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STOCK-BASED COMPENSATION (Tables) | 3 Months Ended
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Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] | During the three months ended March 31, 2018 and 2017, WESCO granted the following stock-settled stock appreciation rights, restricted stock units and performance-based awards at the following weighted-average fair values:
| Three Months Ended
| March 31, 2018 |  | March 31, 2017
Stock-settled stock appreciation rights granted | 491,229 |  |  | 443,731 |
Weighted-average fair value | $ | 18.40 |  |  | $ | 20.65 |
Restricted stock units granted | 114,269 |  |  | 98,680 |
Weighted-average fair value | $ | 62.81 |  |  | $ | 71.65 |
Performance-based awards granted | 44,144 |  |  | 39,978 |
Weighted-average fair value | $ | 62.80 |  |  | $ | 76.63 |
Schedule of Share-based Compensation, Stock Options and Stock Appreciation Rights Award Activity [Table Text Block] | The following table sets forth a summary of stock-settled stock appreciation rights and related information for the three months ended March 31, 2018:
| Awards |  | Weighted- Average Exercise Price |  | Weighted- Average Remaining Contractual Term (In years) |  | Aggregate Intrinsic Value (In thousands)
Outstanding at December 31, 2017 | 2,238,607 |  |  | $ | 57.75 |  |  |  |  |
Granted | 491,229 |  |  | 62.81 |  |  |  |  |
Exercised | (93,145 | ) |  | 42.52 |  |  |  |  |
Forfeited | (48,248 | ) |  | 65.57 |  |  |  |  |
Outstanding at March 31, 2018 | 2,588,443 |  |  | 59.11 |  |  | 6.6 |  | $ | 19,575 |
Exercisable at March 31, 2018 | 1,639,025 |  |  | $ | 57.81 |  |  | 5.1 |  | $ | 16,000 |
Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block] | The following table sets forth a summary of time-based restricted stock units and related information for the three months ended March 31, 2018:
| Awards |  | Weighted- Average Fair Value
Unvested at December 31, 2017 | 290,054 |  |  | $ | 58.11 |
Granted | 114,269 |  |  | 62.81 |
Vested | (57,175 | ) |  | 69.44 |
Forfeited | (8,572 | ) |  | 56.00 |
Unvested at March 31, 2018 | 338,576 |  |  | $ | 57.84 |
Schedule of Nonvested Performance-based Units Activity [Table Text Block] | The following table sets forth a summary of performance-based awards for the three months ended March 31, 2018:
| Awards |  | Weighted- Average Fair Value
Unvested at December 31, 2017 | 148,508 |  |  | $ | 60.23 |
Granted | 44,144 |  |  | 62.80 |
Vested | — |  |  | — |
Forfeited | (46,858 | ) |  | 65.44 |
Unvested at March 31, 2018 | 145,794 |  |  | $ | 59.33 |
Stock Appreciation Rights (SARs) [Member] |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] | The fair value of stock-settled stock appreciation rights was estimated using the following weighted-average assumptions:
| Three Months Ended
| March 31, 2018 |  | March 31, 2017
Risk free interest rate | 2.5 | % |  | 1.9 | %
Expected life (in years) | 5 |  |  | 5 |
Expected volatility | 28 | % |  | 29 | %
Performance Shares [Member] |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] | The fair value of the performance shares granted during the three months ended March 31, 2018 and 2017 was estimated using the following weighted-average assumptions:
| Three Months Ended
| March 31, 2018 |  | March 31, 2017
Grant date share price | $ | 62.80 |  |  | $ | 71.65 |
WESCO expected volatility | n/a |  |  | 29 | %
Peer group median volatility | n/a |  |  | 24 | %
Risk-free interest rate | n/a |  |  | 1.5 | %
Correlation of peer company returns | n/a |  |  | 114 | %

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- Definition Tabular disclosure of the changes in outstanding nonvested performance-based units. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the number and weighted-average exercise prices (or conversion ratios) for stock options and stock appreciation rights that were outstanding at the beginning and end of the year, exercisable at the end of the year, and the number of stock options and stock appreciation rights that were granted, exercised or converted, forfeited, and expired during the year. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Disclosure of the number and weighted-average grant date fair value for restricted stock and restricted stock units that were outstanding at the beginning and end of the year, and the number of restricted stock and restricted stock units that were granted, vested, or forfeited during the year. + References No definition available. + Details
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- Definition Tabular disclosure of the weighted-average grant-date fair value of equity options or other equity instruments granted during the year. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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