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Stock-Based Compensation
3 Months Ended
Mar. 31, 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 months ended March 31, 2019 and 2018:


Three Months Ended March 31,
(in millions)
2019
 
2018
Stock option compensation expense
$
0.5

 
$
0.5

RSU compensation expense
1.0

 
0.9

PSU compensation expense
0.5

 
1.8

Total stock-based compensation expense
$
2.0

 
$
3.2



We generally recognize compensation expense for stock-based awards ratably over the vesting period. During the first quarter of 2019, the Company's Board of Directors approved stock compensation grants which consisted of 1,303,255 stock options, 354,695 RSUs and 365,893 PSUs.

The following table summarizes our unrecognized compensation expense and the weighted-average period over which the expense will be recognized as of March 31, 2019:

March 31, 2019

Unrecognized
 
Weighted Average

Compensation
 
Years Expense To Be
(in millions, except weighted average years)
Expense
 
Recognized Over
Stock options
$5.8
 
2.5
RSUs
$7.4
 
2.2
PSUs
$5.5
 
2.1