Net Income Per Share Available to Common Stock - Antidilutive Securities Excluded from Earnings Per Share (Details) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share (in shares)	180	14	146	73
Stock options and RSUs to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share (in shares)	82	14	87	73
Employee stock purchase plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share (in shares)	98	0	59	0