Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2016

RSU activity for the six months ended June 30, 2016 is as follows:



	Number of		Weighted-Average Grant-Date
(in thousands, except weighted-average fair value prices)	Shares		Fair Value
Unvested RSUs outstanding at December 31, 2015	1,537		$	32.92
RSUs granted	943		$	34.68
RSUs vested	(690	)	$	32.57
RSUs forfeited	(57	)	$	34.48
Unvested RSUs outstanding at June 30, 2016	1,733		$	33.96

The fair values of the 2016 and 2015 awards were estimated using Monte-Carlo simulation with the following weighted-average assumptions:



	For the Six Months Ended June 30,
	2016		2015

Expected dividend yield	—	%	—	%
Risk-free interest rate ⁽¹⁾	0.99	%	0.93	%
Expected volatility ⁽²⁾	25.12	%	24.01	%
Average total stockholder return ⁽²⁾	1.48	%	8.37	%


(1)	The risk-free interest rate for the periods within the contractual term of the PBRSUs is based on the U.S. Treasury yield curve in effect at the time of the grant.


(2)	The expected volatility and average total stockholder return are measures of the amount by which a stock price has fluctuated or is expected to fluctuate based primarily on our and our peers' historical data.

PBRSU activity for the six months ended June 30, 2016 is as follows:



	Number of		Weighted-Average Grant-Date
(in thousands, except weighted-average fair value prices)	Shares		Fair Value
Unvested PBRSUs outstanding at December 31, 2015	659		$	29.15
PBRSUs granted	279		$	35.30
PBRSUs vested	(88	)	$	26.04
PBRSUs forfeited	(91	)	$	19.50
Unvested PBRSUs outstanding at June 30, 2016	758		$	34.10

Option activity for the six months ended June 30, 2016 is as follows:



(in thousands, except weighted-average price)	Number of Shares		Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Term	Aggregate Intrinsic Value
Options outstanding at December 31, 2015	1,826		$	21.33
Options exercised	(251	)	$	22.03
Options outstanding at June 30, 2016	1,575		$	21.22	5.0	$	27,152
Options vested and expected to vest at June 30, 2016	1,571		$	21.20	5.0	$	27,136
Options exercisable at June 30, 2016	1,495		$	20.68	4.9	$	26,618

The following table sets forth the stock-based compensation expense recognized for the three and six months ended June 30, 2016 and 2015.



	For the Three Months Ended				For the Six Months Ended
	June 30,				June 30,
(in thousands)	2016		2015		2016		2015
RSUs	$	6,560	$	6,819	$	13,578	$	12,833
PBRSUs	2,701		2,272		4,514		4,052
Stock options	217		474		601		1,037
Employee stock purchase plan	297		242		625		617
	$	9,775	$	9,807	$	19,318	$	18,539