Stock-Based Compensation

6 Months Ended

Jun. 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 six months ended June 30, 2018 and 2017:

	Three Months Ended June 30,				Six Months Ended June 30,
(in millions of dollars)	2018		2017		2018		2017
Stock option compensation expense	$	0.5	$	0.6	$	1.0	$	1.2
RSU compensation expense	2.0		1.7		2.9		2.7
PSU compensation expense	1.5		3.1		3.3		3.9
Total stock-based compensation expense	$	4.0	$	5.4	$	7.2	$	7.8

During the second quarter of 2018, the Company's Board of Directors approved the annual stock compensation grant to eligible non-employee directors, which consisted of 77,589 RSUs.

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, 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 June 30, 2018:

	June 30, 2018
	Unrecognized	Weighted Average
	Compensation	Years Expense To Be
(in millions of dollars, except weighted average years)	Expense	Recognized Over
Stock options	$4.4	2.3
RSUs	$7.5	2.2
PSUs	$13.4	1.9