The Company is authorized to grant up to 25,000,000 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 7,145,866 shares are available for future grant under the Plan as of December 31, 2018. The foregoing share numbers provided as of December 31, 2018 do not reflect any adjustment with respect to the 15-to-1 reverse stock split that occurred on February 28, 2019.

Our stock based compensation expense is as follows for the years ended December 31, 2018, 2017, and 2016 (in thousands):

	Year Ended December 31,
	2018		2017		2016
Restricted stock units	$	4,014	$	5,301	$	4,006
Performance units		3,497		3,622		1,922
Restricted stock issued to directors		380		378		556
Incentive units		—		—		(268	)
Total expense	$	7,891	$	9,301	$	6,216

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 December 31, 2018, there was $4.0 million of total unrecognized compensation cost related to 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 year ended December 31, 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		270,490		$	37.20	$	9,738
Granted		99,901			25.50
Vested		(132,406	)		42.42
Forfeited		(4,025	)		31.30
Total awarded and unvested, December 31, 2018		233,960		$	29.27	$	3,685

Performance unit awards vest subject to the satisfaction of a three-year service requirement and based on Total Shareholder Return (“TSR”), 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 December 31, 2018, there was $3.5 million of total unrecognized compensation cost related to performance units. The weighted average period for the shares to vest is approximately 1 years.

A summary of performance stock unit awards activity during the year ended December 31, 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		264,425		$	27.30	$	11,257
Granted		99,901			28.80
Vested		(11,536	)		27.51
Forfeited		(6,201	)		27.61
Total awarded and unvested, December 31, 2018		346,589		$	27.68	$	716

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 public peer group. The following table presents the assumptions used to determine the fair value for performance stock units granted during the years ended December 31, 2018, 2017, and 2016:

	Year Ended December 31,
	2018			2017			2016
Volatility		89.70	%		50.41	%		49.84	%
Risk-free interest rate		2.37	%		1.34	%		0.96	%

The fair value of the performance stock units vested during the year ended December 31, 2017 was approximately $0.8 million.

On May 11, 2015, the Company issued an aggregate of 8,833 restricted shares of common stock to its seven non-employee members of its Board of Directors, which became fully vested on May 11, 2016. For the year ended December 31, 2016, the Company recognized expense of approximately $0.3 million related to these awards.

On May 18, 2016, the Company issued an aggregate of 9,963 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 years ended December 31, 2017 and 2016, the Company recognized expense of approximately $0.2 million and $0.3 million, respectively, related to these awards.

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 years ended December 31, 2017 and 2018, the Company recognized expense of approximately $0.2 million and $0.1 million, respectively, 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 year ended December 31, 2018, the Company recognized expense of approximately $0.3 million related to these awards. As of December 31, 2018, there was

approximately $0.1 million of total unrecognized compensation cost related to outstanding restricted stock issued to the Company’s Directors.