Capital lease obligation period.

No definition available.

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

- Definition

Debt instrument maximum authorized value.

+ References

No definition available.

+ Details

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

- Definition

Line of credit facility additional borrowing capacity.

+ References

No definition available.

+ Details

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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

Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.

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

- 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

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
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+ Details

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

- 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

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

- 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

+ 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

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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
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-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
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+ Details

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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

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

- Details

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Balance Type:
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Balance Type:
Period Type:

- Details

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Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
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Balance Type:
Period Type:

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Debt and Financing Arrangements - Additional Information (Detail) - USD ($)		6 Months Ended
	Mar. 06, 2015	Jun. 30, 2018	Dec. 31, 2017	Jan. 31, 2008	Nov. 30, 2007	Apr. 30, 2005	Dec. 31, 2002
Debt Instrument [Line Items]
Letter of credit facility outstanding amount		$ 1,800,000
Capital Leases, described below		$ 105,000,000	$ 89,916,000
Lease expiration period		7 years
Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Weighted average interest rates for capital lease		3.33%	3.07%
Restated Credit Agreement [Member]
Debt Instrument [Line Items]
Amendment line of credit facility expiration year and month	2020-03
Restated Master Shelf Agreement [Member] \| Senior Notes under Master Shelf Agreement [Member]
Debt Instrument [Line Items]
Senior notes issued, value				$ 25,000,000	$ 25,000,000		$ 100,000,000
Senior notes authorized amount						$ 150,000,000	$ 125,000,000
Revolving Credit Facility [Member] \| Restated Credit Agreement [Member]
Debt Instrument [Line Items]
Borrowing capacity under credit agreement	$ 250,000,000
Additional borrowing capacity under revolving credit facility		$ 75,000,000
Credit Agreement with Banks		50,000,000	$ 43,000,000
Letter of credit facility outstanding amount		$ 27,700,000	$ 33,900,000
Revolving Credit Facility [Member] \| Restated Credit Agreement [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Rate margin		1.125%
Letter of credit unused portion fee		0.20%
Revolving Credit Facility [Member] \| Restated Credit Agreement [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Rate margin		2.25%
Letter of credit unused portion fee		0.30%
Revolving Credit Facility [Member] \| Restated Credit Agreement [Member] \| LIBOR Rate Margin [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Rate margin		1.125%
Revolving Credit Facility [Member] \| Restated Credit Agreement [Member] \| LIBOR Rate Margin [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Rate margin		2.25%
Revolving Credit Facility [Member] \| Restated Credit Agreement [Member] \| Base Rate Margin [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Rate margin		0.125%
Revolving Credit Facility [Member] \| Restated Credit Agreement [Member] \| Base Rate Margin [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Rate margin		0.50%