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Stock-Based Compensation (Tables) - 2008 Equity Incentive Plan | 9 Months Ended
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Sep. 30, 2018
Activity Under Equity Incentive Plans Related to Stock Options | Activity under the Company’s equity incentive plans related to stock options is set forth below (in thousands except per share amounts):
|  | Number of Options Outstanding |  |  | Weighted Average Exercise Price per Share |
Balances at December 31, 2017 |  |  | 17,138 |  |  | $ | 4.27 |
Granted |  |  | 3,145 |  |  |  | 4.61 |
Exercised |  |  | (1,906 | ) |  |  | 3.43 |
Forfeited |  |  | (604 | ) |  |  | 4.66 |
Expired |  |  | (151 | ) |  |  | 5.93 |
Balances at September 30, 2018 |  |  | 17,622 |  |  |  | 4.39 |
Activity Under Equity Incentive Plans Related to RSUs | Activity under the Company’s equity incentive plans related to RSUs is set forth below (in thousands except per share amounts):
|  | Number of RSUs Unvested |  |  | Weighted Average Grant Date Fair Value per Share |
Balances at December 31, 2017 |  |  | 1,256 |  |  | $ | 4.53 |
Granted (1) |  |  | 1,382 |  |  |  | 4.45 |
Vested |  |  | (521 | ) |  |  | 4.45 |
Forfeited |  |  | (139 | ) |  |  | 4.20 |
Balances at September 30, 2018 |  |  | 1,978 |  |  |  | 4.52 |
| (1) | Includes the number of shares issuable under the performance-based restricted stock unit awards granted during the nine months ended September 30, 2018.

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- Definition Tabular disclosure of the number and weighted-average grant date fair value for restricted stock units that were outstanding at the beginning and end of the year, and the number of restricted stock units that were granted, vested, or forfeited during the year. + 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 (c)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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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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