Cash, Cash Equivalents, and Available for Sale Marketable Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Amortized Cost Basis [Abstract]
Cash and Cash Equivalents, at Carrying Value	$ 48,201	$ 36,641	$ 63,507	$ 31,309
Marketable debt securities, amortized cost	77,399
Total marketable securities, amortized cost	78,619	139,061
Total money market funds and marketable securities	125,865	175,064
Fair Value Disclosure [Abstract]
Cash and cash equivalents, fair value	48,201	36,641	$ 63,507	$ 31,309
Marketable debt securities, fair value	77,386
Total marketable securities	78,346	139,137
Total money market funds and marketable securities	125,592	175,140
Unrealized Gain Loss [Abstract]
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	1
Unrealized gains	1	142
Total money market funds and marketable securities	1	142
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	14
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	274	66
Total money market funds and marketable securities	274	66
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis, Rolling Maturity [Abstract]
Due in one year or less	77,386
Money market funds
Amortized Cost Basis [Abstract]
Cash and Cash Equivalents, at Carrying Value	47,246	36,003
Fair Value Disclosure [Abstract]
Cash and cash equivalents, fair value	47,246	36,003
Debt Securities [Member]
Amortized Cost Basis [Abstract]
Marketable debt securities, amortized cost		137,841
Fair Value Disclosure [Abstract]
Marketable debt securities, fair value		137,777
Unrealized Gain Loss [Abstract]
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		2
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		66
U.S. Treasury notes
Amortized Cost Basis [Abstract]
Marketable debt securities, amortized cost	67,421	130,237
Fair Value Disclosure [Abstract]
Marketable debt securities, fair value	67,407	130,173
Unrealized Gain Loss [Abstract]
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		2
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	14	66
U. S. Government agency securities
Amortized Cost Basis [Abstract]
Marketable debt securities, amortized cost	9,978	7,604
Fair Value Disclosure [Abstract]
Marketable debt securities, fair value	9,979	7,604
Unrealized Gain Loss [Abstract]
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	1
Equity Securities
Amortized Cost Basis [Abstract]
Marketable equity securities, amortized cost	1,220	1,220
Fair Value Disclosure [Abstract]
Marketable equity securities, fair value	$ 960	1,360
Unrealized Gain Loss [Abstract]
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax		$ 140