Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2015

The following summarizes the assumptions used for estimating the fair value of stock options granted during the periods indicated:



	Year Ended December 31,
	2015	2014	2013
Risk-free interest rate	1.5 - 1.6%	1.2 - 1.8%	0.7 - 2.2%
Expected life (in years)	4.3 - 4.8	3.8 - 6.1	4.8 - 6.9
Expected volatility	45.7 - 46.9%	45.1 - 46.8%	46.4 - 49.4%
Dividend yield	—	—	—
Weighted-average grant date fair value per share	$9.38	$5.63	$3.15

Stock option activity was as follows:



	Number of Options		Weighted Average Exercise Price
Balance as of January 1, 2013	5,732		$	1.94
Granted	834		7.69
Exercised	(738	)	0.59
Forfeited	(406	)	5.31
Balance as of December 31, 2013	5,422		2.76
Granted	2,331		10.10
Exercised	(1,583	)	1.27
Forfeited	(59	)	7.56
Balance as of December 31, 2014	6,111		5.90
Granted	582		23.12
Exercised	(1,578	)	2.71
Forfeited	(71	)	8.95
Balance as of December 31, 2015	5,044		$	8.84

The summary of stock options outstanding as of December 31, 2015 is as follows:



	Options Outstanding				Options Exercisable
Range of Exercise Prices	Number of Options	Weighted Average Exercise Price		Weighted Average Remaining Contractual Life (in years)	Number of Options	Weighted Average Exercise Price		Weighted Average Remaining Contractual Life (in years)
$0.29 - $0.84	639	$	0.47	2.8	639	$	0.47	2.8
$1.74 - $3.10	637	2.79		5.4	637	2.79		5.4
$4.00 - $7.82	995	6.98		4.7	591	6.66		4.7
$8.35	1,650	8.35		5.1	104	8.35		4.9
$13.00 - $19.44	541	15.40		5.6	155	14.97		5.5
$20.19 - $29.38	582	23.12		6.3	6	24.89		6.4
	5,044	$	8.84	5.0	2,132	$	4.39	4.4

The summary of restricted stock unit activity is as follows:



	Number of Shares		Weighted Average Grant Date Fair Value
Nonvested as of January 1, 2014	—		$	—
Granted	28		19.44
Vested	—		—
Forfeited	—		—
Nonvested as of December 31, 2014	28		19.44
Granted	707		26.39
Vested	(7	)	19.44
Forfeited	(12	)	26.14
Nonvested as of December 31, 2015	716		$	26.19