Financial Instruments (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012
Amortized cost and estimated fair value of marketable debt securities
Amortized Cost Due in one year or less	$ 1,621,892
Amortized Cost Due after one year through five years	255,577
Amortized Cost Due after five years through ten years	266,223
Amortized Cost Due after ten years	653,151
Amortized Cost Total	2,796,843
Estimated Fair Value Due in one year or less	1,621,793
Estimated Fair Value Due after one year through five years	259,584
Estimated Fair Value Due after five years through ten years	270,742
Estimated Fair Value Due after ten years	659,121
Estimated Fair Value Total	$ 2,811,240