Stock-based compensation

Service-based vesting stock options

The following table summarizes the activity for options that vest solely based upon the satisfaction of a service condition for the six months ended June 30, 2018:



	Options outstanding		Weighted-average exercise price		Weighted-average remaining contractual life (in years)	Aggregate intrinsic values (in thousands)
Balance as of December 31, 2017	2,597,294		$	10.66
Granted	585,250		18.43
Exercised	(122,090	)	3.38
Canceled or forfeited	(115,934	)	7.63
Balance as of June 30, 2018	2,944,520		$	12.62	8.0	$	13,101

Exercisable, June 30, 2018	1,237,480		$	7.26	7.2	$	10,755

The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying awards and the Company's closing stock of $15.24, as reported on the New York Stock Exchange on June 30, 2018.

The Company recognized stock-based compensation cost related to service-based vesting options of $0.9 million and $1.6 million in the three and six months ended June 30, 2018, respectively, and $0.6 million and $1.1 million in the three and six months ended June 30, 2017, respectively. As of June 30, 2018, there was $9.3 million of total unrecognized stock-based compensation cost related to unvested service-based stock options, which is expected to be recognized over the remaining weighted-average period of 2.8 years. All stock-based compensation expense is recorded in selling, general and administrative expenses.

Performance-based and market-based vesting stock options

The following table summarizes the activity for options that vest based upon the satisfaction of performance or market conditions for the six months ended June 30, 2018:



	Options outstanding		Weighted-average exercise price		Weighted-average remaining contractual life (in years)	Aggregate intrinsic values (in thousands)
Balance as of December 31, 2017	2,406,537		$	6.58
Exercised	(37,260	)	2.24
Canceled or forfeited	(22,250	)	12.95
Balance as of June 30, 2018	2,347,027		$	6.59	7.0	$	25,117

The Company recognized stock-based compensation cost related to performance-based and market-based vesting options of $0.4 million and $0.8 million in the three and six months ended June 30, 2018, respectively, and $1.1 million and $1.9 million in the three and six months ended June 30, 2017, respectively. As of June 30, 2018, there was $0.3 million of total unrecognized stock-based compensation cost related to unvested market-based stock options, which is expected to be recognized over a weighted-average period of 0.4 years.

Restricted stock

The following table summarizes the activities for restricted stock awards (“RSAs”) and restricted stock units (“RSUs”) for the six months ended June 30, 2018:



	Shares of restricted stock outstanding		Weighted-average grant date fair value
Balance as of December 31, 2017	1,248,824		$	23.37
Granted	1,129,355		18.49
Vested	(221,181	)	26.19
Canceled or forfeited	(60,564	)	21.09
Balance as of June 30, 2018	2,096,434		$	20.50

As of June 30, 2018, there were 885,520 unvested shares subject to RSAs outstanding.

The Company recognized stock-based compensation cost related to RSAs and RSUs of $3.3 million and $5.8 million in the three and six months ended June 30, 2018, respectively, and $1.8 million and $2.9 million in the three and six months ended June 30, 2017, respectively. As of June 30, 2018, there was $37.7 million of total unrecognized stock-based compensation cost related to unvested RSAs and RSUs, which is expected to be recognized over a weighted-average period of 3.0 years.