Financial Highlights (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Financial Highlights [Line Items]
Units (in shares)	28,866,793	29,562,144
Net Assets	$ 106,651,649	$ 100,673,266	$ 99,791,032	$ 96,815,891
Company Net Assets
Financial Highlights [Line Items]
Net Assets	$ 25,788,000	$ 23,442,000	$ 24,688,000	$ 25,731,000	$ 24,255,000
Company Units
Financial Highlights [Line Items]
Units (in shares)	7,069,000	6,971,000	7,828,000	8,883,000	8,902,000
Units
Financial Highlights [Line Items]
Units (in shares)	21,798,000	22,591,000	23,400,000	24,124,000	24,958,000
Net Assets
Financial Highlights [Line Items]
Net Assets	$ 80,863,000	$ 77,231,000	$ 75,103,000	$ 71,085,000	$ 69,236,000
Investment Income Ratio
Financial Highlights [Line Items]
Investment Income Ratio	3.16%	3.68%	5.04%	4.84%	4.79%
Minimum | Units
Financial Highlights [Line Items]
Units Value (in dollars per share)	$ 3.07375	$ 2.85193	$ 2.69505	$ 2.49097	$ 2.36048
Minimum | Units | Scenario, Previously Reported
Financial Highlights [Line Items]
Units Value (in dollars per share)				$ 2.30961
Minimum | Expense Ratio
Financial Highlights [Line Items]
Expense Ratio	0.35%	0.35%	0.35%	0.35%	0.35%
Minimum | Total Return
Financial Highlights [Line Items]
Total Return	7.78%	5.82%	8.19%	5.53%	12.33%
Maximum | Units
Financial Highlights [Line Items]
Units Value (in dollars per share)	$ 3.99015	$ 3.66932	$ 3.43650	$ 3.14796	$ 2.95634
Maximum | Units | Scenario, Previously Reported
Financial Highlights [Line Items]
Units Value (in dollars per share)				$ 2.63416
Maximum | Expense Ratio
Financial Highlights [Line Items]
Expense Ratio	1.25%	1.25%	1.25%	1.25%	1.25%
Maximum | Total Return
Financial Highlights [Line Items]
Total Return	8.74%	6.77%	9.17%	6.48%	13.33%