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Employee Benefit Plans - Summary of Weighted-Average Assumptions for Disclosure (Detail) - Supplemental Employee Retirement Plan [Member] | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Weighted-average assumptions for Disclosure |  |
Discount rate | 2.10% | 2.95%
Mortality table | Pri-2012 WC | Pri-2012
mortality improvement scale | MP-2020 | MP-2019

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- Definition Defined benefit plan description of mortality. + References No definition available. + Details
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Balance Type: | na
Period Type: | duration

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- Definition Defined benefit plan description of mortality improvement scale. + References No definition available. + Details
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Namespace Prefix: | bbgi_
Data Type: | xbrli:stringItemType
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- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- References No definition available. + Details
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |