Financial Instruments (Details 3) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Proceeds from Marketable Debt Securities
Proceeds from maturities and sales	$ 78,300,000	$ 599,600,000	$ 4,404,707,000	$ 1,423,931,000
Realized gains	0	600,000	6,300,000	1,300,000
Realized losses	$ 0	$ (1,300,000)	$ (2,000,000)	$ (2,000,000)