Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2019

Share-based Payment Arrangement [Abstract]

Summary of Expense Related to Share-Based Payments

The table below summarizes the expense related to share-based payments recognized in the three and six months ended June 30 (in thousands): 

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Stock options	$	9,698	$	11,352	$	19,863	$	22,051
Restricted stock	8,608		7,750		10,984		11,454
Stock-based compensation	$	18,306	$	19,102	$	30,847	$	33,505

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 June 30, 2019 (cost in thousands):

	Unrecognized Compensation Cost		Weighted Average Period of Expense Recognition (in Years)
Stock options	$	59,483	1.59
Restricted stock	26,932		1.38
Total	$	86,415

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 six months ended June 30, 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	419		243.19					$	57.51
Exercised	(617	)	96.08							114,085
Forfeited	(49	)	149.92
Outstanding at June 30, 2019	7,369		$	126.32	5,061	$	102.96			$	1,138,765
Expected to vest as of June 30, 2019	2,308		$	177.52

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 six months ended June 30, 2019 and 2018:

	June 30,
	2019		2018
Risk-free interest rate	2.42	%	2.55	%
Dividend yield	—		—
Expected volatility	26.40	%	26.96	%
Expected life (in years)	3.7		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 six months ended June 30, 2019 (shares in thousands): 

	Shares		Weighted Average Grant Date Fair Value
Outstanding at December 31, 2018	174		$	190.73
Granted	164		235.43
Vested	(112	)	200.29
Canceled or forfeited	(28	)	218.58
Outstanding at June 30, 2019	198		$	233.20