Stock-Based Compensation (Tables)

Compensation expense for the stock plans, as reported in our consolidated statements of income, is as follows:


	Year Ended December 31
	2020		2019		2018
	(in millions of dollars)
Performance Share Units	$	5.6	$	5.0	$	6.8
Restricted Stock Units and Cash-Settled Restricted Stock Units	23.9		21.0		19.0

Stock Success Units	0.4		—		—
Other	0.5		0.6		0.5
Total Compensation Expense, Before Income Tax	$	30.4	$	26.6	$	26.3

Total Compensation Expense, Net of Income Tax	$	26.1	$	22.7	$	20.9

Stock Success Units (SSUs)

Activity for SSUs classified as equity is as follows:


		Weighted Average
	Shares	Grant Date
	(000s)	Fair Value
Outstanding at December 31, 2019	—	$	—
Granted	321	18.78


Outstanding at December 31, 2020	321	18.78

Activity for PSUs classified as equity is as follows:


		Weighted Average
	Shares	Grant Date
	(000s)	Fair Value
Outstanding at December 31, 2019	337	$	44.11
Granted	323	23.49
Vested	(135)	48.20
Forfeited	(8)	35.81
Outstanding at December 31, 2020	517	30.31

The fair value of PSUs is estimated on the date of initial grant using the Monte-Carlo simulation model. Key assumptions used to value PSUs granted during the years shown are as follows:


	Year Ended December 31
	2020		2019		2018
Expected Volatility (based on our and our peer group historical daily stock prices)	23	%	23	%	24	%
Expected Life (equals the performance period)	3 years		3 years		3 years
Risk Free Interest Rate (based on U.S. Treasury yields at the date of grant)	0.85	%	2.53	%	2.32	%

Activity for RSUs classified as equity is as follows:


		Weighted Average
	Shares	Grant Date
	(000s)	Fair Value
Outstanding at December 31, 2019	1,044	$	41.06
Granted	1,262	22.71
Vested	(626)	40.11
Forfeited	(87)	28.75
Outstanding at December 31, 2020	1,593	27.57

Activity for cash-settled RSUs classified as a liability is as follows:


		Weighted Average
	Shares	Grant Date
	(000s)	Fair Value
Outstanding at December 31, 2019	—	$	—
Granted	68	22.94


Outstanding at December 31, 2020	68	22.94

Stock option activity is summarized as follows:


				Remaining	Intrinsic
	Shares	Weighted Average		Contractual Term	Value
	(000s)	Exercise Price		(in years)	(in millions)
Outstanding at December 31, 2019	74	$	24.09

Exercised	(13)	23.35
Forfeited	(21)	24.25
Outstanding at December 31, 2020	40	24.25		0.1	$	—

Exercisable at December 31, 2020	40	$	24.25	0.1	$	—