Stock-based compensation and warrants (Tables)

12 Months Ended

Dec. 31, 2015

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Summary of Stock-Based Compensation Expense by Award Type

Share-based compensation expense recognized by award type is as follows (in thousands):

	Year ended December 31,
	2015		2014		2013
Stock options	$	37,536	$	9,487	$	6,399
Restricted stock awards		—		52		92
Restricted stock units		3,325		1,158		—
Employee stock purchase plan		259		66		—
	$	41,120	$	10,763	$	6,491

Assumptions Used for the Black-Scholes Option-Pricing Model to Determine the Per Share Weighted Average Fair Value for Options Granted

The fair value of each option issued to employees was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions:

	Year ended December 31,
	2015			2014			2013
Expected volatility		72.6	%		82.3	%		82.0	%
Expected term (in years)		5.9			6.0			6.1
Risk-free interest rate		1.7	%		1.8	%		1.1	%
Expected dividend yield	0.0%			0.0%			0.0%

Summary of Stock Option Activity Under Plan

The following table summarizes the stock option activity under the Company’s equity awards plans (shares and aggregate intrinsic value in thousands):

	Shares			Weighted-average exercise price per share		Weighted-average contractual life (in years)		Aggregate intrinsic value (a) (in thousands)
Outstanding at December 31, 2014		3,652		$	12.30
Granted		1,245		$	114.32
Exercised		(1,283	)	$	7.42
Canceled or forfeited		(82	)	$	68.16
Outstanding at December 31, 2015		3,532		$	48.74		7.8	$	114,870
Exercisable at December 31, 2015		1,159		$	11.53		6.7	$	61,163
Vested and expected to vest at December 31, 2015		3,423		$	50.02		8.0	$	110,888

(a) The aggregate intrinsic value is calculated as the difference between the exercise price of the underlying options and the estimated fair value of the common stock for the options that were in the money at December 31, 2015.

Restricted Stock Units [Member]

Share Based Compensation Arrangement By Share Based Payment Award [Line Items]

Summary of Restricted Common Stock Awards

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, 2014		179		$	30.47
Granted		41			156.83
Vested		(62	)		30.47
Forfeited		(10	)		30.47
Unvested balance at December 31, 2015		148		$	65.79