Stock-Based Compensation (Tables)

RSU activity for the nine months ended September 30, 2015 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, 2014	1,380		$	27.17
RSUs granted	943		$	35.34
RSUs vested	(664	)	$	24.86
RSUs forfeited	(78	)	$	31.41
Unvested RSUs outstanding at September 30, 2015	1,581		$	32.80

The fair values of the 2015 and 2014 awards were estimated using Monte-Carlo simulation with the following weighted-average assumptions:



	For the Nine Months Ended September 30,
	2015		2014

Expected dividend yield	—	%	—	%
Risk-free interest rate ⁽¹⁾	0.93	%	0.74	%
Expected volatility ⁽²⁾	24.01	%	27.88	%
Average total stockholder return ⁽²⁾	8.37	%	(0.90	)%


(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 is a measure 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 nine months ended September 30, 2015 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, 2014	903		$	22.19
PBRSUs granted	223		$	34.01
PBRSUs vested	(415	)	$	16.51
PBRSUs forfeited	(49	)	$	30.83
Unvested PBRSUs outstanding at September 30, 2015	662		$	29.21

The fair values of these stock options were estimated using the Black-Scholes valuation model with the following weighted-average assumptions:


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


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


(3)	The expected life is the period of time, on average, that participants are expected to hold their options before exercise based primarily on our historical data.

Option activity for the nine months ended September 30, 2015 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, 2014	2,562		$	22.32
Options exercised	(676	)	$	24.56
Options canceled	(28	)	$	29.27
Options outstanding at September 30, 2015	1,858		$	21.40	5.3	$	29,409
Options vested and expected to vest at September 30, 2015	1,849		$	21.36	5.3	$	29,348
Options exercisable at September 30, 2015	1,604		$	20.14	4.9	$	27,419

The following table sets forth the stock-based compensation expense recognized for the three and nine months ended September 30, 2015 and 2014.



	For the Three Months Ended				For the Nine Months Ended
	September 30,				September 30,
(in thousands)	2015		2014		2015		2014
RSUs	$	5,699	$	4,316	$	18,532	$	14,746
PBRSUs	1,433		1,227		5,485		3,973
Stock options	443		778		1,480		2,999
Employee stock purchase plan	305		151		922		563
	$	7,880	$	6,472	$	26,419	$	22,281