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STOCK-BASED COMPENSATION (Tables) | 12 Months Ended
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Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
Schedule of Share-based Compensation, Stock Options and Stock Appreciation Rights Award Activity | The following table sets forth a summary of stock-settled stock appreciation rights and related information for the years presented:
| Year Ended December 31,
| 2018 |  | 2017 |  | 2016
| Awards |  | Weighted-Average Exercise Price |  | Weighted-Average Remaining Contractual Life |  | Aggregate Intrinsic Value (In thousands) |  | Awards |  | Weighted-Average Exercise Price |  | Awards |  | Weighted-Average Exercise Price
Beginning of year | 2,238,607 |  |  | $ | 57.75 |  |  |  |  |  |  | 2,439,487 |  |  | $ | 52.62 |  |  | 2,567,021 |  |  | $ | 54.47 |
Granted | 509,046 |  |  | 62.68 |  |  |  |  |  |  | 455,807 |  |  | 71.21 |  |  | 709,999 |  |  | 42.63 |
Exercised | (192,700 | ) |  | 40.74 |  |  |  |  |  |  | (495,181 | ) |  | 42.19 |  |  | (526,818 | ) |  | 41.54 |
Canceled | (203,320 | ) |  | 68.69 |  |  |  |  |  |  | (161,506 | ) |  | 66.06 |  |  | (310,715 | ) |  | 63.71 |
End of year | 2,351,633 |  |  | 59.26 |  |  | 6.1 |  | $ | 6,514 |  |  | 2,238,607 |  |  | 57.75 |  |  | 2,439,487 |  |  | 52.62 |
Exercisable at end of year | 1,453,932 |  |  | $ | 57.93 |  |  | 4.6 |  | $ | 5,623 |  |  | 1,331,580 |  |  | $ | 56.96 |  |  | 1,549,350 |  |  | $ | 53.35 |
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] | the fair value of stock-settled stock appreciation rights granted during the years presented:
| Year Ended December 31,
| 2018 |  | 2017 |  | 2016
Stock-settled stock appreciation rights granted | 509,046 |  | 455,807 |  | 709,999
Risk free interest rate | 2.5% |  | 1.9% |  | 1.2%
Expected life (in years) | 5 |  | 5 |  | 5
Expected volatility | 28% |  | 29% |  | 32%
Schedule of Share-based Compensation, Restricted Stock Units Award Activity | The following table sets forth a summary of performance-based awards and related information for the years presented:
| Year Ended December 31,
| 2018 |  | 2017 |  | 2016
| Awards |  | Weighted-Average Fair Value |  | Awards |  | Weighted-Average Fair Value |  | Awards |  | Weighted-Average Fair Value
Unvested at beginning of year | 148,508 |  |  | $ | 60.23 |  |  | 149,320 |  |  | $ | 60.36 |  |  | 114,520 |  |  | $ | 76.48 |
Granted | 44,144 |  |  | 62.80 |  |  | 39,978 |  |  | 76.63 |  |  | 91,768 |  |  | 47.00 |
Vested | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | — |
Forfeited | (53,756 | ) |  | 64.67 |  |  | (40,790 | ) |  | 76.77 |  |  | (56,968 | ) |  | 71.25 |
Unvested at end of year | 138,896 |  |  | $ | 59.33 |  |  | 148,508 |  |  | $ | 60.23 |  |  | 149,320 |  |  | $ | 60.36 |
The following table sets forth a summary of time-based restricted stock units and related information for the years presented:
| Year Ended December 31,
| 2018 |  | 2017 |  | 2016
| Awards |  | Weighted-Average Fair Value |  | Awards |  | Weighted-Average Fair Value |  | Awards |  | Weighted-Average Fair Value
Unvested at beginning of year | 290,054 |  |  | $ | 58.11 |  |  | 257,096 |  |  | $ | 57.47 |  |  | 175,411 |  |  | $ | 74.52 |
Granted | 122,062 |  |  | 62.40 |  |  | 100,993 |  |  | 71.33 |  |  | 162,256 |  |  | 44.45 |
Vested | (64,166 | ) |  | 67.91 |  |  | (44,720 | ) |  | 84.57 |  |  | (60,015 | ) |  | 72.41 |
Forfeited | (20,152 | ) |  | 58.15 |  |  | (23,315 | ) |  | 57.52 |  |  | (20,556 | ) |  | 59.15 |
Unvested at end of year | 327,798 |  |  | $ | 57.87 |  |  | 290,054 |  |  | $ | 58.11 |  |  | 257,096 |  |  | $ | 57.47 |
Schedule of Share-based Payment Award, Performance-based Awards, Valuation Assumptions [Table Text Block] | The following table sets forth the assumptions used to estimate the fair value of performance shares granted during the years presented:
| Year ended December 31,
| 2018 |  | 2017 |  | 2016
Grant date share price | $ | 62.80 |  |  | $ | 71.67 |  |  | $ | 42.44 |
WESCO expected volatility | n/a |  |  | 29 | % |  | 26 | %
Peer group median volatility | n/a |  |  | 24 | % |  | 24 | %
Risk-free interest rate | n/a |  |  | 1.5 | % |  | 0.9 | %
Correlation of peer company returns | n/a |  |  | 114 | % |  | 122 | %

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