Marketable Securities (Details 1) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Summary of proceeds from sales and maturities of marketable securities and gross realized gains and losses
Proceeds from maturities	$ 750,000	$ 1,950,000	$ 1,250,000	$ 3,300,000
Proceeds from sales	50,017	73	150,050	12,089
Gross realized gains				784
Gross realized losses		$ (1)	$ (5)	$ (2)