Net Loss Per Share Available to Common Stockholders, Potential Common Shares Outstanding (Details) - shares shares in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Common Shares Outstanding, Excluded from Computation of Diluted Net Loss Per Share [Abstract]
Shares excluded from computation of diluted net loss per share (in shares)	1,705	1,486
Options to Purchase Common Stock [Member]
Common Shares Outstanding, Excluded from Computation of Diluted Net Loss Per Share [Abstract]
Shares excluded from computation of diluted net loss per share (in shares)	553	548
Unvested RSUs [Member]
Common Shares Outstanding, Excluded from Computation of Diluted Net Loss Per Share [Abstract]
Shares excluded from computation of diluted net loss per share (in shares)	1,152	938