Statements of Cash Flows (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash Flows from Operating Activities:
Net income (loss)	$ (18,806,073)us-gaap_NetIncomeLoss	$ 5,578,244us-gaap_NetIncomeLoss	$ (10,398,778)us-gaap_NetIncomeLoss
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
(Increase) decrease in commodity futures trading account - cash and cash equivalents	(16,237,347)	7,985,297	30,603,108
Unrealized (gain) loss on futures contracts	18,738,510	(85,540)	2,445,210
(Increase) decrease in dividends receivable	578	1,451	(753)
(Increase) decrease in interest receivable	0	0	14
(Increase) decrease in prepaid registration fees	0	81,536	44,043
(Increase) decrease in ETF transaction fees receivable	0	(350)	0
Increase (decrease) in investment payable	(2,141,524)	2,141,524	(33)
Increase (decrease) in General Partner management fees payable	(14,893)	(17,031)	(38,796)
Increase (decrease) in professional fees payable	(61,346)	(11,128)	(104,591)
Increase (decrease) in brokerage commissions payable	(200)	(1,430)	(2,380)
Increase (decrease) in directors' fees and insurance payable	(1,046)	79	(1,619)
Increase (decrease) in license fees payable	(1,075)	(1,227)	(4,074)
Increase (decrease) in registration fees payable	(153)	0	0
Net cash provided by (used in) operating activities	(18,524,569)	15,671,425	22,541,351
Cash Flows from Financing Activities:
Addition of partnership shares	5,507,498	26,960,147	101,599,752
Redemption of partnership shares	(12,902,604)usl_PaymentsToPartnershipRedemption	(69,955,807)usl_PaymentsToPartnershipRedemption	(161,169,072)usl_PaymentsToPartnershipRedemption
Net cash provided by (used in) financing activities	(7,395,106)	(42,995,660)	(59,569,320)
Net Increase (Decrease) in Cash and Cash Equivalents	(25,919,675)	(27,324,235)	(37,027,969)
Cash and Cash Equivalents, beginning of year	59,384,208	86,708,443	123,736,412
Cash and Cash Equivalents, end of year	$ 33,464,533	$ 59,384,208	$ 86,708,443

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.