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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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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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No definition available.

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Fair Value of Financial Instruments - Cash and Cash Equivalents (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	$ 990,612	$ 1,020,854	$ 1,051,099	$ 1,033,444	$ 1,114,910	$ 974,435
Cost
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total cash and cash equivalents	990,612		1,051,099
Cost \| Cash and Cash Equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and overnight deposits	444,650		383,021
Corporate commercial paper	520,740		635,345
Time deposits	25,222		32,733
Fair Value \| Cash and Cash Equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and overnight deposits	444,650		383,021
Corporate commercial paper	521,292		635,919
Time deposits	25,222		32,733
Total cash and cash equivalents	$ 991,164		$ 1,051,673