Debt (Narrative) (Details) - USD ($)		3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 27, 2025	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Line of Credit Facility [Line Items]
Total debt		$ 2,100,000		$ 2,100,000		$ 2,200,000
Long-term debt		1,100,000		1,100,000		1,300,000
Short-term borrowings		0		0		22,000,000.0
Remaining borrowing capacity						724,200,000
Interest income (expense), net		100,000	$ 600,000	500,000	$ 1,600,000
Letters of credit outstanding		10,800,000		10,800,000		$ 9,600,000
Minimum [Member]
Line of Credit Facility [Line Items]
Short-term borrowings		0	0	0	0
Maximum [Member]
Line of Credit Facility [Line Items]
Short-term borrowings		37,000,000.0	$ 38,000,000.0	37,000,000.0	$ 38,000,000.0
Credit Agreement [Member]
Line of Credit Facility [Line Items]
Debt instrument, term						5 years
Maximum borrowing capacity						$ 750,000,000.0
Expiration date						Aug. 01, 2026
Accordion feature, increase limit						$ 375,000,000.0
Credit Agreement [Member] | Graybar Canada [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity						100,000,000.0
Credit Agreement [Member] | Letter of Credit Sub Facility [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity						25,000,000.0
Credit Agreement [Member] | Swing-Line Loan [Member] | United States [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity						75,000,000.0
Credit Agreement [Member] | Swing-Line Loan [Member] | Canada [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity						20,000,000.0
Amended Credit Agreement [Member]
Line of Credit Facility [Line Items]
Debt instrument, term	5 years
Accordion feature, increase limit	$ 375,000,000.0
Debt maturity date	Jun. 01, 2030
Remaining borrowing capacity		744,200,000		$ 744,200,000
Credit agreement commitment fee percentage				0.40%
Amended Credit Agreement [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Credit agreement commitment fee percentage	0.25%
Amended Credit Agreement [Member] | Minimum [Member] | Base Rate [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.00%
Amended Credit Agreement [Member] | Minimum [Member] | Other Rates [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.00%
Amended Credit Agreement [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Credit agreement commitment fee percentage	0.40%
Amended Credit Agreement [Member] | Maximum [Member] | Base Rate [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.60%
Amended Credit Agreement [Member] | Maximum [Member] | Other Rates [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.60%
Amended Credit Agreement [Member] | Graybar Canada [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 100,000,000.0
Letters of credit outstanding		5,800,000		$ 5,800,000		3,800,000
Amended Credit Agreement [Member] | Letter of Credit Sub Facility [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 35,000,000.0
Amended Credit Agreement [Member] | Letter of Credit Sub Facility [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Letter of credit fees, percentage	1.00%
Amended Credit Agreement [Member] | Letter of Credit Sub Facility [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Letter of credit fees, percentage	1.60%
Amended Credit Agreement [Member] | Swing-Line Loan [Member] | United States [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 75,000,000.0
Amended Credit Agreement [Member] | Swing-Line Loan [Member] | Canada [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 20,000,000.0
Prudential Private Placement Shelf Agreement [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity		200,000,000.0		$ 200,000,000.0
Expiration date				Aug. 01, 2026
Issuance period				3 years
Notes issued face amount		0		$ 0		0
MetLife Private Placement Shelf Agreement [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity		200,000,000.0		$ 200,000,000.0
Expiration date				Jun. 01, 2027
Issuance period				3 years
Successive issuance period				3 years
Notes issued face amount		$ 0		$ 0		$ 0