Stockholders' Equity and Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2018

Summary of Nonvested Restricted Stock Awards Activity

The following table summarizes restricted stock awards activity for the three months ended March 31, 2018:

	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, 2017		888,680		$	42.17		—		$	—
Granted		227,243		$	92.94		284,118		$	82.84
Vested		(14,211	)	$	0.29		(283,080	)	$	82.84
Forfeited		(19,726	)	$	47.19		(1,038	)	$	82.64
Unvested shares of restricted stock   outstanding at March 31, 2018		1,081,986		$	53.29		—		$	—

Market-based Stock [Member]

Summary of Market-based Stock Vesting Activity

The following table summarizes market-based stock vesting activity during the three months ended March 31, 2018, the associated compensation cost recognized in connection with the 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 = $5.9 billion)	March 14, 2018		141,599	$9.7 million		54,000
Market-based (TEV = $6.2 billion)	March 23, 2018		141,481	$10.1 million		54,909

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