Fair Value Measurements - Fair Value of Assets and Liabilities (Details) - Fair Value Measurements on Recurring Basis [Member] - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Cash Equivalents and Marketable Securities
Total cash equivalents and marketable securities	$ 104,883	$ 377,671
Money Market Funds [Member]
Cash Equivalents and Marketable Securities
Total cash equivalents and marketable securities	72,866	68,425
Certificates of Deposit [Member]
Cash Equivalents and Marketable Securities
Total cash equivalents and marketable securities		19,007
Corporate Notes [Member]
Cash Equivalents and Marketable Securities
Total cash equivalents and marketable securities	4,551	152,319
Commercial Paper [Member]
Cash Equivalents and Marketable Securities
Total cash equivalents and marketable securities	21,471	21,991
Securities of Government-sponsored Entities [Member]
Cash Equivalents and Marketable Securities
Total cash equivalents and marketable securities	5,995	115,929
Quoted Price in Active Market (Level 1) [Member]
Cash Equivalents and Marketable Securities
Total cash equivalents and marketable securities	72,866	87,432
Quoted Price in Active Market (Level 1) [Member] | Money Market Funds [Member]
Cash Equivalents and Marketable Securities
Total cash equivalents and marketable securities	72,866	68,425
Quoted Price in Active Market (Level 1) [Member] | Certificates of Deposit [Member]
Cash Equivalents and Marketable Securities
Total cash equivalents and marketable securities		19,007
Significant Other Observable Inputs (Level 2) [Member]
Cash Equivalents and Marketable Securities
Total cash equivalents and marketable securities	32,017	290,239
Significant Other Observable Inputs (Level 2) [Member] | Corporate Notes [Member]
Cash Equivalents and Marketable Securities
Total cash equivalents and marketable securities	4,551	152,319
Significant Other Observable Inputs (Level 2) [Member] | Commercial Paper [Member]
Cash Equivalents and Marketable Securities
Total cash equivalents and marketable securities	21,471	21,991
Significant Other Observable Inputs (Level 2) [Member] | Securities of Government-sponsored Entities [Member]
Cash Equivalents and Marketable Securities
Total cash equivalents and marketable securities	$ 5,995	$ 115,929