Cash and Cash Equivalents and Investments in Marketable Securities (Available-For-Sale Securities) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012
Schedule of Trading Securities and Other Trading Assets [Line Items]
Marketable securities, Amortized Cost	$ 635,730
Marketable securities, Gross Unrealized Gain	59
Marketable securities, Gross Unrealized Loss	(87)
Marketable securities, Fair Value (Net Carrying Amount)	635,702
Certificates of deposit
Schedule of Trading Securities and Other Trading Assets [Line Items]
Marketable securities, Amortized Cost	82,791
Marketable securities, Gross Unrealized Gain	14
Marketable securities, Gross Unrealized Loss	(6)
Marketable securities, Fair Value (Net Carrying Amount)	82,799
Commercial paper
Schedule of Trading Securities and Other Trading Assets [Line Items]
Marketable securities, Amortized Cost	45,737
Marketable securities, Gross Unrealized Gain	17
Marketable securities, Gross Unrealized Loss	0
Marketable securities, Fair Value (Net Carrying Amount)	45,754
Corporate debt securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Marketable securities, Amortized Cost	49,587
Marketable securities, Gross Unrealized Gain	2
Marketable securities, Gross Unrealized Loss	(30)
Marketable securities, Fair Value (Net Carrying Amount)	49,559
State and political subdivisions debt securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Marketable securities, Amortized Cost	457,615
Marketable securities, Gross Unrealized Gain	26
Marketable securities, Gross Unrealized Loss	(51)
Marketable securities, Fair Value (Net Carrying Amount)	$ 457,590