Cash and Cash Equivalents and Investments in Marketable Securities (Available-For-Sale Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Marketable securities, Amortized Cost	$ 144,122	$ 474,297
Available-for-sale Securities, Gross Unrealized Gain	7	23
Available-for-sale Securities, Gross Unrealized Loss	(110)	(210)
Marketable securities, Fair Value (Net Carrying Amount)	144,019	474,110
State and political subdivisions debt securities
Schedule of Available-for-sale Securities [Line Items]
Marketable securities, Amortized Cost	88,935	231,819
Available-for-sale Securities, Gross Unrealized Gain	6	5
Available-for-sale Securities, Gross Unrealized Loss	(67)	(112)
Marketable securities, Fair Value (Net Carrying Amount)	88,874	231,712
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Marketable securities, Amortized Cost	32,311	136,181
Available-for-sale Securities, Gross Unrealized Gain	0	1
Available-for-sale Securities, Gross Unrealized Loss	(41)	(94)
Marketable securities, Fair Value (Net Carrying Amount)	32,270	136,088
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Marketable securities, Amortized Cost	22,876	52,297
Available-for-sale Securities, Gross Unrealized Gain	1	7
Available-for-sale Securities, Gross Unrealized Loss	(2)	(4)
Marketable securities, Fair Value (Net Carrying Amount)	$ 22,875	52,300
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Marketable securities, Amortized Cost		54,000
Available-for-sale Securities, Gross Unrealized Gain		10
Available-for-sale Securities, Gross Unrealized Loss		0
Marketable securities, Fair Value (Net Carrying Amount)		$ 54,010

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 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
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-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
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49167202&loc=d3e26610-111562

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity 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)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

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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)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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