Equity Incentive Plans and Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2019

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

Schedule of Share Based Compensation Stock Options Awards, Number of Shares Activity [Table Text Block]

A summary of shares available for grant under the Company’s plan is as follows:

	Shares Available for Grant
Shares available as of December 31, 2018	10,074,046
Stock options granted	—
Stock options forfeited	33,967
Nonvested equity stock and stock units granted (1) (2)	(5,907,629	)
Nonvested equity stock and stock units forfeited (1)	315,349
Total available for grant as of March 31, 2019	4,515,733

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(1) For purposes of determining the number of shares available for grant under the 2015 Equity Incentive Plan (the “2015 Plan”) against the maximum number of shares authorized, each share of restricted stock granted reduces the number of shares available for grant by 1.5 shares and each share of restricted stock forfeited increases shares available for grant by 1.5 shares.

(2) Amount includes approximately 0.9 million shares that have been reserved for potential future issuance related to certain performance unit awards granted in the first quarter of 2019 and discussed under the section titled "Nonvested Equity Stock and Stock Units" below.

Schedule of stock option activity

The following table summarizes stock option activity under the 2015 Plan for the three months ended March 31, 2019 and information regarding stock options outstanding, exercisable, and vested and expected to vest as of March 31, 2019.

	Options Outstanding
	Number of Shares		Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Term (years)	Aggregate Intrinsic Value
	(In thousands, except per share amounts)
Outstanding as of December 31, 2018	3,235,891		$	10.25
Options granted	—		$	—
Options exercised	(704,851	)	$	6.89
Options forfeited	(33,967	)	$	8.58
Outstanding as of March 31, 2019	2,497,073		$	11.22	4.72	$	2,779
Vested or expected to vest at March 31, 2019	2,473,989		$	11.20	4.69	$	2,779
Options exercisable at March 31, 2019	1,972,769		$	10.82	3.70	$	2,779

Weighted-average assumptions for Stock Option Plans

The following table presents the weighted-average assumptions used to estimate the fair value of stock options granted that contain only service conditions in the periods presented.

	Stock Option Plan
	Three Months Ended
	March 31, 2018
Stock Option Plan
Expected stock price volatility	29		%
Risk free interest rate	2.6		%
Expected term (in years)	5.8
Weighted-average fair value of stock options granted to employees	$	4.24

Schedule of nonvested equity stock and stock units activity

The following table reflects the activity related to nonvested equity stock and stock units for the three months ended March 31, 2019:

Nonvested Equity Stock and Stock Units	Shares		Weighted- Average Grant-Date Fair Value
Nonvested at December 31, 2018	4,859,135		$	12.71
Granted	3,389,337		$	9.56
Vested	(1,076,198	)	$	12.49
Forfeited	(170,752	)	$	11.89
Nonvested at March 31, 2019	7,001,522		$	11.24