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SHARE-BASED AWARDS AND OPTIONS | 3 Months Ended
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Mar. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
SHARE-BASED AWARDS AND OPTIONS | SHARE-BASED AWARDS AND OPTIONS The following table summarizes share-based compensation expense and the related income tax benefit recognized for our share-based awards and stock options:
| Three Months Ended
| March 31, 2018 |  | March 31, 2017
| (in thousands)
Share-based compensation expense | $ | 14,898 |  |  | $ | 8,816 |
Income tax benefit | $ | 3,285 |  |  | $ | 3,065 |
Share-Based Awards The following table summarizes the changes in unvested restricted stock and performance awards for the three months ended March 31, 2018:
| Shares |  | Weighted-Average Grant-Date Fair Value
| (in thousands) |  |
Unvested at December 31, 2017 | 1,226 |  |  |  | $78.29 |
Granted | 367 |  |  | 114.68 |
Vested | (168 | ) |  | 77.15 |
Forfeited | (16 | ) |  | 82.91 |
Unvested at March 31, 2018 | 1,409 |  |  |  | $87.84 |
The total fair value of restricted stock and performance awards vested during the three months ended March 31, 2018 and March 31, 2017 was $13.0 million and $0.1 million, respectively. For restricted stock and performance awards, we recognized compensation expense of $13.8 million and $8.0 million during the three months ended March 31, 2018 and March 31, 2017, respectively. As of March 31, 2018, there was $82.6 million of unrecognized compensation expense related to unvested restricted stock and performance awards that we expect to recognize over a weighted-average period of 2.2 years. Our restricted stock and performance award plans provide for accelerated vesting under certain conditions. Stock Options The following summarizes changes in stock option activity for the three months ended March 31, 2018:
| Options |  | Weighted-Average Exercise Price |  | Weighted-Average Remaining Contractual Term |  | Aggregate Intrinsic Value
| (in thousands) |  |  |  | (years) |  | (in millions)
Outstanding at December 31, 2017 | 723 |  |  | $47.79 |  | 6.4 |  | $37.9
Granted | 103 |  |  | 114.7 |  |  |  |
Exercised | (23 | ) |  | 21.89 |  |  |  |
Outstanding at March 31, 2018 | 803 |  |  | $57.11 |  | 6.8 |  | $44.0
Options vested and exercisable at March 31, 2018 | 520 |  |  | $40.66 |  | 5.6 |  | $36.8
We recognized compensation expense for stock options of $0.7 million and $0.6 million during the three months ended March 31, 2018 and March 31, 2017, respectively. The aggregate intrinsic value of stock options exercised during the three months ended March 31, 2018 and March 31, 2017 was $2.1 million and $2.8 million, respectively. As of March 31, 2018, we had $6.3 million of unrecognized compensation expense related to unvested stock options that we expect to recognize over a weighted-average period of 2.4 years. The weighted-average grant-date fair value of each stock option granted during the three months ended March 31, 2018 and March 31, 2017 was $35.09 and $23.68, respectively. Fair value was estimated on the date of grant using the Black-Scholes valuation model with the following weighted-average assumptions:
| Three Months Ended
| March 31, 2018 |  | March 31, 2017
Risk-free interest rate | 2.60% |  | 1.99%
Expected volatility | 29% |  | 30%
Dividend yield | 0.04% |  | 0.06%
Expected term (years) | 5 |  | 5
The risk-free interest rate is based on the yield of a zero coupon U.S. Treasury security with a maturity equal to the expected life of the option from the date of the grant. Our assumption on expected volatility is based on our historical volatility. The dividend yield assumption is calculated using our average stock price over the preceding year and the annualized amount of our most current quarterly dividend per share. We based our assumptions on the expected term of the options on our analysis of the historical exercise patterns of the options and our assumption on the future exercise pattern of options.

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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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -URI http://asc.fasb.org/topic&trid=2228938 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -URI http://asc.fasb.org/subtopic&trid=2208855 + Details
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