XML 14 R54.htm IDEA: XBRL DOCUMENT v2.4.1.9
Equity-Based Compensation (Schedule of Compensation Expense Recognized from Change in Fair Value and Vesting of Performance Stock Units) (Details) (Performance Stock Units, USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Performance Stock Units
    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Performance stock unit compensation expense $ 581us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_StatementEquityComponentsAxis
= rock_PerformanceStockUnitsMember		 $ 1,137us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_StatementEquityComponentsAxis
= rock_PerformanceStockUnitsMember