XML 49 R31.htm IDEA: XBRL DOCUMENT
v3.8.0.1

Summary of Significant Accounting Policies (Tables) | 12 Months Ended
--- | ---
Dec. 31, 2017
Accounting Policies [Abstract] |
Summary Of Activity Recorded Within The Allowance For Doubtful Accounts | The following table summarizes activity recorded within the allowance for doubtful accounts balances for the years ended December 31 (in thousands):
| 2017 |  | 2016 |  | 2015
Beginning balance | $ | 5,272 |  |  | $ | 4,868 |  |  | $ | 4,280 |
Bad debt expense | 1,253 |  |  | 2,519 |  |  | 1,404 |
Accounts written off and other adjustments | (91 | ) |  | (2,115 | ) |  | (816 | )
Ending balance | $ | 6,434 |  |  | $ | 5,272 |  |  | $ | 4,868 |
Summary Of Interest Capitalized And Depreciation Expense | The table below sets forth the amount of interest capitalized and depreciation expense recognized during the years ended December 31 (in thousands):
| 2017 |  | 2016 |  | 2015
Capitalized interest | $ | 137 |  |  | $ | 138 |  |  | $ | 166 |
Depreciation expense | $ | 12,929 |  |  | $ | 14,477 |  |  | $ | 17,869 |

X
- Definition Summary of interest capitalized and depreciation expense. + References No definition available. + Details
Name: | rock_SummaryOfInterestCapitalizedAndDepreciationExpenseTableTextBlock
Namespace Prefix: | rock_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_AccountingPoliciesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Tabular disclosure of credit losses for current financing receivables (such as trade and accounts receivable), including a rollforward of the balance. An alternative caption includes allowance for doubtful accounts. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524 + Details
Name: | us-gaap_ScheduleOfCreditLossesForFinancingReceivablesCurrentTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration