| Summary of Valuation of Investments and Financial Instruments Pricing Levels |
The following table summarizes the valuation of our investments and financial instruments by the above pricing levels as of the valuation dates listed: | | | | | | | | | | | | | | | | | | September 30, 2017 | | | Total | | (Level 1) | | (Level 2) | | | (Thousands of dollars) | Investments: | | | | | | | Money market mutual funds | | $ | 5,278 |
| | $ | 5,278 |
| | $ | — |
| Commercial paper | | 9,413 |
| | — |
| | 9,413 |
| U.S. Government agencies | | 13,491 |
| | — |
| | 13,491 |
| Corporate bonds and notes | | 188,235 |
| | — |
| | 188,235 |
| | | 216,417 |
| | 5,278 |
| | 211,139 |
| Deferred compensation plan assets | | 2,630 |
| | 2,630 |
| | — |
| Total | | $ | 219,047 |
| | $ | 7,908 |
| | $ | 211,139 |
| | | | | | | | | | | | December 31, 2016 | | | Total | | (Level 1) | | (Level 2) | | | (Thousands of dollars) | Investments: | | | | | Money market mutual funds | | $ | 18,118 |
| | $ | 18,118 |
| | $ | — |
| Commercial paper | | 27,867 |
| | — |
| | 27,867 |
| U.S. government agencies | | 13,263 |
| | — |
| | 13,263 |
| Corporate bonds and notes | | 243,582 |
| | — |
| | 243,582 |
| | | 302,830 |
| | 18,118 |
| | 284,712 |
| Deferred compensation plan assets | | 2,394 |
| | 2,394 |
| | — |
| Total | | $ | 305,224 |
| | $ | 20,512 |
| | $ | 284,712 |
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