Stock-based compensation (Tables)

3 Months Ended

Mar. 31, 2016

Stock-based compensation expense by award type was as follows (in thousands):

	Three months ended March 31,
		2016		2015
Stock options	$	8,926	$	4,755
Restricted stock units		1,127		617
Employee stock purchase plan		90		60
	$	10,143	$	5,432

The following table summarizes the stock option activity under the Company’s equity award plans (shares in thousands):

	Shares			Weighted-average exercise price per share
Outstanding at December 31, 2015		3,532		$	48.74
Granted		713		$	50.93
Exercised		(33	)	$	14.34
Canceled or forfeited		(108	)	$	28.54
Outstanding at March 31, 2016		4,104		$	49.94
Exercisable at March 31, 2016		1,538		$	24.29
Vested and expected to vest at March 31, 2016		4,017		$	50.48

Restricted stock units

The following table summarizes the restricted stock unit activity under the Company’s equity award plans (shares in thousands):

	Shares			Weighted-average grant date fair value
Unvested balance at December 31, 2015		148		$	65.79
Granted		178		$	50.93
Vested		—		$	—
Forfeited		(6	)	$	30.47
Unvested balance at March 31, 2016		320		$	58.20