Consolidated Statement of Shareholders' Equity (Parenthetical) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Statement of Stockholders' Equity [Abstract]
Adjustment to pension benefit liability, taxes	$ 4	$ 10

Other Comprehensive Income Minimum Pension Liability Net Adjustment Tax 1

No definition available.

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Namespace Prefix:	rock_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration