Financial Instruments (Details 3) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Proceeds from Marketable Debt Securities
Proceeds from maturities and sales	$ 617,500,000	$ 491,300,000	$ 5,025,218,000	$ 1,913,381,000
Realized gains	200,000	400,000	6,600,000	1,700,000
Realized losses	$ 0	$ (800,000)	$ (2,100,000)	$ (2,700,000)