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Fair Value of Financial Instruments (Tables) | 9 Months Ended
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Sep. 30, 2018
Fair Value Disclosures [Abstract] |
Schedule of Fair value of Cash and Cash Equivalents by Balance Sheet Grouping | Cash and cash equivalents consist of the following:
| September 30, 2018 |  | December 31, 2017
| Cost |  | Fair Value |  | Cost |  | Fair Value
Cash and Cash Equivalents: |  |  |  |  |  |  |
Cash and overnight deposits | $ | 444,650 |  |  | 444,650 |  |  | 383,021 |  |  | 383,021 |
Corporate commercial paper | 520,740 |  |  | 521,292 |  |  | 635,345 |  |  | 635,919 |
Time deposits | 25,222 |  |  | 25,222 |  |  | 32,733 |  |  | 32,733 |
Total cash and cash equivalents | $ | 990,612 |  |  | 991,164 |  |  | 1,051,099 |  |  | 1,051,673 |

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- Definition Tabular disclosure of the fair value of financial instruments, including financial assets and financial liabilities, and the measurements of those instruments, assets, and liabilities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13467-108611 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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