Investment Securities (Details 4) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Realized gains (losses) on marketable investment securities
Realized gains on the sales of marketable investment securities	$ 1,800,000	$ 19,100,000	$ 36,300,000	$ 147,100,000
Proceeds from sales of marketable investment securities	47,400,000	49,500,000	142,700,000	499,800,000
Fair value of marketable investment securities in a loss position
Less than 12 months	256,380,000		256,380,000		275,587,000
12 months or more	54,907,000		54,907,000		12,963,000
Total	311,287,000		311,287,000		288,550,000
Unrealized losses on marketable investment securities in a loss position
Less than 12 months	(517,000)		(517,000)		(288,000)
12 or more	(48,000)		(48,000)		(19,000)
Total	$ (565,000)		$ (565,000)		$ (307,000)