Cash, Cash Equivalents, And Available-For-Sale Securities (Fair Value Hierarchy for Cash Equivalents and Available-For-Sale Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair value measurements
Cash and Cash Equivalents, Fair Value Disclosure	$ 57,181	$ 66,392
Available-for-sale Securities, Debt Securities, Current	66,361	159,943
Cash and cash equivalents and available for sale securities, Fair Value Disclosure	123,542	226,335
U.S. Treasury securities
Fair value measurements
Available-for-sale Securities, Debt Securities, Current	14,341	20,932
Corporate Debt Securities [Member]
Fair value measurements
Available-for-sale Securities, Debt Securities, Current	52,020	139,011
Level 1
Fair value measurements
Cash and Cash Equivalents, Fair Value Disclosure	57,181	66,392
Cash and cash equivalents and available for sale securities, Fair Value Disclosure	71,522	87,324
Level 1 \| U.S. Treasury securities
Fair value measurements
Available-for-sale Securities, Debt Securities, Current	14,341	20,932
Level 1 \| Corporate Debt Securities [Member]
Fair value measurements
Available-for-sale Securities, Debt Securities, Current	0	0
Level 2
Fair value measurements
Cash and Cash Equivalents, Fair Value Disclosure	0	0
Cash and cash equivalents and available for sale securities, Fair Value Disclosure	52,020	139,011
Level 2 \| U.S. Treasury securities
Fair value measurements
Available-for-sale Securities, Debt Securities, Current	0	0
Level 2 \| Corporate Debt Securities [Member]
Fair value measurements
Available-for-sale Securities, Debt Securities, Current	52,020	139,011
Level 3
Fair value measurements
Cash and Cash Equivalents, Fair Value Disclosure	0	0
Cash and cash equivalents and available for sale securities, Fair Value Disclosure	0	0
Level 3 \| U.S. Treasury securities
Fair value measurements
Available-for-sale Securities, Debt Securities, Current	0	0
Level 3 \| Corporate Debt Securities [Member]
Fair value measurements
Available-for-sale Securities, Debt Securities, Current	$ 0	$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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- Definition

Amount of investments in debt securities classified as available-for-sale, expected to be converted to cash, sold or exchanged within one year or the normal 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=75030427&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=82896770&loc=d3e26626-111562

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- Definition

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

No definition available.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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