Related Party Transactions (Details)	6 Months Ended
Jun. 30, 2012
Related Party Transactions [Abstract]
Cash holdings percentage for margin requirements used in interest income calculation (in hundredths)	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