Short-term Investments (Available-For-Sale Securities Reported At Fair Value) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Non-current Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Restricted Cash	$ 4,400
Level 1 Securities [Member] \| Cash and Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Money Market Funds, at Carrying Value	449,700
Level 1 Securities [Member] \| Non-current Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Money Market Funds, at Carrying Value	1,300
Level 1 Securities [Member] \| Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and Money market funds, Amortized Cost		$ 204,967
Cash and Money market funds, Fair Value Disclosure		204,967
Level 2 Securities [Member] \| Cash and Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Foreign deposits	$ 300,800
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and Money market funds, Amortized Cost		1,267,523
Cash and Money market funds, Fair Value Disclosure		1,267,523
Fair Value, Measurements, Recurring [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Gross Unrealized Gains		110
Available-for-sale Securities, Gross Unrealized Losses		(1,087)
Total, Amortized Cost		1,739,673
Total, Estimated Fair Value		1,738,696
Fair Value, Measurements, Recurring [Member] \| Cash and Cash Equivalents [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value		1,467,576
Fair Value, Measurements, Recurring [Member] \| Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value		266,206
Fair Value, Measurements, Recurring [Member] \| Non-current Assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Total, Estimated Fair Value		4,914
Fair Value, Measurements, Recurring [Member] \| Level 1 Securities [Member] \| Money market funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash, Cash Equivalents and Restricted Cash, Gross Unrealized Gains		0
Cash, Cash Equivalents and Restricted Cash, Gross Unrealized Losses		0
Cash and Money market funds, Fair Value Disclosure		203,450
Cash and Money market funds, Non-current assets		1,517
Fair Value, Measurements, Recurring [Member] \| Level 2 Securities [Member] \| Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost		199,843
Available-for-sale Securities, Gross Unrealized Gains		110
Available-for-sale Securities, Gross Unrealized Losses		(731)
Available-for-sale Securities, Estimated Fair Value		199,222
Fair Value, Measurements, Recurring [Member] \| Level 2 Securities [Member] \| US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost		35,944
Available-for-sale Securities, Gross Unrealized Gains		0
Available-for-sale Securities, Gross Unrealized Losses		(128)
Available-for-sale Securities, Estimated Fair Value		35,816
Fair Value, Measurements, Recurring [Member] \| Level 2 Securities [Member] \| Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost		9,833
Available-for-sale Securities, Estimated Fair Value		9,833
Fair Value, Measurements, Recurring [Member] \| Level 2 Securities [Member] \| Agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost		21,563
Available-for-sale Securities, Gross Unrealized Gains		0
Available-for-sale Securities, Gross Unrealized Losses		(228)
Available-for-sale Securities, Estimated Fair Value		21,335
Fair Value, Measurements, Recurring [Member] \| Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash, Cash Equivalents and Restricted Cash, Gross Unrealized Gains		0
Cash, Cash Equivalents and Restricted Cash, Gross Unrealized Losses		0
Cash and Money market funds, Fair Value Disclosure		1,264,126
Cash and Money market funds, Non-current assets		$ 3,397

Available-for-sale Securities including Cash, Cash Equivalents and Restricted Cash, Amortized Cost

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Available-for-sale Securities including Cash, Cash Equivalents and Restricted Cash, Fair Value Disclosure

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Cash, Cash Equivalents and Restricted Cash, Amortized Cost

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Cash, Cash Equivalents and Restricted Cash, Gross Unrealized Gains

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Cash, Cash Equivalents And Restricted Cash, Noncurrent Assets

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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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.

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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.

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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.

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The aggregate of all foreign interest-bearing and noninterest-bearing deposit liabilities.

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Investment in short-term money-market instruments (such as commercial paper, banker's acceptances, repurchase agreements, government securities, certificates of deposit, and so forth) which are highly liquid (that is, readily convertible to known amounts of cash) and so near their maturity that they present an insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify as cash equivalents by definition. Original maturity means an original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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Amount of cash restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits.

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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.

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