Common Stock (Tables)

12 Months Ended

Dec. 31, 2020

Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]

The following schedule provides information related to our stock repurchase program for each of the three years ended December 31, 2020. During 2020, 1,951,899 shares ($196.6 million in the aggregate) were repurchased pursuant to the terms of the stock repurchase program and 81,057 shares ($10.2 million in the aggregate) were repurchased in connection with the income tax withholding obligations resulting from stock-based compensation programs. During 2019, 5,397,753 shares ($706.2 million in the aggregate) were repurchased pursuant to the terms of our stock repurchase program and 336,943 shares ($47.7 million in the aggregate) were repurchased in connection with the income tax withholding obligations resulting from stock-based compensation programs. During 2018, 3,321,968 shares ($401.3 million) were repurchased pursuant to the terms of our stock repurchase program and 102,800 shares ($12.7 million in the aggregate) were repurchased in connection with the income tax withholding obligations resulting from stock-based compensation programs.

	Additional dollars authorized for repurchase (in thousands)		Total number of shares purchased (a.)		Total number of shares cancelled		Average price paid per share for forfeited restricted shares		Total number of shares purchased as part of publicly announced programs		Average price paid per share for shares purchased as part of publicly announced program		Aggregate purchase price paid (in thousands)		Aggregate purchase price paid for shares purchased as part of publicly announced program		Maximum number of dollars that may yet be purchased under the program (in thousands)
Balance as of January 1, 2018																	$	363,660
2018	$	500,000		3,435,992		11,224	$	0.01		3,321,968	$	120.81	$	414,002	$	401,316	$	462,344
2019	$	1,000,000		5,762,409		27,713	$	0.01		5,397,753	$	130.84	$	753,928	$	706,221	$	756,123
2020	$	—		2,050,735		17,779	$	0.01		1,951,899	$	100.70	$	206,719	$	196,560	$	559,563
Total for three year period ended December 31, 2020	$	1,500,000		11,249,136		56,716	$	0.01		10,671,620	$	122.20	$	1,374,649	$	1,304,097

(a.)

Includes 17,779, 27,713 and 11,224 of restricted shares that were forfeited by former employees pursuant to the terms of our restricted stock purchase plan during 2020, 2019 and 2018, respectively.

Weighted-Average Assumptions Fair Value of Option Grants

The weighted-average assumptions reflected below were based upon twenty-six option grants for the five-year period ending December 31, 2020, twenty-nine option grants for the five-year period ending December 31, 2019 and twenty-seven option grants for the five-year period ending December 31, 2018.

Year Ended December 31,	2020			2019			2018
Expected volatility		28	%		27	%		27	%
Risk free Interest rate		2	%		2	%		1	%
Expected life (years)		3.5			3.4			3.4
Forfeiture rate		8	%		9	%		13	%
Dividend yield		0.5	%		0.3	%		0.3	%

Stock Option Activity

The table below summarizes our stock option activity during the year ended December 31, 2020:

Outstanding Options	Number of Shares			Weighted Average Exercise Price
Balance, January 1, 2020		8,133,176		$	124.52
Granted		2,589,966		$	70.36
Exercised		(1,534,741	)	$	118.99
Expired		(551,750	)	$	117.30
Cancelled		(397,685	)	$	114.79
Balance, December 31, 2020		8,238,966		$	109.47
Outstanding options vested and exercisable as of December 31, 2020		2,522,906		$	124.62

Information about Unvested Options

The following table provides information about unvested options for the year ended December 31, 2020:

	Shares			Weighted Average Grant Date Fair Value
Unvested options as of January 1, 2020		5,581,909		$	28.45
Granted		2,589,966		$	14.60
Vested		(2,184,896	)	$	27.40
Cancelled		(270,919	)	$	24.92
Unvested options as of December 31, 2020		5,716,060		$	22.74

Information about all Outstanding Options

The following table provides information regarding all options outstanding at December 31, 2020:

	Options Outstanding		Options Exercisable
Number of options outstanding		8,238,966		2,522,906
Weighted average exercise price	$	109.47	$	124.62
Aggregate intrinsic value as of December 31, 2020	$	231,848,241	$	32,525,812
Weighted average remaining contractual life		2.9		1.9

Weighted Average Remaining Contractual Life for Options Outstanding and Weighted Average Exercise Price Per Share for Exercisable Options

The weighted average remaining contractual life for options outstanding and weighted average exercise price per share for exercisable options at December 31, 2018, 2019 and 2020 were as follows:

Year Ended:	Options Outstanding		Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Life (in Years)		Exercisable Options		Weighted Average Exercise Price Per Share		Expected to Vest Options		Weighted Average Exercise Price Per Share
	Shares						Shares				Shares
2018		9,674,791	$	115.39		2.6		3,724,179	$	106.77		4,414,324	$	120.82
2019		8,133,176		124.52		2.7		2,551,267		119.86		5,073,423		126.62
2020		8,238,966		109.47		2.9		2,522,906		124.62		5,099,823		110.47