The Company is authorized to grant up to 25,000,000 shares of common stock under its Amended and Restated 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 7,074,218 shares were available for future grants under the Plan as of June 30, 2018.

Our stock-based compensation expense was as follows for the three and six months ended June 30, 2018 and 2017 (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Restricted stock units	$	974	$	1,344	$	2,137	$	2,591
Performance units		910		907		1,639		1,641
Restricted stock issued to directors		95		97		184		197
Total expense	$	1,979	$	2,348	$	3,960	$	4,429

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 June 30, 2018, there was $5.9 million of total unrecognized compensation cost related to outstanding restricted stock units. The weighted average period for the shares to vest is approximately 1 year. A summary of employee restricted stock unit awards activity during the six months ended June 30, 2018 is as follows:

	Number of shares			Weighted average grant date fair value		Aggregate intrinsic value (in thousands)
Total awarded and unvested, December 31, 2017		4,057,354		$	2.48	$	9,738
Granted		1,498,524			1.70
Vested		(1,862,526	)		2.88
Forfeited		(38,077	)		2.11
Total awarded and unvested, June 30, 2018		3,655,275		$	1.95	$	5,848

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 June 30, 2018, there was $5.4 million of total unrecognized compensation cost related to outstanding performance units. The weighted average period for the shares to vest is approximately 2 years. A summary of performance stock unit awards activity during the six months ended June 30, 2018 is as follows:

	Number of shares			Weighted average grant date fair value		Aggregate intrinsic value (in thousands)
Total awarded and unvested, December 31, 2017		3,966,377		$	1.82	$	11,257
Granted		1,498,524			1.92
Vested		—			—
Forfeited		(60,127	)		1.83
Total awarded and unvested, June 30, 2018		5,404,774		$	1.84	$	3,173

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 six months ended June 30, 2018 and 2017:

	Six Months Ended June 30,
	2018			2017
Volatility		89.70	%		50.41	%
Risk-free interest rate		2.37	%		1.34	%

On May 18, 2016, the Company issued an aggregate of 149,448 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 18, 2017. For the three and six months ended June 30, 2017, the Company recognized expense of approximately $0.1 million and $0.2 million, respectively, related to these awards.

On May 17, 2017, the Company issued an aggregate of 153,192 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 each of the three and six months ended June 30, 2018 and 2017, the Company recognized expense of approximately $0.1 million related to these awards.

On May 16, 2018, the Company issued an aggregate of 232,143 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 each of the three and six months ended June 30, 2018, the Company recognized expense of approximately $0.1 million related to these awards. As of June 30, 2018, there was approximately $0.3 million of total unrecognized compensation cost related to outstanding restricted stock issued to the Company’s directors.