Fair Value Measurements - Financial Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Financial Assets:
Financial Assets, fair value	$ 372,160	$ 415,870
Financial Liabilities:
Financial Liabilities, fair value	3,542	3,750
Contingent Consideration Related To Acquisition
Financial Liabilities:
Financial Liabilities, fair value	3,542	3,750
Level 1
Financial Assets:
Financial Assets, fair value	57,500	104,602
Level 2
Financial Assets:
Financial Assets, fair value	314,660	311,268
Level 3
Financial Liabilities:
Financial Liabilities, fair value	3,542	3,750
Level 3 | Contingent Consideration Related To Acquisition
Financial Liabilities:
Financial Liabilities, fair value	3,542	3,750
Money Market Funds
Financial Assets:
Financial Assets, fair value	57,500	104,602
Money Market Funds | Level 1
Financial Assets:
Financial Assets, fair value	57,500	104,602
U S Government Agencies Debt Securities
Financial Assets:
Financial Assets, fair value	186,948	194,055
U S Government Agencies Debt Securities | Level 2
Financial Assets:
Financial Assets, fair value	186,948	194,055
Corporate Debt Securities
Financial Assets:
Financial Assets, fair value	68,191	74,918
Corporate Debt Securities | Level 2
Financial Assets:
Financial Assets, fair value	68,191	74,918
Commercial Paper
Financial Assets:
Financial Assets, fair value	59,521	42,295
Commercial Paper | Level 2
Financial Assets:
Financial Assets, fair value	$ 59,521	$ 42,295