Financial Instruments - Additional Information (Detail) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Investments, All Other Investments [Abstract]
Cash held by acting commodity futures brokers administrating open contracts	$ 75,208,450	$ 120,025,451

This number represents total cash plus exchange-traded net unrealized gain(loss).

No definition available.

Name:	dwsf_RestrictedCashHeldByActingCommodityFuturesBroker
Namespace Prefix:	dwsf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant