Debt - Credit Agreement (Details) - USD ($)		3 Months Ended
	Jun. 06, 2014	Mar. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity		$ 370,000,000	$ 380,357,000
Commitment fee percentage		0.40%
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
Letter of Credit
Line of Credit Facility [Line Items]
Letters of credit outstanding, amount		$ 5,371,000	5,371,000
Letter of Credit | Revolving Credit Facility
Line of Credit Facility [Line Items]
Letters of credit outstanding, amount		$ 0	$ 0