Investments (Schedule of Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 312,489	$ 317,488
Gross unrealized gains	572	261
Gross unrealized losses	(46)	(577)
Fair value	313,015	317,172
Subtotal [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	311,785	317,006
Gross unrealized gains	298	0
Gross unrealized losses	(46)	(577)
Fair value	312,037	316,429
Federal agency notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	279,850	285,276
Gross unrealized gains	259	0
Gross unrealized losses	(46)	(519)
Fair value	280,063	284,757
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	31,935	31,730
Gross unrealized gains	39	0
Gross unrealized losses	0	(58)
Fair value	31,974	31,672
Mutual funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	704	482
Gross unrealized gains	274	261
Gross unrealized losses	0	0
Fair value	$ 978	$ 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
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-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
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-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
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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
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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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