At the Company’s 2019 Annual Meeting of Stockholders held on June 14, 2019, the Company’s stockholders approved the Company’s 2019 Long-Term Incentive Plan (the “2019 Plan”), which was previously approved by the Company’s Board of Directors. The 2019 Plan replaces the Company’s 2014 Long-Term Incentive Plan, as amended (the “Prior Plan”). Upon stockholder approval, (i) the 2019 Plan became effective and (ii) the Prior Plan terminated and no additional awards will be granted under the Prior Plan; provided that awards outstanding under the Prior Plan as of the date the 2019 Plan became effective will remain in full force and effect under the Prior Plan according to their respective terms.

The Company is authorized to grant up to 2,650,000 shares of common stock under the 2019 Plan. The 2019 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 1,858,580 shares were available for future grants under the Plan as of June 30, 2019.

Our stock-based compensation expense was as follows for the three and six months ended June 30, 2019 and 2018 (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Restricted stock units	$	235	$	974	$	3,328	$	2,137
Performance units		231		910		2,990		1,639
Restricted stock		86		95		235		184
Total expense	$	552	$	1,979	$	6,553	$	3,960

Restricted stock unit awards vest subject to the satisfaction of service requirements. The Company recognizes expense related to 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, 2019, there was $3.2 million of total unrecognized compensation cost related to outstanding restricted stock units. The weighted average period for the shares to vest is approximately two years. A summary of employee restricted stock unit awards activity during the six months ended June 30, 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		407,714			6.46
Vested		(204,418	)		29.31
Forfeited		(485	)		31.78
Total awarded and unvested, June 30, 2019		436,771		$	7.95	$	2,664

Performance unit awards vest subject to the satisfaction of a three-year service requirement and based on Total Shareholder Return (as defined in the award agreements), 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, 2019, there was $2.3 million of total unrecognized compensation cost related to outstanding performance units. The weighted average period for the shares to vest is approximately two years. A summary of performance stock unit awards activity during the six months ended June 30, 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		243,600			7.25
Vested		(266,411	)		27.55
Forfeited		(16,483	)		24.47
Total awarded and unvested, June 30, 2019		307,295		$	11.76	$	—

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, 2019 and 2018:

	Six Months Ended June 30,
	2019			2018
Volatility		65.10	%		89.70	%
Risk-free interest rate		1.83	%		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 who were not affiliated with the Company’s then controlling stockholder, which shares became fully vested on May 17, 2018.

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 who were not affiliated with the Company’s then controlling stockholder, which shares became fully vested on May 16, 2019.

Effective February 28, 2019, the Company issued an aggregate of 70,409 restricted shares of common stock to two of its officers in connection with retention bonus arrangements entered into between the Company and each of these officers. The restricted shares of common stock vest in substantially equal installments on August 28, 2019, February 28, 2020, August 28, 2020 and February 28, 2021.

Pursuant to the Company’s Non-Employee Director Compensation Policy, on June 18, 2019, the Company awarded an aggregate of 53,328 restricted shares of common stock to eight of the non-employee members of its Board of Directors, which shares are scheduled to fully vest on June 18, 2020. The other non-employee member of the Company’s Board of Directors declined to receive any compensation for his service on the Company’s Board of Directors for 2019.