Cash and Cash Equivalents and Investments in Marketable Securities (Available-For-Sale Securities) (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Certificates of Deposit [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-Sale Securities, Amortized Cost	$ 64,703,000		$ 74,802,000
Available-for-Sale Securities, Gross Unrealized Gain	20,000		21,000
Available-for-sale Securities, Gross Unrealized Losses	0		(1,000)
Available-for-sale Securities, Fair Value (Net Carrying Amount)	64,723,000		74,822,000
Commercial Paper [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-Sale Securities, Amortized Cost	72,568,000		78,216,000
Available-for-Sale Securities, Gross Unrealized Gain	20,000		28,000
Available-for-sale Securities, Gross Unrealized Losses	0		0
Available-for-sale Securities, Fair Value (Net Carrying Amount)	72,588,000		78,244,000
Corporate Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-Sale Securities, Amortized Cost	136,056,000		96,889,000
Available-for-Sale Securities, Gross Unrealized Gain	15,000		6,000
Available-for-sale Securities, Gross Unrealized Losses	(64,000)		(44,000)
Available-for-sale Securities, Fair Value (Net Carrying Amount)	136,007,000		96,851,000
US States and Political Subdivisions Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-Sale Securities, Amortized Cost	442,167,000		475,235,000
Available-for-Sale Securities, Gross Unrealized Gain	66,000		49,000
Available-for-sale Securities, Gross Unrealized Losses	(33,000)		(41,000)
Available-for-sale Securities, Fair Value (Net Carrying Amount)	442,200,000		475,243,000
Total Marketable Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Income, Interest	600,000	500,000
Available-for-Sale Securities, Amortized Cost	715,494,000		725,142,000
Available-for-Sale Securities, Gross Unrealized Gain	121,000		104,000
Available-for-sale Securities, Gross Unrealized Losses	(97,000)		(86,000)
Available-for-sale Securities, Fair Value (Net Carrying Amount)	$ 715,518,000		$ 725,160,000