Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Payable for LME contracts		$ 22,459,170
LME contracts	$ 2,261,914
Average charges paid to broker per round-turn trade, maximum	$ 9.00	$ 9.00	$ 10.00

The carrying amount of liabilities as of the balance sheet date that pertain to principal and customer trading transactions, or which may be incurred with the objective of generating a profit from short-term fluctuations in price as part of an entity's market-making, hedging and proprietary trading. Examples include short positions in securities, derivatives and commodities, obligations under repurchase agreements, and securities borrowed arrangements.

No definition available.

Name:	us-gaap_TradingLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant