Long-Term Debt (5.75% Senior Notes Due in 2022) (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 29, 2017	Dec. 31, 2016	Sep. 25, 2012
Debt Instrument [Line Items]
Total debt	$ 1,091.0	$ 938.6
North America
Debt Instrument [Line Items]
Debt issuance costs	(9.5)	(10.6)
North America \| 5.75% Senior Notes due 2022
Debt Instrument [Line Items]
Face value	600.0	600.0
Debt issuance costs	(6.1)	(7.0)
Total debt	593.9	593.0
Fair Value (Level 1)	$ 603.0	$ 579.0
Interest rate	5.75%	5.75%	5.75%
Interest payments	Semi-Annual: Apr 1 & Oct 1
Maturity date	October 2022
Call Option, Beginning Date, year one	Oct. 01, 2017
Call Option, Percentage, year one	102.875%
Call Option, Beginning Date, year two	Oct. 01, 2018
Call Option, Percentage, year two	101.917%
Call Option, Beginning Date, year three	Oct. 01, 2019
Call Option, Percentage, year three	100.958%
Call Option, Percentage, year four and thereafter	100.00%
Redemption price in cash equal to principal plus accrued and unpaid interest, percent	105.75%
North America \| 5.75% Senior Notes due 2022 \| Maximum
Debt Instrument [Line Items]
Percent of aggregate principal amount of senior notes that can be redeemed on or prior to October 1, 2015	35.00%
North America \| 5.75% Senior Notes due 2022 \| Minimum
Debt Instrument [Line Items]
Percent of aggregate principal amount of senior notes issued remains outstanding	65.00%

Call Option Percentage, Year Four and Thereafter

No definition available.

Name:	bgc_CallOptionPercentageYearFourAndThereafter
Namespace Prefix:	bgc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Call Option Percentage, Year One

+ References

No definition available.

+ Details

Name:	bgc_CallOptionPercentageYearOne
Namespace Prefix:	bgc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Call Option Percentage, Year Three

+ References

No definition available.

+ Details

Name:	bgc_CallOptionPercentageYearThree
Namespace Prefix:	bgc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Call Option Percentage, Year Two

+ References

No definition available.

+ Details

Name:	bgc_CallOptionPercentageYearTwo
Namespace Prefix:	bgc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument Call Date Earliest, Year One

+ References

No definition available.

+ Details

Name:	bgc_DebtInstrumentCallDateEarliestYearOne
Namespace Prefix:	bgc_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument Call Date Earliest, Year Three

+ References

No definition available.

+ Details

Name:	bgc_DebtInstrumentCallDateEarliestYearThree
Namespace Prefix:	bgc_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument Call Date Earliest, Year Two

+ References

No definition available.

+ Details

Name:	bgc_DebtInstrumentCallDateEarliestYearTwo
Namespace Prefix:	bgc_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt instrument interest payment terms.

+ References

No definition available.

+ Details

Name:	bgc_DebtInstrumentInterestPaymentTerms
Namespace Prefix:	bgc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percent of Aggregate Principal Amount of Senior Notes Issued Remains Outstanding

+ References

No definition available.

+ Details

Name:	bgc_PercentOfAggregatePrincipalAmountOfSeniorNotesIssuedRemainsOutstanding
Namespace Prefix:	bgc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percent of Aggregate Principal Amount of Senior Notes that Can be Redeemed

+ References

No definition available.

+ Details

Name:	bgc_PercentOfAggregatePrincipalAmountOfSeniorNotesThatCanBeRedeemed
Namespace Prefix:	bgc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Redemption Price in Cash Equal to Principal Plus Accrued and Unpaid Interest, Percent

+ References

No definition available.

+ Details

Name:	bgc_RedemptionPriceInCashEqualToPrincipalPlusAccruedAndUnpaidInterestPercent
Namespace Prefix:	bgc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_DebtInstrumentCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13476-108611

+ Details

Name:	us-gaap_DebtInstrumentFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Description of the maturity date of the debt instrument including whether the debt matures serially and, if so, a brief description of the serial maturities.

+ References

+ Details

Name:	us-gaap_DebtInstrumentMaturityDateDescription
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ References

+ Details

Name:	us-gaap_UnamortizedDebtIssuanceExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=us-gaap_NorthAmericaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=bgc_FivePointSevenFivePercentSeniorNotesDue2022Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: