Financial Instruments (Details 2) $ in Thousands	Sep. 30, 2017 USD ($)
Amortized cost and estimated fair value of marketable debt securities
Amortized Cost Due in one year or less	$ 1,200,344
Amortized Cost Due after one year through five years	469,451
Amortized Cost Due after five years through ten years	312,956
Amortized Cost Due after ten years	1,202,586
Amortized Cost Total	3,185,337
Estimated Fair Value Due in one year or less	1,200,345
Estimated Fair Value Due after one year through five years	467,424
Estimated Fair Value Due after five years through ten years	312,503
Estimated Fair Value Due after ten years	1,198,067
Estimated Fair Value Total	3,178,339
Marketable debt securities with contractual maturities greater than one year but classified as short-term investment	$ 1,930,000