Marketable Securities Marketable Securities (Details 1) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	$ 232,142	$ 261,208
Cash, fair value	59,078	68,168
Cash, Cash Equivalents and Short Term Investments, Fair Value Disclosure	291,220	329,376
Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	63,283	157,676
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	$ 168,859	$ 103,532