Investments (Narrative) (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012 M Y	Jun. 30, 2011	Jun. 30, 2012 M Y	Jun. 30, 2011	Dec. 31, 2011
Proceeds from available-for-sale of investments	$ 598,500,000	$ 215,300,000	$ 702,700,000	$ 395,700,000
Realized gains on the sales of available-for-sale investments	3,500,000	300,000	3,500,000	400,000
Realized losses on the sales of available-for-sale investments	400,000	400,000	600,000	1,100,000
Gross unrealized losses	(269,000)		(269,000)		(1,504,000)
Average remaining maturities for short-term available for sale investments, in months	5		5
Average remaining maturities for long-term available for sale investments, in years	6		6
Cost method investments	$ 28,400,000		$ 28,400,000		$ 32,200,000