Debt Commitments - Senior Unsecured Promissory Notes Payable (Details) - Senior Unsecured Promissory Notes	Jul. 20, 2016 USD ($)	Jun. 30, 2018 USD ($) debt_instrument_series
Debt Instrument [Line Items]
Debt issuance period after effective date	3 years
Maximum aggregate principal amount outstanding	$ 200,000,000
Number of debt instrument series \| debt_instrument_series		3
Fixed Rate Notes Payable
Debt Instrument [Line Items]
Term of debt (no later than 12 years)	12 years
Floating Rate Notes Payable
Debt Instrument [Line Items]
Term of debt (no later than 12 years)	10 years
2.00% Senior Unsecured Promissory Note Payable
Debt Instrument [Line Items]
Debt issuance, aggregate principal amount		$ 40,000,000
Fixed interest rate per annum		2.00%
2.45% Senior Unsecured Promissory Note Payable
Debt Instrument [Line Items]
Debt issuance, aggregate principal amount		$ 35,000,000
Fixed interest rate per annum		2.45%
3.22% Senior Unsecured Promissory Note Payable
Debt Instrument [Line Items]
Debt issuance, aggregate principal amount		$ 60,000,000
Fixed interest rate per annum		3.22%

Debt Instrument, Debt Issuance Allowable Period

No definition available.

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

- Definition

Debt Instrument, Maximum Face Amount Allowable

+ References

No definition available.

+ Details

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

- Definition

Debt Instrument, Number of Debt Instrument Series

+ References

No definition available.

+ Details

Name:	fast_DebtInstrumentNumberofDebtInstrumentSeries
Namespace Prefix:	fast_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
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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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

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Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

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- Details

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- Details

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Namespace Prefix:
Data Type:	na
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- Details

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- Details

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- Details

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