The Company is authorized to grant up to 1,666,667 shares of common stock under its 2014 Long-Term Incentive Plan (as amended, the “Plan”). The Plan allows stock-based compensation awards to be granted in a variety of forms, including stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, dividend equivalent rights, qualified performance-based awards and other types of awards. The terms and conditions of the awards granted are established by the Compensation Committee of the Company’s Board of Directors. A total of 430,656 shares were available for future grants under the Plan as of March 31, 2019.

Our stock-based compensation expense was as follows for the three months ended March 31, 2019 and 2018 (in thousands):

	Three Months Ended March 31,
	2019		2018
Restricted stock units	$	3,147	$	1,164
Performance units		2,759		728
Restricted stock issued to directors		95		89
Total expense	$	6,001	$	1,981

Restricted stock unit awards vest subject to the satisfaction of service requirements. The Company recognizes expense related to restricted stock and restricted stock unit awards on a straight-line basis over the requisite service period, which is three years. The grant date fair values of these awards are determined based on the closing price of the Company’s common stock on the date of the grant. As of March 31, 2019, there was $0.8 million of total unrecognized compensation cost related to outstanding restricted stock units. The weighted average period for the shares to vest is approximately 2 years. A summary of employee restricted stock unit awards activity during the three months ended March 31, 2019 is as follows:

	Number of shares			Weighted average grant date fair value		Aggregate intrinsic value (in thousands)
Total awarded and unvested, December 31, 2018		233,960		$	29.27	$	3,685
Granted		70,409			17.55
Vested		(198,279	)		29.28
Forfeited		(485	)		31.78
Total awarded and unvested, March 31, 2019		105,605		$	21.42	$	1,588

Performance unit awards vest subject to the satisfaction of a three-year service requirement and based on Total Shareholder Return, as compared to an industry peer group over that same period. The performance unit awards are measured at the grant date at fair value using a Monte Carlo valuation method. As of March 31, 2019, there was $2.0 million of total unrecognized compensation cost related to outstanding performance units. The weighted average period for the shares to vest is approximately 1 year. A summary of performance stock unit awards activity during the three months ended March 31, 2019 is as follows:

	Number of shares			Weighted average grant date fair value		Aggregate intrinsic value (in thousands)
Total awarded and unvested, December 31, 2018		346,589		$	27.68	$	716
Granted		—			—
Vested		(265,311	)		27.54
Forfeited		(16,001	)		24.47
Total awarded and unvested, March 31, 2019		65,277		$	29.02	$	1,201

The determination of the fair value of the performance unit awards noted above uses significant Level 3 assumptions in the fair value hierarchy including an estimate of the timing of forfeitures, the risk free rate and a volatility estimate tied to the Company’s stock price. Prior to 2018, the volatility estimate was tied to the Company’s public peer group. The following table presents the assumptions used to determine the fair value for performance stock units granted during the three months ended March 31, 2018:

	Three Months Ended March 31,
	2018
Volatility		89.70	%
Risk-free interest rate		2.37	%

On May 17, 2017, the Company issued an aggregate of 10,212 restricted shares of common stock to its three non-employee members of its Board of Directors that are not affiliated with the Company’s controlling stockholder, which became fully vested on May 17, 2018. For the three months ended March 31, 2018, the Company recognized expense of approximately $0.1 million related to these awards.

On May 16, 2018, the Company issued an aggregate of 15,476 restricted shares of common stock to its three non-employee members of its Board of Directors that are not affiliated with the Company’s controlling stockholder, which are scheduled to fully vest on May 16, 2019. For the three months ended March 31, 2019, the Company recognized expense of approximately $0.1 million related to these awards. As of March 31, 2019, there was approximately less than $0.1 million of total unrecognized compensation cost related to outstanding restricted stock issued to the Company’s directors.