Investments (Narrative) (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Investments [Abstract]
Proceeds from available-for-sale of investments	$ 231,800,000		$ 104,200,000
Realized gains on the sales of available-for-sale investments	200,000	0
Realized losses on the sales of available-for-sale investments	100,000		200,000
Average remaining maturities for short-term available for sale investments	6 months
Average remaining maturities for long-term available for sale investments	4 years
Gross unrealized losses	(404,000)		(192,000)
Cost method investments	$ 27,300,000		$ 26,200,000