In September 2020, our board of directors authorized the purchase of up to $500 million, in the aggregate, of our common stock. This program is funded with cash on hand and extends through December 31, 2021. The exact timing and amount of any repurchase is determined by management based on market conditions and share price, in addition to other factors, and subject to the restrictions relating to volume, price, and timing under applicable law. Under this program, pursuant to a Rule 10b5-1 trading plan, we purchased approximately 766,000 shares of our common stock for $159 million in November and December 2020 (average cost of $208.37 per share), including approximately 29,000 shares purchased for $6 million in late December 2020, and settled in early January 2021.

In December 2019, our board of directors authorized the purchase of up to $500 million, in the aggregate, of our common stock. This program was funded with existing cash on hand and was completed in March 2020. Under this program, pursuant to a Rule 10b5-1 trading plan, we purchased approximately 3.4 million shares of our common stock for $446 million in the first quarter of 2020 (average cost of $132.45 per share). In the first quarter of 2020, we also paid $7 million to settle shares purchased in late December 2019.

In connection with the 1.125% Convertible Notes settlement transaction described in Note 11, “Debt,” in the first quarter of 2020 we entered into privately negotiated agreements to terminate the associated 310,000 warrants outstanding for $30 million, which resulted in a reduction of additional paid-in-capital for the same amount.

In connection with our employee stock plans, approximately 244,000 shares and 242,000 shares of common stock were issued, net of shares used to settle employees’ income tax obligations, during the years ended December 31, 2020, and 2019, respectively. Total share-based compensation expense is reported in “General and administrative expenses” in the accompanying consolidated statements of income, and summarized below.


	Year Ended December 31,
	2020				2019				2018
	(In millions)
	Pretax Charges		Net-of-Tax Amount		Pretax Charges		Net-of-Tax Amount		Pretax Charges		Net-of-Tax Amount
RSAs, PSAs and PSUs (defined below)	$	47	$	44	$	29	$	28	$	17	$	17
Employee stock purchase plan and stock options	10		9		10		9		10		9
Total	$	57	$	53	$	39	$	37	$	27	$	26

At December 31, 2020, we had employee equity incentives outstanding under our 2019 Equity Incentive Plan (the “2019 EIP”). The 2019 EIP provides for awards, in the form of restricted and performance stock awards (“RSAs” and “PSAs”), performance units (“PSUs”), stock options, and other stock– or cash–based awards, to eligible persons who perform services for us. The 2019 EIP provides for the issuance of up to 2.9 million shares of our common stock.

Stock-based awards. RSAs, PSAs and PSUs are granted with a fair value equal to the market price of our common stock on the date of grant, and generally vest in equal annual installments over periods up to four years from the date of grant. Certain PSUs may vest in their entirety at the end of three-year performance periods, if their performance conditions are met. We generally recognize expense for RSAs, PSAs and PSUs on a straight-line basis. Activity for stock-based awards in the year ended December 31, 2020, is summarized below.


	RSAs	Weighted Average Grant Date Fair Value		PSUs	Weighted Average Grant Date Fair Value
Unvested balance, December 31, 2019	447,680	$	102.41	324,078	$	101.45
Granted	344,739	127.89		188,522	123.61
Vested	(172,675)	98.05		(7,368)	68.16
Forfeited	(34,660)	113.10		(33,323)	98.34
Unvested balance, December 31, 2020	585,084	$	118.07	471,909	$	111.04

As of December 31, 2020, total unrecognized compensation expense related to unvested RSAs and PSUs was $42 million, and $23 million, respectively, which we expect to recognize over a remaining weighted-average period of 2.2 years, and 1.2 years, respectively. This unrecognized compensation cost assumes an estimated forfeiture rate of 12.2% for non-executive employees as of December 31, 2020, based on actual forfeitures over the last 4 years.


	Year Ended December 31,
	2020		2019		2018
	(In millions)
Granted:
RSAs	$	44	$	33	$	28
PSUs	23		20		16
Total granted	$	67	$	53	$	44
Vested:
RSAs	$	22	$	19	$	15
PSAs	—		—		3
PSUs	1		2		—
Total vested	$	23	$	21	$	18

Stock Options. Stock option awards generally have an exercise price equal to the fair market value of our common stock on the date of grant, vest in equal annual installments over periods up to four years from the date of grant, and have a maximum term of ten years from the date of grant. Stock option activity for the year ended December 31, 2020, is summarized below.


	Number of Shares	Weighted Average Exercise Price		Aggregate Intrinsic Value		Weighted Average Remaining Contractual term
		(Per share)		(In millions)		(Years)
Stock options outstanding as of December 31, 2019 and December 31, 2020	405,000	$	64.79	$	60	6.4
Stock options exercisable and expected to vest as of December 31, 2020	405,000	$	64.79	$	60	6.4
Exercisable as of December 31, 2020	405,000	$	64.79	$	60	6.4

No stock options were granted or exercised in 2020, 2019, or 2018, and as of December 31, 2020, there was no unrecognized compensation expense related to unvested stock options.


	Options Outstanding and Exercisable
	Number of Shares	Weighted-Average Remaining Contractual Life	Weighted-Average Exercise Price
		(Years)	(Per share)
Range of Exercise Prices
$33.02	30,000	2.2	$	33.02
$67.33	375,000	6.8	$	67.33
Total	405,000

Under our ESPP, eligible employees may purchase common shares at 85% of the lower of the fair market value of our common stock on either the first or last trading day of each six-month offering period. Each participant is limited to a maximum purchase of $25,000 (as measured by the fair value of the stock acquired) per year through payroll deductions. We estimate the fair value of the stock issued using a standard option pricing model. For the years ended December 31, 2020, 2019, and 2018, the inputs to this model were as follows: risk-free interest rates of approximately 0.2% to 2.3%; expected volatility of approximately 31% to 51%, dividend yields of 0%, and an average expected life of 0.5 years.