13. Stock-based Compensation

Long-term Incentive Plan

We have a long-term incentive plan (“LTIP”) to incentivize individuals providing services to us or our affiliates. The LTIP provides for the grant, from time to time, at the discretion of our compensation committee of our board of directors, of stock options, stock appreciation rights, restricted stock, restricted stock units, stock awards, dividend equivalents, other stock-based awards, cash awards, substitute awards and performance awards. Any individual who is an officer or employee or an officer or employee of any of our affiliates, and any other person who provides services to us or our affiliates, including members of our board of directors, will be eligible to receive awards under the LTIP at the discretion of our board of directors.

Restricted Stock Units

Restricted stock units (“RSU’s”) granted pursuant to the LTIP are expected to be settled in shares of our Class A common stock if they vest.

The following table is a summary of restricted stock unit activity:


	Nine Months Ended September 30, 2019
	No. of RSU's		Weighted Average Grant Date Fair Value
	(in thousands)
Nonvested as of December 31, 2018		782	$	19.84
Granted		204	$	37.37
Vested		(271)	$	19.35
Forfeited		(30)	$	22.90
Nonvested as of September 30, 2019		685	$	25.13

Stock-based Compensation

We measure the cost of equity-based awards based on the grant date fair value, and we allocate the compensation expense over the corresponding service period, which is usually the vesting period, using the straight-line method. All grant date fair values are expensed immediately for awards that are fully vested as of the grant date.

During the three and nine months ended September 30, 2019 we recorded $1.7 million and $5.3 million, respectively, of stock-based compensation expense. During the three and nine months ended September 30, 2018, we recorded $1.3 million and $3.4 million, respectively, of stock-based compensation expense. Stock-based compensation expense is primarily recorded in selling, general and administrative expenses. There was approximately $12.5 million of unrecognized compensation expense relating to the unvested RSU’s as of September 30, 2019. The unrecognized compensation expense will be recognized over the weighted average remaining vesting period of 2.1 years.