Marketable Securities (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Cash equivalents and marketable securities
Maximum maturity period of available-for-sale securities (in years)	3 years	3 years
Maximum remaining maturity period of available-for-sale securities (in years)	1 year	1 year
Fair Value	$ 142,258	$ 121,151
Amortized Cost	142,273	121,171
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(15)	(21)
Cash, fair value	41,374	51,031
Cash, amortized cost	41,374	51,031
Cash, cash equivalents and marketable securities
Fair Value	183,632	172,182
Amortized Cost	183,647	172,202
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(15)	(21)
Money market funds
Cash equivalents and marketable securities
Fair Value	12,882	10,681
Amortized Cost	12,882	10,681
Gross Unrealized Gains	0	0
Gross Unrealized Losses	$ 0	$ 0
Weighted Rate of Return (as a percent)	0.90%	0.41%
US Treasury and Government Short-term Debt Securities [Member]
Cash equivalents and marketable securities
Fair Value	$ 30,489	$ 48,292
Amortized Cost	30,491	48,291
Gross Unrealized Gains	0	1
Gross Unrealized Losses	$ (2)	$ 0
Weighted Rate of Return (as a percent)	0.95%	0.39%
Corporate notes, bonds and commercial paper
Cash equivalents and marketable securities
Fair Value	$ 98,887	$ 62,178
Amortized Cost	98,900	62,199
Gross Unrealized Gains	0	0
Gross Unrealized Losses	$ (13)	$ (21)
Weighted Rate of Return (as a percent)	1.13%	0.66%