Four equity compensation plans currently permit us to grant various types of equity awards. We currently grant incentive stock options, nonqualified stock options, service-based restricted stock units and performance-based restricted stock units to associates, including some with market-based performance objectives under our shareholder-approved plans. We also have a Holiday Stock Plan that permits annual awards of one share of common stock to each full-time associate for each full calendar year of service up to a maximum of 10 shares. One of our equity compensation plans permits us to grant stock to our outside directors as a component of their annual compensation. We used treasury shares for share-based compensation award issues or exercises during 2020 and 2019.

$6 million, and $5 million for the years ended December 31, 2020, 2019 and 2018, respectively. Options exercised during the years ended December 31, 2020, 2019 and 2018, had intrinsic value of $15 million, $26 million and $15 million, respectively. Intrinsic value is the market price less the exercise price. Options vested during the years ended December 31, 2020, 2019 and 2018, had total intrinsic value of $7 million, $23 million and $6 million, respectively.

As of December 31, 2020, we had $34 million of unrecognized total compensation cost related to nonvested stock options and restricted stock unit awards. That cost will be recognized over a weighted-average period of 1.7 years.

Stock options are granted to associates at an exercise price equal to the fair value as determined by the average high and low sales price reported on the Nasdaq Global Select Market for the grant date and are exercisable over 10-year periods. The stock options generally vest ratably over a three-year period. In determining the share-based compensation amounts, we estimate the fair value of each option granted on the date of grant using the Black Scholes pricing model. We make the following assumptions to develop the Black Scholes pricing model as follows:


	2020	2019	2018
Weighted-average expected term	7-8 years	7-8 years	7-8 years
Expected volatility	16.89-17.13%	14.49-15.39%	15.04-15.10%
Dividend yield	2.15%	2.61%	2.98%
Risk-free rates	1.40-1.41%	2.62-2.64%	2.77-2.83%
Weighted-average fair value of options granted during the period	$15.45	$11.73	$9.87


(Dollars in millions, except exercise price. Shares in thousands)	Shares	Weighted- average exercise price		Aggregate intrinsic value		Weighted-average remaining contractual life
Outstanding option shares at January 1, 2020	3,437	$	63.99
Granted	508	111.53
Exercised	(274)	41.86
Forfeited or expired	(70)	55.60
Outstanding option shares at December 31, 2020	3,601	72.55		$	66	6.01 years



Options exercisable at end of period	2,509	$	63.11	$	61	4.97 years

Cash received from the exercise of options was $7 million, $11 million and $9 million for the years ended December 31, 2020, 2019 and 2018, respectively. We acquired 50,751, 103,237 and 69,649 shares totaling

$5 million, $9 million and $5 million, respectively, from associates in consideration for option exercises during 2020, 2019 and 2018. The weighted-average remaining contractual life for options expected to vest as of December 31, 2020, was 8.38 years.

Under all active shareholder approved plans, a total of 17.3 million shares were authorized to be granted. At December 31, 2020, 7.1 million shares remained available for future issuance under the plans. During 2020, we granted 12,928 shares of common stock to our directors for 2019 board service fees.

Service-based restricted stock units granted to associates are valued at fair value of the shares on the date of grant less the present value of the dividends that holders of restricted stock units do not receive on the shares underlying the restricted stock units during the vesting period. Service-based restricted stock units generally cliff vest three years after the date of grant. We also grant restricted stock units which vest on a three year ratable vesting schedule. Service-based restricted stock units vested during the year had an intrinsic value of $30 million, $25 million and $24 million for the years ended December 31, 2020, 2019 and 2018, respectively.

We have performance-based awards that vest on the first day of March after a three-calendar-year performance period. These awards vest according to the level of three-year total shareholder return achieved compared with a peer group over a three-year performance period with payouts ranging from 0% to 200% for awards granted in 2020, 2019 and 2018. Three-year total shareholder return is calculated by using annualized total return of a stock to an investor due to capital gain appreciation plus reinvestment of all dividends.

For the three-year performance period ended December 31, 2020, our total shareholder return exceeded eight of our nine peers. We expect payout of these shares at the maximum level to occur in March of 2021. During 2020, we issued 56,722 shares of performance-based restricted stock units at the target-level performance hurdle for the three-year performance period ended December 31, 2019, as our total shareholder return exceeded five of nine peers in our 2017 peer group. We issued 20,549 shares of performance-based restricted stock units during 2019 at the threshold-level performance hurdle for the three-year performance period ended December 31, 2018, as our total shareholder return exceeded four of nine peers in our 2016 peer group. Performance-based awards vested during the year had an intrinsic value of $5 million, $2 million and $6 million for the years ended December 31, 2020, 2019 and 2018, respectively.

These performance-based awards are valued using a Monte-Carlo valuation on the date of grant, which uses a risk-neutral framework to model future stock price movements based upon the risk-free rate of return, the volatility of each peer and the pairwise correlations of each peer being modeled. Compensation cost is recognized regardless of whether the market-based performance objective has been satisfied. We make assumptions to develop the Monte-Carlo model as follows:

•Correlation coefficients are based upon the stock price data used to calculate the historical volatilities. The correlation coefficients are used to model the way the price of each entity's stock tends to move in relation to each other.


	2020	2019	2018
Expected term	2.86 years	2.86 years	2.89 years
Expected volatility	15.88-25.13%	15.10-25.00%	16.01-26.32%
Dividend yield	2.15%	2.61%	2.81%
Risk-free rates	1.30%	2.48%	2.22%


(Shares in thousands)	Service-based shares	Weighted- average grant date fair value		Performance-based shares	Weighted- average grant date fair value
Nonvested at January 1, 2020	783	$	70.27	165	$	64.23
Granted	204	104.81		38	111.77
Vested	(269)	66.53		(56)	43.26
Forfeited or canceled	(22)	78.96		—	—
Nonvested at December 31, 2020	696	81.56		147	84.64