Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) - USD ($)	3 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Adjustments resulting from observable price changes	$ 0
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Total assets	944,608,000	$ 1,112,346,000
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	0	0
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Total assets	1,296,103,000	1,621,733,000
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	512,000	814,000
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Total assets	213,000	393,000
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	0	0
Cash and Cash Equivalents [Member] \| Fair Value, Measurements, Recurring [Member] \| Cash [Member] \| Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	777,798,000	556,520,000
Cash and Cash Equivalents [Member] \| Fair Value, Measurements, Recurring [Member] \| Cash [Member] \| Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and Cash Equivalents [Member] \| Fair Value, Measurements, Recurring [Member] \| Cash [Member] \| Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and Cash Equivalents [Member] \| Fair Value, Measurements, Recurring [Member] \| Money Market Funds [Member] \| Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	166,810,000	555,826,000
Cash and Cash Equivalents [Member] \| Fair Value, Measurements, Recurring [Member] \| Money Market Funds [Member] \| Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and Cash Equivalents [Member] \| Fair Value, Measurements, Recurring [Member] \| Money Market Funds [Member] \| Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and Cash Equivalents [Member] \| Fair Value, Measurements, Recurring [Member] \| Corporate Securities [Member] \| Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Cash and Cash Equivalents [Member] \| Fair Value, Measurements, Recurring [Member] \| Corporate Securities [Member] \| Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	10,089,000	2,784,000
Cash and Cash Equivalents [Member] \| Fair Value, Measurements, Recurring [Member] \| Corporate Securities [Member] \| Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Available-for-sale Securities [Member] \| Fair Value, Measurements, Recurring [Member] \| Corporate Securities [Member] \| Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Available-for-sale Securities [Member] \| Fair Value, Measurements, Recurring [Member] \| Corporate Securities [Member] \| Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	633,171,000	807,299,000
Available-for-sale Securities [Member] \| Fair Value, Measurements, Recurring [Member] \| Corporate Securities [Member] \| Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	213,000	393,000
Available-for-sale Securities [Member] \| Fair Value, Measurements, Recurring [Member] \| Agency Securities [Member] \| Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Available-for-sale Securities [Member] \| Fair Value, Measurements, Recurring [Member] \| Agency Securities [Member] \| Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	375,865,000	439,064,000
Available-for-sale Securities [Member] \| Fair Value, Measurements, Recurring [Member] \| Agency Securities [Member] \| Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Available-for-sale Securities [Member] \| Fair Value, Measurements, Recurring [Member] \| Municipal securities [Member] \| Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Available-for-sale Securities [Member] \| Fair Value, Measurements, Recurring [Member] \| Municipal securities [Member] \| Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	2,505,000	3,965,000
Available-for-sale Securities [Member] \| Fair Value, Measurements, Recurring [Member] \| Municipal securities [Member] \| Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Available-for-sale Securities [Member] \| Fair Value, Measurements, Recurring [Member] \| Government securities [Member] \| Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Available-for-sale Securities [Member] \| Fair Value, Measurements, Recurring [Member] \| Government securities [Member] \| Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	272,167,000	366,123,000
Available-for-sale Securities [Member] \| Fair Value, Measurements, Recurring [Member] \| Government securities [Member] \| Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	0	0
Foreign Exchange Contract [Member] \| Prepaid Expenses and Other Current Assets [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Prepaid expenses and other current assets	0	0
Foreign Exchange Contract [Member] \| Prepaid Expenses and Other Current Assets [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Prepaid expenses and other current assets	2,306,000	2,498,000
Foreign Exchange Contract [Member] \| Prepaid Expenses and Other Current Assets [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Prepaid expenses and other current assets	0	0
Foreign Exchange Contract [Member] \| Accrued Expenses and Other Current Liabilities [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 1 [Member]
Liabilities, Fair Value Disclosure [Abstract]
Foreign currency derivatives	0	0
Foreign Exchange Contract [Member] \| Accrued Expenses and Other Current Liabilities [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 2 [Member]
Liabilities, Fair Value Disclosure [Abstract]
Foreign currency derivatives	512,000	814,000
Foreign Exchange Contract [Member] \| Accrued Expenses and Other Current Liabilities [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 3 [Member]
Liabilities, Fair Value Disclosure [Abstract]
Foreign currency derivatives	0	0
Estimate of Fair Value Measurement [Member] \| Fair Value, Measurements, Recurring [Member]
Assets, Fair Value Disclosure [Abstract]
Total assets	2,240,924,000	2,734,472,000
Liabilities, Fair Value Disclosure [Abstract]
Total liabilities	512,000	814,000
Estimate of Fair Value Measurement [Member] \| Cash and Cash Equivalents [Member] \| Fair Value, Measurements, Recurring [Member] \| Cash [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	777,798,000	556,520,000
Estimate of Fair Value Measurement [Member] \| Cash and Cash Equivalents [Member] \| Fair Value, Measurements, Recurring [Member] \| Money Market Funds [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	166,810,000	555,826,000
Estimate of Fair Value Measurement [Member] \| Cash and Cash Equivalents [Member] \| Fair Value, Measurements, Recurring [Member] \| Corporate Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	10,089,000	2,784,000
Estimate of Fair Value Measurement [Member] \| Available-for-sale Securities [Member] \| Fair Value, Measurements, Recurring [Member] \| Corporate Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	633,384,000	807,692,000
Estimate of Fair Value Measurement [Member] \| Available-for-sale Securities [Member] \| Fair Value, Measurements, Recurring [Member] \| Agency Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	375,865,000	439,064,000
Estimate of Fair Value Measurement [Member] \| Available-for-sale Securities [Member] \| Fair Value, Measurements, Recurring [Member] \| Municipal securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	2,505,000	3,965,000
Estimate of Fair Value Measurement [Member] \| Available-for-sale Securities [Member] \| Fair Value, Measurements, Recurring [Member] \| Government securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale securities	272,167,000	366,123,000
Estimate of Fair Value Measurement [Member] \| Foreign Exchange Contract [Member] \| Prepaid Expenses and Other Current Assets [Member] \| Fair Value, Measurements, Recurring [Member]
Assets, Fair Value Disclosure [Abstract]
Prepaid expenses and other current assets	2,306,000	2,498,000
Estimate of Fair Value Measurement [Member] \| Foreign Exchange Contract [Member] \| Accrued Expenses and Other Current Liabilities [Member] \| Fair Value, Measurements, Recurring [Member]
Liabilities, Fair Value Disclosure [Abstract]
Foreign currency derivatives	$ 512,000	$ 814,000

Amount of impairment loss on investment in equity security without readily determinable fair value.

No definition available.

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- Definition

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

+ References

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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- References

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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

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624163-113959

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226000-175313

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=82924980&loc=SL51823488-111719

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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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- Definition

Fair value portion of liability contracts related to the exchange of different currencies, including, but not limited to, foreign currency options, forward (delivery or nondelivery) contracts, and swaps entered into.

+ References

No definition available.

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- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ 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(12))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(5))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82895926&loc=d3e24584-111560

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-Name Accounting Standards Codification
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-Name Accounting Standards Codification
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Reference 7: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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- Definition

Fair value of financial and nonfinancial obligations.

+ References

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