Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets:
Assets	$ 36,399	$ 18,720
Liabilities:
Total liabilities	25,275	49,920
Other derivative instruments, net
Assets:
Fair value of other derivative instruments, net	753	914
Liabilities:
Fair value of derivative liability	275	644
Silvertip Mine
Liabilities:
Fair value of derivative liability	25,000	49,276
Equity and debt securities
Assets:
Fair value of equity and debt securities		17,806
Level 1
Assets:
Assets	35,646	15,589
Liabilities:
Total liabilities	0	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	0
Level 1 | Equity and debt securities
Assets:
Fair value of equity and debt securities	35,646	15,589
Level 2
Assets:
Assets	753	914
Liabilities:
Total liabilities	275	644
Level 2 | Other derivative instruments, net
Assets:
Fair value of other derivative instruments, net	753	914
Liabilities:
Fair value of derivative liability	275	644
Level 2 | Silvertip Mine
Liabilities:
Fair value of derivative liability	0	0
Level 2 | Equity and debt securities
Assets:
Fair value of equity and debt securities	0	0
Level 3
Assets:
Assets	0	2,217
Liabilities:
Total liabilities	25,000	49,276
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		2,217
Silvertip Mine | Level 3
Liabilities:
Fair value of derivative liability	25,000	$ 49,276
Available-for-sale Securities | Level 3 | Equity and debt securities
Assets:
Fair value of equity and debt securities	$ 0