Marketable Securities (Details 1) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Marketable securities by contractual maturity
Cost	$ 429,568	$ 417,845
Estimated fair value	429,051	417,126
One year or less [Member]
Marketable securities by contractual maturity
Cost	222,701	233,326
Estimated fair value	222,625	233,284
One to five years [Member]
Marketable securities by contractual maturity
Cost	206,867	184,519
Estimated fair value	$ 206,426	$ 183,842