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Equity Incentive Plans and Stock-Based Compensation (Tables) | 3 Months Ended
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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 |
_________________________________________
(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 |

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- Definition Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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