Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Fair Value of Financial Instruments
Money Market Funds	$ 11,220	$ 14,528	$ 7,877	$ 12,590
Total Fair Value	377,574	401,085
Money Market Funds
Fair Value of Financial Instruments
Money Market Funds	10,063	12,137
Corporate Securities
Fair Value of Financial Instruments
Securities	154,098	181,483
Government Securities
Fair Value of Financial Instruments
Securities	213,413	207,465
Level 1
Fair Value of Financial Instruments
Total Fair Value	10,063	12,137
Level 1 | Money Market Funds
Fair Value of Financial Instruments
Money Market Funds	10,063	12,137
Level 2
Fair Value of Financial Instruments
Total Fair Value	367,511	388,948
Level 2 | Corporate Securities
Fair Value of Financial Instruments
Securities	154,098	181,483
Level 2 | Government Securities
Fair Value of Financial Instruments
Securities	$ 213,413	$ 207,465