Stock-Based Compensation

The Company's Amended and Restated Omnibus Incentive and Equity Plan (the "Plan") provides for the grant of equity-based awards, including restricted stock units ("RSUs"), stock options and unrestricted shares of common stock. As of June 30, 2018, a maximum of 2,400,000 shares of common stock were authorized for issuance under the Plan, and 300,637 shares remained available for issuance. Shares that are issued upon exercise of stock options and vesting of RSUs are newly issued shares from the Plan and are not issued from treasury stock. The Company recognized total stock compensation expense of $12.1 million and $9.5 million for the six months ended June 30, 2018 and 2017, respectively.

Restricted Stock Units

Each RSU entitles the holder to one share of common stock when the restriction expires. RSUs generally have a term of one to three years and may be time-vested or performance-contingent. The fair value of each RSU is estimated using the intrinsic value method, which is based on the fair market value price on the date of grant unless it contains a performance metric that is considered a market condition. RSUs that contain a market condition are valued using a simulation valuation model.

RSU activity for the six months ended June 30, 2018 is summarized as follows:



	Number of Shares		Weighted Average Grant Date Fair Value
Outstanding at December 31, 2017	483,021		$	104.16
Granted	175,284		$	133.37
Settled	(80,032	)	$	110.81
Outstanding at June 30, 2018	578,273		$	112.16

For the six months ended June 30, 2018 and 2017, a total of 30,598 and 28,444 RSUs, respectively, were withheld by the Company as a result of net share settlements to settle minimum employee tax withholding obligations. The Company paid $5.2 million and $3.0 million for the six months ended June 30, 2018 and 2017, respectively, in minimum employee tax withholding obligations related to RSUs withheld. These net share settlements had the effect of share repurchases by the Company as they reduced the number of shares that would have been otherwise issued as a result of the vesting.

During the six months ended June 30, 2018, the Company granted 23,356 performance based stock awards ("PSUs") which contain performance-based metrics in addition to a service condition. Compensation expense for these PSUs is recognized over a three-year service period based upon the value determined using a combination of the intrinsic value method, for awards that contain a performance metric that represents a "performance condition" in accordance with ASC 718, and the Monte Carlo simulation valuation model, for awards under the performance metric that represents a "market condition" under ASC 718. Compensation expense for the awards that contain a market condition is fixed at the date of grant and will not be adjusted in future periods based upon the achievement of the market condition. Compensation expense for the awards with a performance condition is recorded each period based upon a probability assessment of the expected outcome of the performance metric with a final adjustment upon the final outcome. For the six months ended June 30, 2018, total stock-based compensation expense for PSUs was $4.3 million.

As of June 30, 2018, unamortized stock-based compensation expense for unvested RSUs and PSUs was $42.3 million, with a weighted-average remaining amortization period of 1.8 years.

Stock Options

Stock options generally cliff vest after three years and have a contractual life of 10 years. Stock options are granted with an exercise price equal to the fair market value of the shares at the date of grant.

Stock option activity for the six months ended June 30, 2018 is summarized as follows:



	Number of Shares		Weighted Average Exercise Price
Outstanding at December 31, 2017	109,808		$	16.44
Exercised	(22,245	)	$	31.38
Outstanding at June 30, 2018	87,563		$	12.64