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Note 9 - Business Segment Information - Components of Customer Sales Determined Based on the Location of Customers (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues	$ 379,389	$ 382,071	$ 406,150
Revenues, Percent	100.00%	100.00%	100.00%
UNITED STATES
Revenues	$ 241,746	$ 250,872	$ 269,628
Revenues, Percent	64.00%	66.00%	66.00%
Foreign Countries [Member]
Revenues	$ 137,643	$ 131,199	$ 136,522
Revenues, Percent	36.00%	34.00%	34.00%

X

- Definition

The percentage of revenues by location.

+ References

No definition available.

+ Details

Name:	grc_PercentageRevenueByLocation
Namespace Prefix:	grc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Total revenue from sale of goods and services rendered during the reporting period, in the normal course of business, reduced by sales returns and allowances, and sales discounts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.1)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_SalesRevenueNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Details

Name:	us-gaap_StatementGeographicalAxis=country_US
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_StatementGeographicalAxis=grc_ForeignCountriesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: