Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets:
Assets	$ 18,720	$ 35,088
Liabilities:
Total liabilities	49,920	48,187
Other derivative instruments, net
Assets:
Fair value of other derivative instruments, net	914	251
Liabilities:
Fair value of derivative liability	644	222
Silvertip Mine
Liabilities:
Fair value of derivative liability	49,276	47,965
Equity and debt securities
Assets:
Fair value of equity and debt securities	17,806	34,837
Level 1
Assets:
Assets	15,589	27,946
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	15,589	27,946
Level 2
Assets:
Assets	914	251
Liabilities:
Total liabilities	644	222
Level 2 | Other derivative instruments, net
Assets:
Fair value of other derivative instruments, net	914	251
Liabilities:
Fair value of derivative liability	644	222
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	2,217	6,891
Liabilities:
Total liabilities	49,276	47,965
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		6,891
Silvertip Mine | Level 3
Liabilities:
Fair value of derivative liability	49,276	$ 47,965
Available-for-sale Securities | Level 3 | Equity and debt securities
Assets:
Fair value of equity and debt securities	$ 2,217