Debt - Credit Agreement (Details) - USD ($)	Aug. 10, 2018	Jun. 06, 2014	Sep. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity			$ 504,000,000	$ 380,357,000
Line of Credit | Credit Agreement | Revolving Credit Facility
Line of Credit Facility [Line Items]
Debt Instrument, Term		5 years
Maximum borrowing capacity		$ 550,000,000
Accordion feature, higher borrowing capacity option		300,000,000
Line of Credit | Credit Agreement | Letter of Credit Sub-Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity		50,000,000
Line of Credit | Credit Agreement | Bridge Loan | Graybar Canada
Line of Credit Facility [Line Items]
Maximum borrowing capacity		100,000,000
Line of Credit | Credit Agreement | Bridge Loan | UNITED STATES
Line of Credit Facility [Line Items]
Maximum borrowing capacity		50,000,000
Line of Credit | Credit Agreement | Bridge Loan | CANADA
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 20,000,000
Line of Credit | Amended Credit Agreement [Member] | Revolving Credit Facility
Line of Credit Facility [Line Items]
Debt Instrument, Term	5 years
Maximum borrowing capacity	$ 750,000,000
Accordion feature, higher borrowing capacity option	$ 375,000,000
Line of Credit | Amended Credit Agreement [Member] | Revolving Credit Facility | Minimum
Line of Credit Facility [Line Items]
Commitment fee percentage	0.25%
Line of Credit | Amended Credit Agreement [Member] | Revolving Credit Facility | Maximum
Line of Credit Facility [Line Items]
Commitment fee percentage	0.40%
Line of Credit | Amended Credit Agreement [Member] | Letter of Credit Sub-Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 25,000,000
Line of Credit | Amended Credit Agreement [Member] | Bridge Loan | Graybar Canada
Line of Credit Facility [Line Items]
Maximum borrowing capacity	100,000,000
Line of Credit | Amended Credit Agreement [Member] | Bridge Loan | UNITED STATES
Line of Credit Facility [Line Items]
Maximum borrowing capacity	75,000,000
Line of Credit | Amended Credit Agreement [Member] | Bridge Loan | CANADA
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 20,000,000
Line of Credit | Amended Credit Agreement [Member] | Letter of Credit | Minimum
Line of Credit Facility [Line Items]
Commitment fee percentage	1.00%
Line of Credit | Amended Credit Agreement [Member] | Letter of Credit | Maximum
Line of Credit Facility [Line Items]
Commitment fee percentage	1.60%
Letter of Credit
Line of Credit Facility [Line Items]
Letters of credit outstanding, amount			5,621,000	5,371,000
Letter of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Letters of credit outstanding, amount			$ 0	$ 0
Base Rate [Member] | Line of Credit | Amended Credit Agreement [Member] | Revolving Credit Facility
Line of Credit Facility [Line Items]
Debt Instrument, Description of Variable Rate Basis	base rate
Base Rate [Member] | Line of Credit | Amended Credit Agreement [Member] | Revolving Credit Facility | Minimum
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.00%
Base Rate [Member] | Line of Credit | Amended Credit Agreement [Member] | Revolving Credit Facility | Maximum
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.60%
London Interbank Offered Rate (LIBOR) [Member] | Line of Credit | Amended Credit Agreement [Member] | Revolving Credit Facility
Line of Credit Facility [Line Items]
Debt Instrument, Description of Variable Rate Basis	LIBOR
London Interbank Offered Rate (LIBOR) [Member] | Line of Credit | Amended Credit Agreement [Member] | Revolving Credit Facility | Minimum
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.00%
London Interbank Offered Rate (LIBOR) [Member] | Line of Credit | Amended Credit Agreement [Member] | Revolving Credit Facility | Maximum
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.60%