Percentage points added to the reference rate to compute the variable rate on the debt instrument.

No definition available.

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtWeightedAverageInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

Name:	us-gaap_LineOfCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The percentage of borrowing availability of the maximum revolver amount.

+ References

No definition available.

+ Details

Name:	usak_AvailabilityPercentageOfMaximumRevolverAmount
Namespace Prefix:	usak_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the borrowing base of a credit facility before any additions pertaining to eligible revenue equipment.

+ References

No definition available.

+ Details

Name:	usak_BorrowingBaseBeforeAdditionsOfEligibleRevenueEquipment
Namespace Prefix:	usak_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the percentage of eligible investment grade accounts receivable elements for calculating the maximum amount of the credit facility.

+ References

No definition available.

+ Details

Name:	usak_BorrowingBasedThresholdEligibleInvestmentGradeAccountsReceivablePercentage
Namespace Prefix:	usak_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the percentage of eligible revenue equipment elements for calculating the maximum amount of the credit facility.

+ References

No definition available.

+ Details

Name:	usak_BorrowingBasedThresholdEligibleRevenueEquipmentPercentage
Namespace Prefix:	usak_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the percentage of eligible unbilled accounts receivable elements for calculating the maximum amount of the credit facility.

+ References

No definition available.

+ Details

Name:	usak_BorrowingBasedThresholdEligibleUnbilledAccountsReceivablePercentage
Namespace Prefix:	usak_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the percentage of newly acquired revenue equipment elements for calculating the maximum amount of the credit facility.

+ References

No definition available.

+ Details

Name:	usak_BorrowingBasedThresholdNewlyAcquiredRevenueEquipmentPercentage
Namespace Prefix:	usak_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Description of consolidated fixed charge coverage and achievement of certain financial ratios.

+ References

No definition available.

+ Details

Name:	usak_DebtInstrumentCovenantFixedChargeCoverageRatio
Namespace Prefix:	usak_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Additional borrowing capacity of the line of credit facility.

+ References

No definition available.

+ Details

Name:	usak_LineOfCreditFacilityAdditionalBorrowingCapacity
Namespace Prefix:	usak_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Represents information about the additional borrowing capacity amount.

+ References

No definition available.

+ Details

Name:	usak_LineOfCreditFacilityAdditionalBorrowingCapacityIncrementalAmount
Namespace Prefix:	usak_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The minimum excess availability requirement as a percentage of the maximum revolver amount.

+ References

No definition available.

+ Details

Name:	usak_MinimumExcessAvailabilityPercentageOfMaximumRevolverAmount
Namespace Prefix:	usak_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=usak_LetterOfCreditSubFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=usak_SwingLineSubFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_BaseRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Long-term Debt (Details Textual) - USD ($)		1 Months Ended
Long-term Debt (Details Textual) - USD ($)	Feb. 28, 2015	Feb. 28, 2015	Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Borrowing based threshold for eligible unbilled accounts receivable		85.00%
Remaining borrowing capacity			$ 76,200,000
Weighted average interest rate			3.65%
Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 170,000,000.0	$ 170,000,000.0
Additional borrowing capacity	$ 80,000,000.0
Additional borrowing capacity, incremental amount		$ 20,000,000.0
Debt instrument, term	5 years
Commitment fee percentage		0.25%
Borrowing based threshold eligible investment grade accounts receivable percentage		85.00%
Borrowing base before additions of eligible revenue equipment	$ 10,000,000.0	$ 10,000,000.0
Newly acquired revenue equipment, percentage		85.00%
Eligible revenue equipment, percentage		85.00%
Fixed charge coverage ratio		1.0
Minimum excess availability percentage of maximum revolver amount	10.00%	10.00%
Percentage of maximum revolver amount			20.00%
Remaining borrowing capacity			$ 34,000,000
Long-term line of credit			46,000,000	$ 61,225,000
Letters of credit outstanding, amount			$ 5,400,000
Letter of Credit Sub Facility
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 15,000,000.0	$ 15,000,000.0
Swing Line Sub Facility
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 20,000,000.0	$ 20,000,000.0
Minimum \| Revolving Credit Facility
Debt Instrument [Line Items]
Borrowing based threshold eligible investment grade accounts receivable percentage		85.00%
Maximum \| Revolving Credit Facility
Debt Instrument [Line Items]
Borrowing based threshold eligible investment grade accounts receivable percentage		90.00%
Base Rate \| Minimum \| Revolving Credit Facility
Debt Instrument [Line Items]
Basis spread on variable rate		0.25%
Base Rate \| Maximum \| Revolving Credit Facility
Debt Instrument [Line Items]
Basis spread on variable rate		1.00%
London Interbank Offered Rate (LIBOR) \| Minimum \| Revolving Credit Facility
Debt Instrument [Line Items]
Basis spread on variable rate		1.25%
London Interbank Offered Rate (LIBOR) \| Maximum \| Revolving Credit Facility
Debt Instrument [Line Items]
Basis spread on variable rate		2.00%