 1
Assets:
Assets	15,086	35,646
Liabilities:
Total liabilities		0
Level 1 | Other derivative instruments, net
Assets:
Fair value of other derivative instruments, net	0	0
Liabilities:
Fair value of derivative liability	0	0
Level 1 | Silvertip Mine
Liabilities:
Fair value of derivative liability		0
Level 1 | Equity and debt securities
Assets:
Fair value of equity and debt securities	15,086	35,646
Level 2
Assets:
Assets	666	753
Liabilities:
Total liabilities		275
Level 2 | Other derivative instruments, net
Assets:
Fair value of other derivative instruments, net	666	753
Liabilities:
Fair value of derivative liability		275
Level 2 | Silvertip Mine
Liabilities:
Fair value of derivative liability		0
Level 2 | Equity and debt securities
Assets:
Fair value of equity and debt securities	0	0
Level 3
Assets:
Assets	0	0
Liabilities:
Total liabilities		25,000
Level 3   | Other derivative instruments, net
Assets:
Fair value of other derivative instruments, net	0	0
Liabilities:
Fair value of derivative liability	0	0
Level 3   | Equity and debt securities
Assets:
Fair value of equity and debt securities		0
Silvertip Mine | Level 3
Liabilities:
Fair value of derivative liability		$ 25,000
Available-for-sale Securities | Level 3   | Equity and debt securities
Assets:
Fair value of equity and debt securities	$ 0