Cash, Cash Equivalents, and Available for Sale Marketable Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Amortized Cost Abstract
Amortized cost	$ 202,364	$ 222,888
Unrealized gains	60
Unrealized losses		251
Fair value	202,424	222,637
Money market funds
Amortized Cost Abstract
Amortized cost	65,463	29,601
Fair value	65,463	29,601
Marketable Securities
Amortized Cost Abstract
Amortized cost	136,901	193,287
Unrealized gains	60
Unrealized losses		251
Fair value	136,961	193,036
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis, Rolling Maturity [Abstract]
Due in one year or less	136,961
U.S. Treasury notes
Amortized Cost Abstract
Amortized cost	109,964	158,166
Unrealized gains	55
Unrealized losses		185
Fair value	110,019	157,981
U.S. Government agency bonds
Amortized Cost Abstract
Amortized cost	26,937	35,121
Unrealized gains	5
Unrealized losses		66
Fair value	$ 26,942	$ 35,055