STOCK-BASED COMPENSATION PLANS (Tables)

12 Months Ended

Dec. 31, 2020

Share-based Payment Arrangement [Abstract]

Summary of stock-based compensation expense, net of amounts capitalized

The following summarizes stock-based compensation expense, net of amounts capitalized to development projects, for the years ended December 31:

millions	2020		2019		2018
Stock Options (a)(b)	$	(1.9)	$	1.4	$	2.2
Restricted Stock (c)(d)	6.5		16.7		7.5
Pre-tax stock-based compensation expense	$	4.6	$	18.1	$	9.7
Income tax benefit	$	0.2	$	3.7	$	1.7

(a)Amounts shown are net of $0.2 million capitalized to development projects in 2020, $0.4 million capitalized to development projects in 2019 and $2.4 million capitalized to development projects in 2018.

(b)The credit position for the year ended December 31, 2020, was due to significant forfeitures which exceeded the expense.

(c)Amounts shown are net of $0.9 million capitalized to development projects in 2020, $1.0 million capitalized to development projects in 2019 and $1.0 million capitalized to development projects in 2018.(d)The increase in compensation expense for the year ended December 31, 2019, compared to the year ended December 31, 2018, was generally in connection with the Company’s restructuring.

Summary of stock option plan activity

The following table summarizes stock option activity:

	Stock Options	Weighted-average Exercise Price		Weighted-average Remaining Contractual Term (years)	Aggregate Intrinsic Value
Stock options outstanding at December 31, 2019	721,496	$	104.55
Granted	41,337	$	74.65
Exercised (a)	(106,237)	65.91
Forfeited	(253,600)	121.63
Expired	(30,260)	106.39
Stock options outstanding at December 31, 2020	372,736	$	100.49	4.7	$	1,908,239
Stock options vested and expected to vest at December 31, 2020	369,384	$	100.55	4.7	$	1,899,155
Stock options exercisable at December 31, 2020	257,899	$	99.18	3.3	$	1,719,245

(a)The total intrinsic value of stock options exercised was $2.4 million during 2020, $2.4 million during 2019, and $12.1 million during 2018, based on the difference between the market price at the exercise date and the exercise price.

Summary of significant assumptions used in Black-Scholes option-pricing model

The fair value on the grant date and the significant assumptions used in the Black‑Scholes option‑pricing model are as follows:

	As of December 31,
	2020			2019			2018
Grant date fair value	$	32.10		$	32.51		$	48.27
Assumptions
Expected life of options (in years)	7.5			7.5			8.4
Risk-free interest rate	0.7		%	2.2		%	2.7		%
Expected volatility	40.4		%	22.6		%	24.7		%
Expected annual dividend per share	—			—			—

Summary of restricted stock activity

The following table summarizes restricted stock activity:

	Restricted Stock	Weighted-average Grant Date Fair Value
Restricted stock outstanding at December 31, 2019	406,802	$	76.27
Granted	181,079	71.48
Vested	(77,276)	108.60
Forfeited	(101,495)	71.06
Restricted stock outstanding at December 31, 2020	409,110	$	69.21