Stockholders' Equity and Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2018

Summary of Nonvested Restricted Stock Awards Activity

The following table summarizes restricted stock awards activity for the six months ended June 30, 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		330,150		$	99.49		284,118		$	82.84
Vested		(206,181	)	$	41.51		(283,080	)	$	82.84
Forfeited		(53,373	)	$	51.10		(1,038	)	$	82.64
Unvested shares of restricted stock   outstanding at June 30, 2018		959,276		$	61.55		—		$	—

Market-Based Shares [Member]

Summary of Market-Based Shares Vesting Activity

The following table summarizes vesting activity for Market-Based Shares during the six months ended June 30, 2018, 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 = $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.