Cash, Cash Equivalents, and Available for Sale Marketable Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Amortized Cost Basis [Abstract]
Cash and Cash Equivalents, at Carrying Value	$ 43,604	$ 36,641	$ 33,895	$ 31,309
Marketable debt securities, amortized cost		137,841
Total marketable securities, amortized cost	115,368	139,061
Total money market funds and marketable securities	157,341	175,064
Fair Value Disclosure [Abstract]
Cash and cash equivalents, fair value	43,604	36,641	$ 33,895	$ 31,309
Marketable debt securities, fair value		137,777
Total marketable securities	116,066	139,137
Total money market funds and marketable securities	158,039	175,140
Amortized Cost Abstract
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		2
Unrealized gains	780	142
Total money market funds and marketable securities	780	142
Unrealized losses	82	66
Total money market funds and marketable securities	82	66
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis, Rolling Maturity [Abstract]
Due in one year or less	114,066
Money market funds
Amortized Cost Basis [Abstract]
Cash and Cash Equivalents, at Carrying Value	41,973	36,003
Fair Value Disclosure [Abstract]
Cash and cash equivalents, fair value	41,973	36,003
Debt Securities [Member]
Amortized Cost Basis [Abstract]
Marketable debt securities, amortized cost	114,148
Fair Value Disclosure [Abstract]
Marketable debt securities, fair value	114,066
Amortized Cost Abstract
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	82
U.S. Treasury notes
Amortized Cost Basis [Abstract]
Marketable debt securities, amortized cost	114,148	130,237
Fair Value Disclosure [Abstract]
Marketable debt securities, fair value	114,066	130,173
Amortized Cost Abstract
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		2
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	82	66
U. S. Government agency bonds
Amortized Cost Basis [Abstract]
Marketable debt securities, amortized cost		7,604
Fair Value Disclosure [Abstract]
Marketable debt securities, fair value		7,604
Equity Securities
Amortized Cost Basis [Abstract]
Marketable equity securities, amortized cost	1,220	1,220
Fair Value Disclosure [Abstract]
Marketable equity securities, fair value	2,000	1,360
Amortized Cost Abstract
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax	$ 780	$ 140