Investments (Schedule of Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 281,336	$ 317,488
Gross unrealized gains	318	261
Gross unrealized losses	(85)	(577)
Fair value	281,569	317,172
Subtotal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	280,680	317,006
Gross unrealized gains	63	0
Gross unrealized losses	(85)	(577)
Fair value	280,658	316,429
Federal agency notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	248,200	285,276
Gross unrealized gains	49	0
Gross unrealized losses	(85)	(519)
Fair value	248,164	284,757
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	32,480	31,730
Gross unrealized gains	14	0
Gross unrealized losses	0	(58)
Fair value	32,494	31,672
Mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	656	482
Gross unrealized gains	255	261
Gross unrealized losses	0	0
Fair value	$ 911	$ 743

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

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 gain 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
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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
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-Paragraph 2
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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)
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