Cash and Cash Equivalents and Investments in Marketable Securities (Available-For-Sale Securities) (Details) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Schedule of Trading Securities and Other Trading Assets [Line Items]
Proceeds from sale and maturity of marketable debt securities			$ 646,301,000	$ 212,216,000
Certificates of Deposit
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-Sale Securities, Amortized Cost	69,756,000		69,756,000		82,791,000
Available-for-Sale Securities, Gross Unrealized Gain			16,000		14,000
Available-for-sale Securities, Gross Unrealized Losses			(4,000)		(6,000)
Available-for-sale Securities, Fair Value (Net Carrying Amount)	69,768,000		69,768,000		82,799,000
Commercial Paper [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-Sale Securities, Amortized Cost	40,237,000		40,237,000		45,737,000
Available-for-Sale Securities, Gross Unrealized Gain			10,000		17,000
Available-for-sale Securities, Gross Unrealized Losses			0		0
Available-for-sale Securities, Fair Value (Net Carrying Amount)	40,247,000		40,247,000		45,754,000
Corporate Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-Sale Securities, Amortized Cost	66,791,000		66,791,000		49,587,000
Available-for-Sale Securities, Gross Unrealized Gain			8,000		2,000
Available-for-sale Securities, Gross Unrealized Losses			(16,000)		(30,000)
Available-for-sale Securities, Fair Value (Net Carrying Amount)	66,783,000		66,783,000		49,559,000
US States and Political Subdivisions Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-Sale Securities, Amortized Cost	485,394,000		485,394,000		457,615,000
Available-for-Sale Securities, Gross Unrealized Gain			34,000		26,000
Available-for-sale Securities, Gross Unrealized Losses			(62,000)		(51,000)
Available-for-sale Securities, Fair Value (Net Carrying Amount)	485,366,000		485,366,000		457,590,000
Categories of Investments, Marketable Securities, Available-for-sale Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Income, Interest	500,000	200,000	1,600,000	700,000
Available-for-Sale Securities, Amortized Cost	662,178,000		662,178,000		635,730,000
Available-for-Sale Securities, Gross Unrealized Gain			68,000		59,000
Available-for-sale Securities, Gross Unrealized Losses			(82,000)		(87,000)
Available-for-sale Securities, Fair Value (Net Carrying Amount)	$ 662,164,000		$ 662,164,000		$ 635,702,000