Stockholders' Equity and Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2019

Summary of Nonvested Restricted Stock Awards Activity

The following table summarizes restricted stock awards activity for the three months ended March 31, 2019:

	Time-Based					Market-Based
	Restricted Stock Awards					Restricted Stock Awards
	Shares			Weighted Average Grant Date Fair Value		Shares			Weighted Average Grant Date Fair Value
Unvested shares of restricted stock   outstanding at December 31, 2018		806,469		$	67.54		—		$	—
Granted		239,109		$	131.88		280,960		$	102.76
Vested		—			—		(280,408	)	$	102.76
Forfeited		(20,083	)	$	71.66		(552	)	$	102.57
Unvested shares of restricted stock   outstanding at March 31, 2019		1,025,495		$	82.46		—		$	—

Market-Based Shares [Member]

Summary of Market-Based Shares Vesting Activity

The following table summarizes vesting activity for Market-Based Shares during the three months ended March 31, 2019, the associated compensation cost recognized in connection with each vesting event and the number of shares withheld to satisfy tax withholding obligations:

Vesting Condition	Date Vested	Number of Shares Vested		Compensation Cost Recognized Upon Vesting	Shares Withheld for Taxes1
Market-based (TEV = $8.65 billion)	February 13, 2019		140,218	$14.9 million		54,507
Market-based (TEV = $9.35 billion)	February 22, 2019		140,190	$13.9 million		55,547

¹ All shares withheld to satisfy tax withholding obligations are held as treasury stock.