Investments Amortized Cost (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Total	$ 204,342	$ 228,319
Accumulated Other Comprehensive Income, Gains	4	5
Accumulated Other Comprehensive Income, Losses	381	126
Estimated Fair Value	203,965	228,198
Securities and obligations of U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Accumulated Other Comprehensive Income, Losses	323	126
Cash Equivalents and Short-term Investments [Member] | Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Total	9,996	9,998
Accumulated Other Comprehensive Income, Gains	0	0
Accumulated Other Comprehensive Income, Losses	0	0
Estimated Fair Value	9,996	9,998
Cash Equivalents [Member] | Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total	9,074	26,456
Accumulated Other Comprehensive Income, Gains	0	0
Accumulated Other Comprehensive Income, Losses	0	0
Estimated Fair Value	9,074	26,456
Short-term Investments [Member] | US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total	56,929	3,918
Accumulated Other Comprehensive Income, Gains	1	1
Accumulated Other Comprehensive Income, Losses	58	0
Estimated Fair Value	56,872	3,919
Short-term Investments [Member] | Securities and obligations of U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Total	128,343	187,947
Accumulated Other Comprehensive Income, Gains	3	4
Accumulated Other Comprehensive Income, Losses	323	126
Estimated Fair Value	$ 128,023	$ 187,825