Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets:
Assets	$ 24,290	$ 35,088
Liabilities:
Total liabilities	48,918	48,187
Other derivative instruments, net
Assets:
Fair value of other derivative instruments, net	486	251
Liabilities:
Fair value of derivative liability	302	222
Silvertip Mine
Liabilities:
Fair value of derivative liability	48,616	47,965
Equity and debt securities
Assets:
Fair value of equity and debt securities	23,804	34,837
Level 1
Assets:
Assets	17,577	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	17,577	27,946
Level 2
Assets:
Assets	486	251
Liabilities:
Total liabilities	302	222
Level 2 | Other derivative instruments, net
Assets:
Fair value of other derivative instruments, net	486	251
Liabilities:
Fair value of derivative liability	302	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	6,227	6,891
Liabilities:
Total liabilities	48,616	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	48,616	$ 47,965
Available-for-sale Securities | Level 3 | Equity and debt securities
Assets:
Fair value of equity and debt securities	$ 6,227