Cash and Cash Equivalents and Investments in Marketable Securities (Available-For-Sale Securities) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Certificates of Deposit [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-Sale Securities, Amortized Cost	$ 27,000	$ 54,000
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	4	10
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Available-for-sale Securities, Fair Value (Net Carrying Amount)	27,004	54,010
Commercial Paper [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-Sale Securities, Amortized Cost	46,375	52,297
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	16	7
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(3)	(4)
Available-for-sale Securities, Fair Value (Net Carrying Amount)	46,388	52,300
Corporate Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-Sale Securities, Amortized Cost	112,175	136,181
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	12	1
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(47)	(94)
Available-for-sale Securities, Fair Value (Net Carrying Amount)	112,140	136,088
US States and Political Subdivisions Debt Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-Sale Securities, Amortized Cost	229,039	231,819
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	11	5
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(75)	(112)
Available-for-sale Securities, Fair Value (Net Carrying Amount)	228,975	231,712
Total Marketable Securities [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Available-for-Sale Securities, Amortized Cost	414,589	474,297
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	43	23
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	(125)	(210)
Available-for-sale Securities, Fair Value (Net Carrying Amount)	$ 414,507	$ 474,110

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

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-Section 50

-Paragraph 5

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-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

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-Section 45

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

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