Investments (Narrative) (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Investments [Abstract]
Proceeds from available-for-sale of investments	$ 143,600,000	$ 598,500,000	$ 375,300,000	$ 702,700,000
Realized gains on the sales of available-for-sale investments	100,000	3,500,000	300,000	3,500,000
Realized losses on the sales of available-for-sale investments	100,000	400,000	200,000	600,000
Average remaining maturities for short-term available for sale investments	5 months
Average remaining maturities for long-term available for sale investments	3 years
Gross unrealized loss on available-for-sale investments	2,317,000		2,317,000		192,000
Cost method investments	27,600,000		27,600,000		26,200,000
Cost method investments, recognized gain during period	$ 1,000,000		$ 1,000,000