Stock-Based Compensation

Stock Options

A summary of stock option activity during the nine months ended September 30, 2018 is as follows (in thousands, except for per share amounts):



	Shares		Weighted- Average Exercise Price (in dollars per share)		Weighted- Average Contractual Life (in years)	Aggregate Intrinsic Value
Options outstanding at December 31, 2017	5,648		$	37.71
Granted	1,388		34.43
Exercised	(461	)	16.14
Canceled	(407	)	42.53
Expired	(409	)	52.04
Options outstanding at September 30, 2018	5,759		$	37.29	7.48	$	5,728
Options exercisable at September 30, 2018	3,121		$	36.56	6.38	$	5,728

The weighted-average grant-date fair value per share of options granted during the three and nine months ended September 30, 2018 was $12.86 and $19.07, respectively. As of September 30, 2018, there was approximately $49.3 million of total unrecognized compensation expense related to unvested stock options, which is expected to be recognized over a weighted-average period of approximately 2.64 years.

Restricted Stock Units

A summary of RSU activity during the nine months ended September 30, 2018 is as follows (in thousands, except for per share amounts):



	RSUs		Weighted- Average Grant Date Fair Value (in dollars per share)
RSUs Outstanding at December 31, 2017	147		$	36.69
Granted	221		37.83
Vested	(24	)	41.37
Forfeited	(82	)	37.37
RSUs Outstanding at September 30, 2018	262		$	37.00

As of September 30, 2018, there was approximately $7.3 million of total unrecognized compensation expense related to unvested RSUs, which is expected to be recognized over a weighted-average period of approximately 2.84 years.

Employee Stock Purchase Plan

In September 2016, the Company initiated the first offering period under the Company's 2016 Employee Stock Purchase Plan (the “ESPP”), pursuant to which eligible employees may purchase shares of the Company’s common stock on the last day of each predetermined six-month offering period at 85% of the lower of the fair market value per share at the beginning or end of the applicable offering period. The offering periods run from March 1 through August 31 and from September 1 through February 28 (or February 29, in a leap year) of each year.

As of September 30, 2018, the Company had recorded a liability of $0.2 million related to its ESPP obligations. In accordance with the terms of its ESPP, the Company recorded stock-based compensation expense of $0.2 million and $0.6 million for the three and nine-month periods ended September 30, 2018, respectively.