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Stock-Based Compensation
 9 Months Ended
Sep. 30, 2018
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 nine months ended September 30, 2018 and 2017:


Three Months Ended September 30,
 
Nine Months Ended September 30,
(in millions)
2018
 
2017
 
2018
 
2017
Stock option compensation expense
$
0.5

 
$
0.6

 
$
1.5

 
$
1.8

RSU compensation expense
0.8

 
0.8

 
3.7

 
3.5

PSU compensation expense (income)
(2.5
)
 
2.7

 
0.8

 
6.6

Total stock-based compensation expense (income)
$
(1.2
)
 
$
4.1

 
$
6.0

 
$
11.9



We generally recognize compensation expense for stock-based awards ratably over the vesting period. Stock-based compensation expense for each of the nine months ended September 30, 2018 and 2017 includes $1.0 million and $0.8 million, respectively, of expense related to stock awards granted to eligible non-employee directors, which were fully vested on the grant date.

The following table summarizes our unrecognized compensation expense and the weighted-average period over which the expense will be recognized as of September 30, 2018:

September 30, 2018

Unrecognized
 
Weighted Average

Compensation
 
Years Expense To Be
(in millions, except weighted average years)
Expense
 
Recognized Over
Stock options
$3.9
 
2.1
RSUs
$6.4
 
2.0
PSUs
$3.9
 
1.3