Investments (Narrative) (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Proceeds from available-for-sale of investments	$ 188,300,000	$ 149,900,000	$ 891,000,000	$ 545,600,000
Realized gains on the sales of available-for-sale investments	3,500,000		3,500,000	400,000
Realized losses on the sales of available-for-sale investments	200,000	400,000	800,000	1,400,000
Average remaining maturities for short-term available for sale investments, in months	6 months		6 months
Average remaining maturities for long-term available for sale investments, in years	5 years		5 years
Available-for-sale securities, carrying value	1,077,349,000		1,077,349,000		1,140,422,000
Available-for-sale securities, fair value	1,082,125,000		1,082,125,000		1,144,305,000
Gross unrealized losses	(170,000)		(170,000)		(1,504,000)
Cost method investments	27,600,000		27,600,000		32,200,000
Cost-method investments, impairment charge included in other income	2,800,000
Cost-method investments, gain recorded in other income due to sale of impaired investments	11,800,000
Other-Than-Temporary Impaired Available-for-sale Securities [Member]
Available-for-sale securities, carrying value	5,000,000		5,000,000
Available-for-sale securities, fair value	2,500,000		2,500,000
Other-than-temporary impairment charge included in other income	$ 2,500,000