Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2019

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Summary of Expense Related to Share-Based Payments

The table below summarizes the expense related to share-based payments recognized in the three months ended March 31 (in thousands):

	Three Months Ended March 31,
	2019		2018
Stock options	$	10,165	$	10,699
Restricted stock	2,376		3,704
Stock-based compensation	$	12,541	$	14,403

Summary of Total Unrecognized Compensation Cost Related to Stock-Based Compensation

The following table summarizes the Company’s total unrecognized compensation cost related to stock-based compensation as of March 31, 2019 (cost in thousands):

	Unrecognized Compensation Cost		Weighted Average Period of Expense Recognition (in Years)
Stock options	$	53,415	1.21
Restricted stock	11,967		0.75
Total	$	65,382

Summary of Changes in Number of Shares of Common Stock Under Option

The following summarizes the changes in the number of shares of common stock under option for the three months ended March 31, 2019 (shares/options and aggregate intrinsic value in thousands):

	Shares		Weighted Average Exercise Price		Options Exercisable at End of Period	Weighted Average Exercise Price of Exercisable Options		Weighted Average Fair Value of Options Granted  During the Period		Aggregate Intrinsic Value
Outstanding at December 31, 2018	7,616		$	117.58	5,174	$	98.39			$	518,954
Granted	140		255.47					$	56.47
Exercised	(278	)	107.15							38,752
Forfeited	(18	)	153.37
Outstanding at March 31, 2019	7,460		$	119.91	4,987	$	98.80			$	945,049
Expected to vest as of March 31, 2019	2,473		$	162.48

Schedule of Weighted-Average Assumptions

The fair value of stock option awards granted was estimated using the Black-Scholes option pricing model with the following weighted-average assumptions for grants or modifications during the three months ended March 31, 2019 and 2018:

	March 31,
	2019		2018
Risk-free interest rate	2.49	%	2.46	%
Dividend yield	—		—
Expected volatility	26.64	%	27.11	%
Expected life (in years)	4.0		3.9

Summary of Changes in Number of Shares of Restricted Stock and Restricted Stock Units

The following table summarizes the changes in the number of shares of restricted stock and restricted stock units for the three months ended March 31, 2019 (shares in thousands):

	Shares		Weighted Average Grant Date Fair Value
Outstanding at December 31, 2018	174		$	190.73
Granted	77		228.53
Vested	(104	)	201.05
Canceled or forfeited	(23	)	209.43
Outstanding at March 31, 2019	124		$	216.03