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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Unsecured short-term debt instrument issued by corporations which are 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. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means 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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Cash and Cash Equivalents, Restricted Cash and Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Cash and Cash Equivalents [Line Items]
Cash	$ 140,957	$ 102,194
Total cash and cash equivalents	330,061	371,167	$ 374,006	$ 185,665
Level 1 [Member]
Cash and Cash Equivalents [Line Items]
Money market funds	181,524	266,478
Level 2 [Member]
Cash and Cash Equivalents [Line Items]
Commercial paper	$ 7,580	$ 2,495