Exit Activity Costs and Asset Impairments (Narrative) (Details) - facility	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017
Restructuring and Related Activities [Abstract]
Number of facilities closed	1	3

Restructuring and Related Cost, Number of Facilities to be Closed

No definition available.

Name:	rock_RestructuringandRelatedCostNumberofFacilitiestobeClosed
Namespace Prefix:	rock_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration