Related Party Transactions - Additional Information (Detail) - Morgan Stanley [Member]	9 Months Ended
Sep. 30, 2015
Related Party Transaction [Line Items]
Percentage of interest income earned on any excess cash invested in permitted investments for partnership	100.00%
Percentage of interest paid on average daily equity maintained in cash in partnership account	100.00%
Percentage of monthly average of 4-week U.S. Treasury bill discount rate	80.00%

Percentage of average daily cash balance held on deposit with a related party to meet margin requirements used in a calculation of interest income.

No definition available.

Name:	dwsf_AverageDailyHoldingsPercentageInterestIncomeBasisRelatedPartyHoldingsForMarginRequirement
Namespace Prefix:	dwsf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration