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Equity Incentive Plans and Stock-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
Shares available for grant under stock-based incentive plans | A summary of shares available for grant under the Company’s plans is as follows:
| Shares Available for Grant
Shares available as of December 31, 2015 | 11,173,545
Stock options granted | (500,000)
Stock options forfeited | 1,081,107
Stock options expired under former plans | (412,467)
Nonvested equity stock and stock units granted (1) (2) | (5,316,675)
Nonvested equity stock and stock units forfeited (1) | 1,279,858
Total shares available for grant as of December 31, 2016 | 7,305,368
Stock options granted | (558,426)
Stock options forfeited | 1,978,042
Nonvested equity stock and stock units granted (1) (3) | (5,007,947)
Nonvested equity stock and stock units forfeited (1) | 1,334,110
Total shares available for grant as of December 31, 2017 | 5,051,147
Increase in shares approved for issuance | 5,500,000
Stock options granted | (711,479)
Stock options forfeited | 877,803
Nonvested equity stock and stock units granted (1) (4) | (4,993,802)
Nonvested equity stock and stock units forfeited (1) | 4,350,377
Total shares available for grant as of December 31, 2018 | 10,074,046
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(1) | For purposes of determining the number of shares available for grant under the 2015 Plan against the maximum number of shares authorized, each restricted stock granted reduces the number of shares available for grant by 1.5 shares and each restricted stock forfeited increases shares available for grant by 1.5 shares.
(2) | Amount includes 300,003 shares that had been reserved for potential future issuance related to certain performance unit awards discussed under the section titled "Nonvested Equity Stock and Stock Units" below.
(3) | Amount includes 394,853 shares that had been reserved for potential future issuance related to certain performance unit awards discussed under the section titled "Nonvested Equity Stock and Stock Units" below.
(4) | Amount includes 525,965 shares that have been reserved for potential future issuance related to certain performance unit awards 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 stock option plans for the years ended December 31, 2018, 2017 and 2016 and information regarding stock options outstanding, exercisable, and vested and expected to vest as of December 31, 2018.
| Options Outstanding |  | Weighted Average Remaining Contractual Term |  |
| Number of Shares |  | Weighted Average Exercise Price per Share |  |  | Aggregate Intrinsic Value
| (Dollars in thousands, except per share amounts)
Outstanding as of December 31, 2015 | 8,995,017 |  | $ | 10.01 |  |  |  |  |
Options granted | 500,000 |  | $ | 12.29 |  |  |  |  |
Options exercised | (1,405,077) |  | $ | 7.27 |  |  |  |  |
Options forfeited | (1,081,107) |  | $ | 18.98 |  |  |  |  |
Outstanding as of December 31, 2016 | 7,008,833 |  | $ | 9.34 |  |  |  |  |
Options granted | 558,426 |  | $ | 12.95 |  |  |  |  |
Options exercised | (1,278,856) |  | $ | 7.34 |  |  |  |  |
Options forfeited | (1,978,042) |  | $ | 10.68 |  |  |  |  |
Outstanding as of December 31, 2017 | 4,310,361 |  | $ | 9.78 |  |  |  |  |
Options granted | 711,479 |  | $ | 12.84 |  |  |  |  |
Options exercised | (908,146) |  | $ | 6.70 |  |  |  |  |
Options forfeited | (877,803) |  | $ | 13.73 |  |  |  |  |
Outstanding as of December 31, 2018 | 3,235,891 |  | $ | 10.25 |  |  | 4.01 |  | $ | 1,675 |
Vested or expected to vest at December 31, 2018 | 3,205,109 |  | $ | 10.22 |  |  | 3.97 |  | $ | 1,675 |
Options exercisable at December 31, 2018 | 2,656,079 |  | $ | 9.71 |  |  | 2.98 |  | $ | 1,675 |
Schedule of shares authorized under Stock Option Plans, by exercise price range | The following table summarizes the information about stock options outstanding and exercisable as of December 31, 2018:
| Options Outstanding |  | Options Exercisable
Range of Exercise Prices | Number Outstanding |  | Weighted Average Remaining Contractual Life (in years) |  | Weighted Average Exercise Price |  | Number Exercisable |  | Weighted Average Exercise Price
$4.13 – $5.39 | 53,584 |  | 3.6 |  | $ | 4.43 |  |  | 53,584 |  | $ | 4.43 |
$5.46 – $5.46 | 362,014 |  | 2.2 |  | $ | 5.46 |  |  | 362,014 |  |  | $ | 5.46 |
$5.63 – $5.76 | 324,773 |  | 1.0 |  | $ | 5.71 |  |  | 324,773 |  | $ | 5.71 |
$6.83 – $8.73 | 275,875 |  | 2.5 |  | $ | 7.73 |  |  | 275,875 |  | $ | 7.73 |
$8.76 – $8.76 | 540,878 |  | 3.1 |  | $ | 8.76 |  |  | 540,878 |  | $ | 8.76 |
$9.53 – $11.93 | 325,858 |  | 4.1 |  | $ | 11.23 |  |  | 319,736 |  | $ | 11.23 |
$11.93 – $12.31 | 433,488 |  | 5.4 |  | $ | 12.23 |  |  | 289,796 |  | $ | 12.27 |
$12.46 – $12.84 | 582,596 |  | 7.6 |  | $ | 12.80 |  |  | 215,598 |  | $ | 12.81 |
$13.60 – $21.95 | 274,025 |  | 4.3 |  | $ | 16.00 |  |  | 211,025 |  | $ | 16.71 |
$22.72 – $22.72 | 62,800 |  | 0.8 |  | $ | 22.72 |  |  | 62,800 |  | $ | 22.72 |
$4.13 – $22.72 | 3,235,891 |  | 4.0 |  | $ | 10.25 |  |  | 2,656,079 |  | $ | 9.71 |
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 for Years Ended December 31,
| 2018 |  | 2017 |  | 2016
Stock Option Plan |  |  |  |  |
Expected stock price volatility | 24%-32% |  | 24%-32% |  | 34%-36%
Risk free interest rate | 2.6%-2.8% |  | 1.8%-2.0% |  | 1.3%-1.7%
Expected term (in years) | 5.8 |  | 5.3-5.4 |  | 5.4-6.1
Weighted-average fair value of stock options granted | $4.23 |  | $4.09 |  | $4.59
Weighted-average assumptions for Employee Stock Purchase Plan |
| Employee Stock Purchase Plan for Years Ended December 31,
| 2018 |  | 2017 |  | 2016
Employee Stock Purchase Plan |  |  |  |  |
Expected stock price volatility | 27%-34% |  | 25%-27% |  | 31%-33%
Risk free interest rate | 2.05%-2.5% |  | 0.98%-1.3% |  | 0.41%-0.5%
Expected term (in years) | 0.5 |  | 0.5 |  | 0.5
Weighted-average fair value of purchase rights granted under the purchase plan | $2.59 |  | $3.07 |  | $2.88
Activity related to nonvested equity stock and stock units | The following table reflects the activity related to nonvested equity stock and stock units for the three years ended December 31, 2018:
Nonvested Equity Stock and Stock Units | Shares |  | Weighted-Average Grant-Date Fair Value
Nonvested at December 31, 2015 | 3,008,118 |  | $ | 11.32 |
Granted | 3,344,448 |  | $ | 12.84 |
Vested | (789,864) |  | $ | 10.98 |
Forfeited | (699,646) |  | $ | 11.94 |
Nonvested at December 31, 2016 | 4,863,056 |  | $ | 12.33 |
Granted | 3,075,396 |  | $ | 13.02 |
Vested | (1,216,476) |  | $ | 12.15 |
Forfeited | (860,627) |  | $ | 12.61 |
Nonvested at December 31, 2017 | 5,861,349 |  | $ | 12.68 |
Granted | 2,978,558 |  | $ | 12.77 |
Vested | (1,713,930) |  | $ | 12.39 |
Forfeited | (2,266,842) |  | $ | 12.97 |
Nonvested at December 31, 2018 | 4,859,135 |  | $ | 12.71 |

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