Stock-Based Compensation

3 Months Ended

Mar. 31, 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 months ended March 31, 2018 and 2017:

	Three Months Ended March 31,
(in millions of dollars)	2018		2017
Stock option compensation expense	$	0.5	$	0.6
RSU compensation expense	0.9		1.0
PSU compensation expense	1.8		0.8
Total stock-based compensation expense	$	3.2	$	2.4

We generally recognize compensation expense for stock-based awards ratably over the vesting period. During thefirstquarter of 2018, the Company's Board of Directors approved a stock compensation grant which consisted of769,477 stock options, 387,789RSUs and747,996PSUs.

The following table summarizes our unrecognized compensation expense and the weighted-average period over which the expense will be recognized as of March 31, 2018:

	March 31, 2018
	Unrecognized	Weighted Average
	Compensation	Years Expense To Be
(in millions of dollars, except weighted average years)	Expense	Recognized Over
Stock options	$5.0	2.6
RSUs	$8.6	2.4
PSUs	$17.8	2.1