Debt (Narrative) (Details) - USD ($)		3 Months Ended
	Aug. 10, 2018	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Line of Credit Facility [Line Items]
Remaining borrowing capacity		$ 749,600,000		$ 699,600,000
Credit agreement commitment fee percentage		0.40%
Short-term borrowings				50,000,000.0
Interest expense, net		$ 200,000	$ 900,000
Notes issued face amount		0
Letters of credit outstanding		6,300,000		6,300,000
Minimum
Line of Credit Facility [Line Items]
Short-term borrowings		0.0	55,000,000.0
Maximum
Line of Credit Facility [Line Items]
Short-term borrowings		50,000,000.0	$ 270,000,000.0
Revolving Credit Facility
Line of Credit Facility [Line Items]
Short-term borrowings		0		50,000,000.0
Amended Credit Agreement
Line of Credit Facility [Line Items]
Letters of credit outstanding		$ 400,000		$ 400,000
Prudential Private Placement Shelf Agreement
Line of Credit Facility [Line Items]
Agreement face amount	$ 100,000,000.0
Issuance period		3 years
MetLife Private Placement Shelf Agreement
Line of Credit Facility [Line Items]
Agreement face amount		$ 100,000,000.0
Issuance period		3 years
Line of credit | Amended Credit Agreement | Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	750,000,000.0
Accordian feature, increase limit	$ 375,000,000.0
Expiration date		Aug. 10, 2023
Debt instrument, term	5 years
Line of credit | Amended Credit Agreement | Letter of Credit Sub-Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 25,000,000.0
Line of credit | Amended Credit Agreement | Bridge Loan | Graybar Canada
Line of Credit Facility [Line Items]
Maximum borrowing capacity	100,000,000.0
Line of credit | Amended Credit Agreement | Bridge Loan | UNITED STATES
Line of Credit Facility [Line Items]
Maximum borrowing capacity	75,000,000.0
Line of credit | Amended Credit Agreement | Bridge Loan | CANADA
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 20,000,000.0