Debt Commitments - Unsecured Revolving Credit Facility (Details) - Credit Facility	3 Months Ended
Mar. 31, 2023 USD ($)
Minimum
Debt Instrument [Line Items]
Percentage fee paid for unused portion of credit facility	0.10%
Maximum
Debt Instrument [Line Items]
Percentage fee paid for unused portion of credit facility	0.125%
Daily Simple SOFR
Debt Instrument [Line Items]
Per annum interest rate over Daily Simple SOFR, spread adjustment	0.10%
Per annum interest rate over Daily Simple SOFR	0.95%
Unsecured Revolving Credit Facility
Debt Instrument [Line Items]
Credit facility, maximum borrowing capacity	$ 835,000,000.0
Credit facility, uncommitted accordion option, increase limit	365,000,000.0
Credit facility, uncommitted accordion option, higher borrowing capacity option	1,200,000,000
Letter of Credit Subfacility
Debt Instrument [Line Items]
Credit facility, maximum borrowing capacity	$ 55,000,000.0

Debt Instrument, Basis Spread On Variable Rate, Spread Adjustment

No definition available.

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Namespace Prefix:	fast_
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Balance Type:	na
Period Type:	duration

- Definition

Line of Credit Facility, Uncommitted Accordion Option, Higher Borrowing Capacity Option

+ References

No definition available.

+ Details

Name:	fast_LineOfCreditFacilityUncommittedAccordionOptionHigherBorrowingCapacityOption
Namespace Prefix:	fast_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line of Credit Facility, Uncommitted Accordion Option, Increase Limit

+ References

No definition available.

+ Details

Name:	fast_LineOfCreditFacilityUncommittedAccordionOptionIncreaseLimit
Namespace Prefix:	fast_
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

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types: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

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 https://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

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Details

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- Details

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- Details

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- Details

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Namespace Prefix:
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- Details

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Namespace Prefix:
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- Details

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