Stock-Based Compensation

The Company has an Omnibus Incentive and Equity Plan (the "Plan") under which officers, employees and directors may be granted equity-based awards, including restricted stock units ("RSUs"), performance stock units ("PSUs"), stock options and unrestricted shares of common stock. At December 31, 2018, 297,407 shares of common stock remain available for issuance of the 2,400,000 shares that are authorized for issuance under the Plan.

Stock-based compensation expense is summarized as follows:



	Years Ended December 31,
($ in thousands)	2018		2017		2016
Stock-based compensation expense	$	23,116	$	20,288	$	11,948

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 (PSUs). The fair value of each RSU is based on the closing market price of the Company's common stock 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. Shares that are issued upon vesting are newly issued shares from the Plan and are not issued from treasury stock.

RSU activity for the year ended December 31, 2018 is summarized as follows:



	Number of shares		Weighted Average Grant Date Fair Value
Outstanding at December 31, 2017	483,021		$	104.16
Granted	198,180		$	131.16
Forfeited	(19,814	)	$	134.65
Settled	(109,149	)	$	114.28
Outstanding at December 31, 2018	552,238		$	111.49

The grant-date intrinsic value of RSUs granted during the year ended December 31, 2018 was $26.0 million. The weighted-average grant-date fair value of RSUs granted during the years ended December 31, 2018, 2017 and 2016 was $131.16, $108.32 and $80.33 per share, respectively. The total fair value of RSUs vested during the years ended December 31, 2018, 2017 and 2016 was $12.5 million, $11.3 million and $9.3 million, respectively. For the years ended December 31, 2018, 2017 and 2016, a total of 41,101, 32,716 and 37,488 RSUs, respectively, were withheld through net share settlement by the Company to settle minimum employee tax withholding obligations. The Company paid $5.3 million, $3.5 million and $1.5 million for the years ended December 31, 2018, 2017 and 2016, 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.

As of December 31, 2018 and 2017, unamortized stock-based compensation expense for outstanding RSUs was $32.2 million and $29.3 million with a weighted average remaining contractual life of 1.5 years and 1.6 years, respectively. The Company did not capitalize any stock-based compensation expenses during the years ended December 31, 2018, 2017 and 2016.

During the years ended December 31, 2018 and 2017, the Company granted 68,803 and 122,606 PSUs, which contain performance-based metrics in addition to a service condition. Compensation expense for these PSUs is generally 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 years ended December 31, 2018 and 2017, total stock-based compensation expense included $8.2 million and $7.8 million respectively, for PSUs. As of December 31, 2018 and 2017, unamortized stock-based compensation expense related to PSUs was $11.4 million and $10.9 million, respectively.

Stock Options

Stock option activity for the year ended December 31, 2018 is summarized as follows:



	Number of shares		Weighted Average Exercise Price
Outstanding at December 31, 2017	109,808		$	16.44
Exercised	(33,057	)	$	24.74
Outstanding at December 31, 2018	76,751		$	12.86
Vested and exercisable at December 31, 2018	76,751		$	12.86

Stock options generally cliff vest after three years and have a contractual life of ten years. Stock options are granted with an exercise price equal to the fair market value of the shares at the date of grant. The weighted-average remaining contractual term for stock options outstanding at December 31, 2018 and December 31, 2017 was 0.5 and 1.2 years, respectively. The weighted-average remaining contractual term for stock options vested and exercisable at December 31, 2018 was 0.5 years. At December 31, 2018, the aggregate intrinsic value of stock options outstanding and vested and exercisable was $5.1 million. There were no unvested stock options at December 31, 2018. The total intrinsic value of stock options exercised for the years ended December 31, 2018, 2017 and 2016 was $3.0 million, $2.5 million and $1.3 million, respectively. Cash received from stock option exercises was $0.8 million, $0.1 million and $0.5 million for 2018, 2017 and 2016, respectively.

Employee Stock Purchase Plan

The Company offers an employee stock purchase plan that allows employees to purchase shares of common stock on the open market at market price through after-tax payroll deductions. The initial transaction fees are paid for by the Company and shares of common stock are purchased on a quarterly basis. The Company does not reserve shares for this plan or discount the purchase price of the shares.