Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2013

A summary of stock option activity for the three months ended March 31, 2013 is as follows:


				Weighted-
				Average
	Stock Options			Exercise Price
Options outstanding at December 31, 2012		1,691,357		$	11.11
Granted		125,325			47.08
Exercised		(351,216	)		4.60
Cancelled		(250	)		30.58

Options outstanding at March 31, 2013		1,465,216		$	15.74

Exercisable at March 31, 2013		491,674		$	7.07

Assumptions Used in Black-Scholes Option Pricing Model

The following table provides the assumptions used in the Black-Scholes pricing model valuation of options:


	Three months ended March 31,
	2013			2012
Risk-free interest rate		1.27	%		1.16	%
Expected life (years)		6.50			6.25
Expected volatility		53.54	%		53.00	%
Expected dividend yield		0	%		0	%

Assumptions Used to Estimate Fair Value of Employee Stock Purchase Plan

The following table presents the assumptions used to estimate the fair value of the ESPP during the three months ended March 31, 2013 and 2012, respectively:


	Three months ended March 31,
	2013			2012
Risk-free interest rate		0.13	%		0.16	%
Expected life (months)		6.0			8.5
Expected volatility		53.14	%		53.00	%
Expected dividend yield		0	%		0	%