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Stock-Based Compensation | 6 Months Ended
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Jun. 30, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
Stock-Based Compensation | 7. Stock-Based Compensation The following table summarizes our stock-based compensation expense (including stock options, restricted stock units ("RSUs") and performance stock units ("PSUs")) for the three and six months ended June 30, 2019 and 2018:
| Three Months Ended June 30, |  | Six Months Ended June 30,
(in millions) | 2019 |  | 2018 |  | 2019 |  | 2018
Stock option compensation expense | $ | 0.8 |  |  | $ | 0.5 |  |  | $ | 1.3 |  |  | $ | 1.0 |
RSU compensation expense | 1.9 |  |  | 2.0 |  |  | 2.9 |  |  | 2.9 |
PSU compensation expense | 0.7 |  |  | 1.5 |  |  | 1.2 |  |  | 3.3 |
Total stock-based compensation expense | $ | 3.4 |  |  | $ | 4.0 |  |  | $ | 5.4 |  |  | $ | 7.2 |
We generally recognize compensation expense for stock-based awards ratably over the vesting period. Stock-based compensation expense for each of the three and six months ended June 30, 2019 and 2018 includes $1.0 million and $1.0 million, respectively, of expense related to stock awards granted to eligible non-employee directors, which were fully vested on the grant date. During the second quarter of 2019, the Company's Board of Directors approved stock compensation grants to executive officers, which consisted of 204,981 RSUs and 529,496 PSUs. The Board of Directors also approved the annual stock compensation grant to eligible non-employee directors, which consisted of 119,925 RSUs. The following table summarizes our unrecognized compensation expense and the weighted-average period over which the expense will be recognized as of June 30, 2019:
| June 30, 2019
| Unrecognized |  | Weighted Average
| Compensation |  | Years Expense To Be
(in millions, except weighted average years) | Expense |  | Recognized Over
Stock options | $5.0 |  | 2.2
RSUs | $8.0 |  | 2.2
PSUs | $8.3 |  | 2.2

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- Definition The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -URI http://asc.fasb.org/topic&trid=2228938 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -URI http://asc.fasb.org/subtopic&trid=2208855 + Details
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