Fair Value Measurements and Disclosures, Investment Percentages (Details)	Sep. 30, 2012	Dec. 31, 2011
TT II, LLC [Member]
Other Ownership Interests [Line Items]
Percentage of investment in trading companies (in hundredths)	33.60%	26.80%
WNT I, LLC [Member]
Other Ownership Interests [Line Items]
Percentage of investment in trading companies (in hundredths)	18.85%	17.00%
Kaiser I, LLC [Member]
Other Ownership Interests [Line Items]
Percentage of investment in trading companies (in hundredths)	9.40%	20.05%
Chesapeake I, LLC [Member]
Other Ownership Interests [Line Items]
Percentage of investment in trading companies (in hundredths)	11.75%	9.30%
Rotella I, LLC [Member]
Other Ownership Interests [Line Items]
Percentage of investment in trading companies (in hundredths)	11.25%	9.65%
GLC I, LLC [Member]
Other Ownership Interests [Line Items]
Percentage of investment in trading companies (in hundredths)	8.00%	7.50%
Augustus I, LLC [Member]
Other Ownership Interests [Line Items]
Percentage of investment in trading companies (in hundredths)	7.15%	9.70%