Stockholders' Equity - Fair Value Assumptions for Options Granted (Details) - 6 months ended Jun. 30, 2015 - $ / shares	Total
April 21, 2015 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free Interest Rate	1.30%
Expected Life of Option in Years	5 years
Expected Dividend Yield	2.70%
Expected Stock Volatility	26.84%
Estimated Fair Value of Stock Option	$ 7.35
April 22, 2014 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free Interest Rate	1.80%
Expected Life of Option in Years	5 years
Expected Dividend Yield	2.00%
Expected Stock Volatility	28.55%
Estimated Fair Value of Stock Option	$ 9.57
April 16, 2013 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free Interest Rate	0.70%
Expected Life of Option in Years	5 years
Expected Dividend Yield	1.60%
Expected Stock Volatility	37.42%
Estimated Fair Value of Stock Option	$ 12.66
April 17, 2012 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free Interest Rate	0.90%
Expected Life of Option in Years	5 years
Expected Dividend Yield	1.40%
Expected Stock Volatility	39.25%
Estimated Fair Value of Stock Option	$ 13.69
April 19, 2011 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free Interest Rate	2.10%
Expected Life of Option in Years	5 years
Expected Dividend Yield	1.60%
Expected Stock Volatility	39.33%
Estimated Fair Value of Stock Option	$ 11.20
April 20, 2010 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free Interest Rate	2.60%
Expected Life of Option in Years	5 years
Expected Dividend Yield	1.50%
Expected Stock Volatility	39.10%
Estimated Fair Value of Stock Option	$ 8.14
April 21, 2009 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free Interest Rate	1.90%
Expected Life of Option in Years	5 years
Expected Dividend Yield	1.00%
Expected Stock Volatility	38.80%
Estimated Fair Value of Stock Option	$ 3.64
April 15, 2008 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free Interest Rate	2.70%
Expected Life of Option in Years	5 years
Expected Dividend Yield	1.00%
Expected Stock Volatility	30.93%
Estimated Fair Value of Stock Option	$ 7.75
April 17, 2007 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free Interest Rate	4.60%
Expected Life of Option in Years	4 years 10 months 6 days
Expected Dividend Yield	1.00%
Expected Stock Volatility	31.59%
Estimated Fair Value of Stock Option	$ 5.63

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (f)(2)(iii)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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

- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (f)(2)(ii)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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

- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (f)(2)(iv)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

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

- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (d)(1)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 14.D.2)

-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (f)(2)(i)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 14

-Section D

-Subsection 2

+ Details

Name:	us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_AwardDateAxis=fast_AprilTwentyFirstTwoThousandFifteenMemberDomain
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=fast_AprilTwentySecondTwoThousandFourteenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=fast_AprilSixteenthTwoThousandThirteenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=fast_AprilSeventeenthTwoThousandTwelveMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=fast_AprilNineteenthTwoThousandElevenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=fast_AprilTwentiethTwoThousandTenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=fast_AprilTwentyFirstTwoThousandNineMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=fast_AprilFifteenthTwoThousandEightMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardDateAxis=fast_AprilSeventeenthTwoThousandSevenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: