Investments Amortized Cost (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Total	$ 183,438	$ 204,342
Accumulated Other Comprehensive Income, Gains	9	4
Accumulated Other Comprehensive Income, Losses	332	381
Estimated Fair Value	183,115	203,965
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Accumulated Other Comprehensive Income, Losses	60
Securities and obligations of U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Accumulated Other Comprehensive Income, Losses	265	323
Cash Equivalents and Short-term Investments [Member] | Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Total	23,744	9,996
Accumulated Other Comprehensive Income, Gains	0	0
Accumulated Other Comprehensive Income, Losses	60	0
Estimated Fair Value	23,684	9,996
Cash Equivalents [Member] | Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Total	2,266	9,074
Accumulated Other Comprehensive Income, Gains	0	0
Accumulated Other Comprehensive Income, Losses	0	0
Estimated Fair Value	2,266	9,074
Short-term Investments [Member] | US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Total	29,467	56,929
Accumulated Other Comprehensive Income, Gains	1	1
Accumulated Other Comprehensive Income, Losses	7	58
Estimated Fair Value	29,461	56,872
Short-term Investments [Member] | Securities and obligations of U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Total	127,961	128,343
Accumulated Other Comprehensive Income, Gains	8	3
Accumulated Other Comprehensive Income, Losses	265	323
Estimated Fair Value	$ 127,704	$ 128,023