Finance lease obligation period.

No definition available.

Name:	saia_FinanceLeaseObligationPeriod
Namespace Prefix:	saia_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Line of credit facility additional borrowing capacity.

+ References

No definition available.

+ Details

Name:	saia_LineOfCreditFacilityAdditionalBorrowingCapacity
Namespace Prefix:	saia_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line of credit facility amended expiration month and year.

+ References

No definition available.

+ Details

Name:	saia_LineOfCreditFacilityAmendedExpirationMonthAndYear
Namespace Prefix:	saia_
Data Type:	xbrli:gYearMonthItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum leverage ratio.

+ References

No definition available.

+ Details

Name:	saia_MaximumLeverageRatio
Namespace Prefix:	saia_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Minimum debt service coverage ratio.

+ References

No definition available.

+ Details

Name:	saia_MinimumDebtServiceCoverageRatio
Namespace Prefix:	saia_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

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

Present value of lessee's discounted obligation for lease payments from finance lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=121609121&loc=SL77918701-209980

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121603541&loc=SL77918627-209977

+ Details

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

- Definition

Amount, after accumulated amortization, of right-of-use asset from finance lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121603541&loc=SL77918627-209977

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

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

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 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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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

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031897-161870

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-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

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-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Definition

Weighted average interest rate of long-term debt outstanding calculated at point in time.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_PropertyPlantAndEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=saia_FinanceLeaseObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=saia_ExistingCreditAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Balance Type:
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- Details

Name:	srt_RangeAxis=srt_MaximumMember
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Data Type:	na
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Period Type:

- Details

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- Details

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Debt and Financing Arrangements - Additional Information (Detail) - USD ($)		3 Months Ended
	Feb. 05, 2019	Mar. 31, 2021	Dec. 31, 2020	Feb. 04, 2019
Debt Instrument [Line Items]
Amendment line of credit facility expiration year and month	2024-02
Letter of credit facility outstanding amount		$ 1,800,000
Liabilities recognized under finance leases		$ 66,017,000	$ 70,976,000
Lease expiration period		7 years
Property and Equipment [Member]
Debt Instrument [Line Items]
Finance leased assets, net		$ 97,300,000	$ 100,100,000
Finance Lease Obligations [Member]
Debt Instrument [Line Items]
Weighted average interest rates for finance lease		3.50%	3.50%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Borrowing capacity under credit agreement	$ 300,000,000			$ 250,000,000
Additional borrowing capacity under revolving credit facility	$ 100,000,000
Minimum debt service coverage ratio	125.00%
Maximum leverage ratio	325.00%
Revolving Credit Facility [Member] \| Existing Credit Agreement [Member]
Debt Instrument [Line Items]
Borrowing capacity under credit agreement		$ 0	$ 0
Letter of credit facility outstanding amount		$ 28,800,000	$ 27,200,000
Revolving Credit Facility [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Rate margin	1.00%
Letter of credit unused portion fee	0.175%
Revolving Credit Facility [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Rate margin	2.00%
Letter of credit unused portion fee	0.30%
Revolving Credit Facility [Member] \| LIBOR Rate Margin [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Rate margin	1.00%
Revolving Credit Facility [Member] \| LIBOR Rate Margin [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Rate margin	2.00%
Revolving Credit Facility [Member] \| Base Rate Margin [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Rate margin	0.50%
Revolving Credit Facility [Member] \| Base Rate Margin [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Rate margin	0.50%