Financial Instruments and Fair Value Measurements - Fair Value of Assets and Liabilities (Details) - Fair value measurements on recurring basis [Member] - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	$ 181,711	$ 377,671
Money Market Funds [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	113,976	68,425
Corporate notes [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	48,176	152,319
Commercial paper [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	10,453	21,991
Securities of government-sponsored entities [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	9,106	115,929
Certificates of deposit [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments		19,007
Quoted price in active market (Level 1) [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	113,976	87,432
Quoted price in active market (Level 1) [Member] | Money Market Funds [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	113,976	68,425
Quoted price in active market (Level 1) [Member] | Certificates of deposit [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments		19,007
Significant other observable inputs (Level 2) [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	67,735	290,239
Significant other observable inputs (Level 2) [Member] | Corporate notes [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	48,176	152,319
Significant other observable inputs (Level 2) [Member] | Commercial paper [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	10,453	21,991
Significant other observable inputs (Level 2) [Member] | Securities of government-sponsored entities [Member]
Marketable Securities and Restricted Investments
Total marketable securities and restricted investments	$ 9,106	$ 115,929