Cash and Cash Equivalents and Investments in Marketable Securities (Available-For-Sale Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Marketable securities, Amortized Cost	$ 424,150	$ 144,122
Available-for-sale Securities, Gross Unrealized Gain	2	7
Available-for-sale Securities, Gross Unrealized Loss	(4)	(110)
Marketable securities, Fair Value (Net Carrying Amount)	424,148	144,019
State and political subdivisions debt securities
Schedule of Available-for-sale Securities [Line Items]
Marketable securities, Amortized Cost	412,462	88,935
Available-for-sale Securities, Gross Unrealized Gain	1	6
Available-for-sale Securities, Gross Unrealized Loss	(3)	(67)
Marketable securities, Fair Value (Net Carrying Amount)	412,460	88,874
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Marketable securities, Amortized Cost	4,001	32,311
Available-for-sale Securities, Gross Unrealized Gain	0	0
Available-for-sale Securities, Gross Unrealized Loss	0	(41)
Marketable securities, Fair Value (Net Carrying Amount)	4,001	32,270
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Marketable securities, Amortized Cost	7,687	22,876
Available-for-sale Securities, Gross Unrealized Gain	1	1
Available-for-sale Securities, Gross Unrealized Loss	(1)	(2)
Marketable securities, Fair Value (Net Carrying Amount)	$ 7,687	$ 22,875

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=73527792&loc=d3e27161-111563

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- Definition

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=65884222&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=73527792&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=66010992&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=73527792&loc=d3e27161-111563

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This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ 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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