Financial Instruments (Details 3) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Proceeds from Marketable Debt Securities
Proceeds from maturities and sales	$ 2,718,923,000	$ 5,190,052,000	$ 2,749,558,000
Realized gains	700,000	6,600,000	2,100,000
Realized losses	$ (500,000)	$ (2,100,000)	$ (3,500,000)