Fair Value of Financial Instruments (Cash and Cash Equivalents, Available-for-sale Securities, Restricted Cash Reconciliation) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost, total	$ 289,739	$ 222,454
Unrealized Gains	7	6
Unrealized Losses	(165)	(217)
Fair value	289,581	222,243
Cash and Cash Equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost, total	86,737	91,696
Unrealized Gains	0	2
Unrealized Losses	0	0
Fair value	86,737	91,698
Cash and Cash Equivalents \| Cash
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost, total	72,673	61,372
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair value	72,673	61,372
Cash and Cash Equivalents \| Money market funds
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost, total	473	3,980
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair value	473	3,980
Cash and Cash Equivalents \| U.S. government agencies
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost, total		8,999
Unrealized Gains		1
Unrealized Losses		0
Fair value		9,000
Cash and Cash Equivalents \| Commercial paper
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost, total	13,591	17,345
Unrealized Gains	0	1
Unrealized Losses	0	0
Fair value	13,591	17,346
Short-term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost, total	157,167	87,317
Unrealized Gains	5	4
Unrealized Losses	(53)	(53)
Fair value	157,119	87,268
Short-term Investments \| U.S. government agencies
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost, total	127,660	64,422
Unrealized Gains	0	3
Unrealized Losses	(42)	(40)
Fair value	127,618	64,385
Short-term Investments \| Commercial paper
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost, total	14,782	10,447
Unrealized Gains	5	1
Unrealized Losses	0	0
Fair value	14,787	10,448
Short-term Investments \| Corporate bonds
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost, total	13,490	12,448
Unrealized Gains	0	0
Unrealized Losses	(11)	(13)
Fair value	13,479	12,435
Short-term Investments \| Asset-backed securities
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost, total	1,235
Unrealized Gains	0
Unrealized Losses	0
Fair value	1,235
Long-Term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost, total	45,835	43,441
Unrealized Gains	2	0
Unrealized Losses	(112)	(164)
Fair value	45,725	43,277
Long-Term Investments \| U.S. government agencies
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost, total	29,501	32,683
Unrealized Gains	0	0
Unrealized Losses	(71)	(142)
Fair value	29,430	32,541
Long-Term Investments \| Corporate bonds
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost, total	11,243	3,751
Unrealized Gains	0	0
Unrealized Losses	(41)	(4)
Fair value	11,202	3,747
Long-Term Investments \| Asset-backed securities
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Amortized cost, total	5,091	7,007
Unrealized Gains	2	0
Unrealized Losses	0	(18)
Fair value	$ 5,093	$ 6,989

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity 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 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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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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Amount of investment in debt and equity securities categorized neither as trading securities nor held-to-maturity securities and intended be sold or mature one year or operating cycle, if longer.

+ 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 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=49167202&loc=d3e26626-111562

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