v3.6.0.2

Fair Value (Tables) | 12 Months Ended
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Dec. 31, 2016
Fair Value Disclosures [Abstract] |
Summary of Valuation of Investments and Financial Instruments Pricing Levels | The following tables summarize the valuation of our investments and financial instruments by the above pricing levels as of the valuation dates listed:
|  |  |  |  | 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 |
|  |  |  |  | December 31, 2015 |
|  | Total |  |  | (Level 1) |  |  | (Level 2) |
|  |  |  |  | (Thousands of dollars) |
Investments: |  |  |  |  |  |  |  |  |  |  |  |
Money market mutual funds |  | $ | 18,181 |  |  | $ | 18,181 |  |  | $ | — |
Commercial paper |  |  | 5,981 |  |  |  | — |  |  |  | 5,981 |
U.S. government agencies |  |  | 11,469 |  |  |  | — |  |  |  | 11,469 |
Corporate bonds and notes |  |  | 264,228 |  |  |  | — |  |  |  | 264,228 |
|  |  | 299,859 |  |  |  | 18,181 |  |  |  | 281,678 |
Deferred compensation plan assets |  |  | 2,294 |  |  |  | 2,294 |  |  |  | — |
Total |  | $ | 302,153 |  |  | $ | 20,475 |  |  | $ | 281,678 |
Summary of Combined Fair Value of Assets that Incurred Impairments | The below table summarizes the combined fair value of the assets that incurred impairments during the years ended December 31, 2016, 2015 and 2014, along with the amount of the impairment. The impairment charges were recorded in Impairment of assets. See Note 1 for additional information.
|  | Year Ended December 31, |
|  | 2016 |  |  | 2015 |  |  | 2014 |
|  | (Thousands of dollars) |
Amount of impairment incurred |  | $ | 407 |  |  | $ | — |  |  | $ | 10,508 |
Combined fair value of assets incurring impairment, after impairment charges |  | $ | 1,380 |  |  | $ | — |  |  | $ | 6,000 |

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- Definition Tabular disclosure of financial instruments measured at fair value, including those classified in shareholders' equity measured on a recurring or nonrecurring basis. Disclosures include, but are not limited to, fair value measurements recorded and the reasons for the measurements, level within the fair value hierarchy in which the fair value measurements are categorized and transfers between levels 1 and 2. Nonrecurring fair value measurements are those that are required or permitted in the statement of financial position in particular circumstances. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19279-110258 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2C -URI http://asc.fasb.org/extlink&oid=66048111&loc=SL7498348-110258 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19190-110258 + Details
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