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Stock-Based Compensation | 6 Months Ended
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Jun. 30, 2018
Share-based Compensation [Abstract] |
Stock-Based Compensation | STOCK-BASED COMPENSATION The Company's Stock Incentive Plan (the "Plan") allows for grants of incentive stock options, non-statutory stock options, restricted stock awards, restricted stock units and other stock-based or cash-based awards. The majority of all stock options and restricted stock unit grants outstanding under the Plan were granted in the first quarter of each fiscal year. Stock compensation is recognized based on an estimated number of awards that are expected to vest. Stock-based compensation expense consisted of the following (in thousands):
| Three Months Ended June 30, |  | Six Months Ended June 30,
| 2018 |  | 2017 |  | 2018 |  | 2017
Stock options | $ | 1,222 |  |  | $ | 1,005 |  |  | $ | 2,294 |  |  | $ | 2,009 |
Restricted stock units | 2,264 |  |  | 1,772 |  |  | 4,305 |  |  | 3,710 |
Total | $ | 3,486 |  |  | $ | 2,777 |  |  | $ | 6,599 |  |  | $ | 5,719 |
Stock Options The Company estimates the fair value of stock options using the Black-Scholes model. Key inputs and assumptions used to estimate the fair value of stock options include the exercise price of the award, the expected option term, the expected stock price volatility of the Company's stock over the option's expected term, the risk-free interest rate over the option's expected term and the Company's expected annual dividend yield. The following table presents the weighted average assumptions for stock options granted in the periods:
| Three Months Ended June 30, |  | Six Months Ended June 30,
| 2018 |  | 2017 |  | 2018 |  | 2017
Expected option term | 6.47 years |  | 6.89 years |  | 4.49 years |  | 4.56 years
Expected stock price volatility | 27.69% |  | 28.50% |  | 28.41% |  | 28.91%
Risk-free interest rate | 2.82% |  | 1.94% |  | 2.47% |  | 1.72%
Expected annual dividend yield | 0.99% |  | 1.26% |  | 1.16% |  | 1.30%
Weighted average grant date fair value | $26.21 |  | $15.98 |  | $18.78 |  | $13.00
During the six months ended June 30, 2018 and 2017, the Company granted a total of 394,098 and 496,384 stock options, respectively. At June 30, 2018, unrecognized costs related to outstanding stock options totaled approximately $11,184,000, before any related tax benefit. The unrecognized costs related to stock options are amortized over the related vesting period using the straight-line attribution method. Unrecognized costs related to stock options at June 30, 2018 are expected to be recognized over a weighted average period of 2.71 years. Restricted Stock Units The Company estimates the fair value of service-based and performance-based restricted stock units using the Black-Scholes model. Key inputs and assumptions used to estimate the fair value of restricted stock units include the vesting period, expected annual dividend yield and closing price of the Company's common stock on the date of grant. The following table presents the weighted average assumptions for restricted stock units granted in the periods:
| Three Months Ended June 30, |  | Six Months Ended June 30,
| 2018 |  | 2017 |  | 2018 |  | 2017
Vesting period | 2.16 years |  | 3.08 years |  | 3.89 years |  | 3.86 years
Expected annual dividend yield | 0.99% |  | 1.26% |  | 1.15% |  | 1.30%
Estimated average grant date fair value per restricted stock unit | $87.20 |  | $55.06 |  | $72.95 |  | $52.54
During the six months ended June 30, 2018 and 2017, the Company granted 176,331 and 245,953 restricted stock units, respectively. At June 30, 2018, unrecognized costs related to outstanding restricted stock units totaled approximately $20,443,000, before any related tax benefit. The unrecognized costs related to restricted stock units are being amortized over the related vesting period using the straight-line attribution method. These unrecognized costs at June 30, 2018 are expected to be recognized over a weighted average period of 2.69 years.

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- Definition The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -URI http://asc.fasb.org/topic&trid=2228938 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -URI http://asc.fasb.org/subtopic&trid=2208855 + Details
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