Financial Instruments (Details 3) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Proceeds from Marketable Debt Securities
Proceeds from maturities and sales	$ 491,300,000	$ 306,200,000	$ 1,913,381,000	$ 1,476,052,000
Realized gains	400,000	300,000	1,700,000	3,400,000
Realized losses	$ (800,000)	$ (400,000)	$ (2,700,000)	$ (1,700,000)