At December 31, 2023, a total of 17,052,156 shares of unissued common stock were authorized for issuance under our stock-based compensation plans. Additionally, a total of 3,467,496 shares are authorized for issuance under a plan whereby substantially all employees may acquire common stock through payroll deductions at prevailing market prices.

The following table summarizes the status of, and changes in, our stock options during 2023.


	Options	Weighted- average exercise price		Weighted-average remaining contractual term in years
Outstanding at December 31, 2022	2,218,506	$	74.31

Exercised	(742,402)	$	72.16


Outstanding at December 31, 2023	1,476,104	$	75.39	1.1
Exercisable at December 31, 2023	1,476,104	$	75.39	1.1

The total intrinsic value of options exercised was $30.6 million in 2023, $40.3 million in 2022, and $177.2 million in 2021. At December 31, 2023, the aggregate intrinsic value of in-the-money options outstanding was $47.7 million. All outstanding options are exercisable.

The following table summarizes the status of, and changes in, our nonvested restricted shares and restricted stock units during 2023.


	Restricted shares	Restricted stock units	Weighted- average fair value
Nonvested at December 31, 2022	8,715	5,901,600	$	142.37
Time-based grants	56,740	2,617,522	$	100.89
Performance-based grants	—	81,556	$	100.55
Vested (value at vest date was $195.5 million)	(8,715)	(1,939,021)	$	132.90
Nonvested dividend equivalents granted to non-employee directors	—	4,039	$	108.28
Forfeited	—	(180,443)	$	140.83
Nonvested at December 31, 2023	56,740	6,485,253	$	127.74

Nonvested at December 31, 2023 includes performance-based restricted stock units of 364,107. These nonvested performance-based restricted units include 67,331 units for which the performance period has lapsed, and the performance threshold has been met.

Compensation and related costs includes expenses for restricted shares and restricted stock units of $265.6 million in 2023, $285.4 million in 2022, and $274.6 million in 2021.

At December 31, 2023, non-employee directors held 86,591 vested stock units that will convert to common shares upon their separation from the Board.

The following table presents the compensation expense to be recognized over the remaining vesting periods of the stock-based awards outstanding at December 31, 2023. Estimated future compensation expense will change to reflect future grants, changes in the probability of performance thresholds being met, and adjustments for actual forfeitures.


(in millions)
First quarter 2024	$	59.8
Second quarter 2024	58.4
Third quarter 2024	57.4
Fourth quarter 2024	50.3
2025	119.1
2026 through 2029	88.5
Total	$	433.5