Fair Value Measurements - Summary of Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets:
Assets	$ 35,088	$ 4,488
Liabilities:
Total liabilities	48,187	10,049
Other derivative instruments, net
Assets:
Other derivative instruments, net	251
Liabilities:
Fair value of derivative liability	222	762
Silvertip Mine
Liabilities:
Fair value of derivative liability	47,965
Rochester Royalty Obligation
Liabilities:
Fair value of derivative liability		9,287
Equity and debt securities
Assets:
Equity and debt securities	28,245	4,488
Level 1
Assets:
Assets	27,946	4,209
Liabilities:
Total liabilities	0	0
Level 1 | Other derivative instruments, net
Assets:
Other derivative instruments, net	0
Liabilities:
Fair value of derivative liability	0	0
Level 1 | Silvertip Mine
Liabilities:
Fair value of derivative liability	0
Level 1 | Rochester Royalty Obligation
Liabilities:
Fair value of derivative liability		0
Level 1 | Equity and debt securities
Assets:
Equity and debt securities	27,946	4,209
Level 2
Assets:
Assets	251	0
Liabilities:
Total liabilities	222	762
Level 2 | Other derivative instruments, net
Assets:
Other derivative instruments, net	251
Liabilities:
Fair value of derivative liability	222	762
Level 2 | Silvertip Mine
Liabilities:
Fair value of derivative liability	0
Level 2 | Rochester Royalty Obligation
Liabilities:
Fair value of derivative liability		0
Level 2 | Equity and debt securities
Assets:
Equity and debt securities	0	0
Level 3
Assets:
Assets	6,891	279
Liabilities:
Total liabilities	47,965	9,287
Level 3 | Other derivative instruments, net
Assets:
Other derivative instruments, net	0
Liabilities:
Fair value of derivative liability	0	0
Level 3 | Rochester Royalty Obligation
Liabilities:
Fair value of derivative liability		9,287
Level 3 | Equity and debt securities
Assets:
Equity and debt securities	6,891	279
Silvertip Mine
Liabilities:
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	$ 47,965	$ 0