Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Fair Value of Financial Instruments
Money Market Funds	$ 13,561	$ 14,528	$ 8,150	$ 12,590
Total Fair Value	390,147	401,085
Money Market Funds
Fair Value of Financial Instruments
Money Market Funds	11,057	12,137
Corporate Securities
Fair Value of Financial Instruments
Securities	164,682	181,483
Government Securities
Fair Value of Financial Instruments
Securities	214,408	207,465
Level 1
Fair Value of Financial Instruments
Total Fair Value	11,057	12,137
Level 1 | Money Market Funds
Fair Value of Financial Instruments
Money Market Funds	11,057	12,137
Level 2
Fair Value of Financial Instruments
Total Fair Value	379,090	388,948
Level 2 | Corporate Securities
Fair Value of Financial Instruments
Securities	164,682	181,483
Level 2 | Government Securities
Fair Value of Financial Instruments
Securities	$ 214,408	$ 207,465