Share-Based Payments (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Summary of share-based compensation expense (income)

The table below presents share-based compensation expense for research and development as well as general and administrative expense, both of which are included in operating expenses, in the years ended December 31, 2017, 2016 and 2015 (in thousands):

	2017		2016		2015
Research and development	$	5,699	$	7,558	$	5,145
General and administrative	10,428		10,764		6,295
Total share-based compensation expense	$	16,127	$	18,322	$	11,440

The following table summarizes share-based compensation expense recorded in each of the years ended December 31, 2017, 2016 and 2015 (in thousands):

	2017		2016		2015
Stock options	$	10,036	$	9,831	$	10,548
Restricted stock awards and restricted stock units	5,608		8,064		504
Employee stock purchase plan	483		427		388
Total share-based compensation expense	$	16,127	$	18,322	$	11,440

Schedule of weighted average assumptions

The following table summarizes the weighted average assumptions the Company used in its fair value calculations at the date of grant:

	Weighted Average Assumptions
	Stock Options						Employee Stock Purchase Plan
	2017		2016		2015		2017		2016		2015
Expected volatility	53	%	58	%	59	%	55	%	57	%	59	%
Expected dividends	—		—		—		—		—		—
Expected life (years)	5.9		6.1		6.1		0.5		0.5		0.5
Risk-free interest rate	2.1	%	1.6	%	1.9	%	0.7	%	0.4	%	0.1	%

Schedule of stock option activity of company's stock plan

The following table presents stock option activity of the 2013 Plan and Prior Plans for the year ended December 31, 2017:

	Number of Stock Options (in thousands)		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in years)	Aggregate Intrinsic Value (in thousands)
Outstanding at December 31, 2016	7,009		$	13.68
Granted	1,531		17.88
Exercised	(803	)	11.46
Forfeited	(369	)	14.10
Expired	(251	)	16.55
Outstanding at December 31, 2017	7,117		$	14.71	5.84	$	6,751
Exercisable at December 31, 2017	4,575		$	14.23	4.55	$	4,593
Vested or expected to vest at December 31, 2017	6,854		$	14.65	5.74	$	6,594

Schedule of nonvested shares of restricted stock

A summary of the status of nonvested shares of restricted stock and restricted stock units as of December 31, 2017 and the changes during the year then ended are presented below (in thousands, except fair values):

	Number of Shares		Weighted Average Grant Date Fair Value
Nonvested at January 1, 2017	1,992		$	10.64
Granted	659		18.12
Vested	(286	)	12.64
Forfeited	(370	)	11.85
Nonvested at December 31, 2017	1,995		$	12.60

Summarized information about time-based or performance-based vesting schedules

Nonvested shares of restricted stock and restricted stock units that have time-based vesting schedules and restricted stock that have performance-based vesting schedules as of December 31, 2017 are summarized below (in thousands):

Vesting Schedule	Nonvested Shares
Time-based	692
Performance-based	1,303
Nonvested at December 31, 2017	1,995