Stockholders' Equity and Equity Compensation Plans (Tables)

6 Months Ended

Jun. 30, 2018

Stockholders Equity Note [Abstract]

Summary of Unvested Restricted Common Stock Activity

Stock-Based Awards. A summary of our unvested restricted common stock activity during the quarter and six months ended June 30, 2018  is as follows (shares in thousands):

	Quarter Ended					Six Months Ended
	June 30, 2018					June 30, 2018
	Shares			Weighted- Average Grant Date Fair Value		Shares			Weighted- Average Grant Date Fair Value
Unvested awards, beginning		1,216		$	41.48		1,222		$	36.84
Awards granted		39			43.80		489			46.75
Awards forfeited/cancelled		(21	)		42.35		(79	)		40.18
Awards vested		(45	)		38.49		(443	)		34.25
Unvested awards, ending		1,189		$	41.67		1,189		$	41.67