| **Note Payable**                      | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
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| **Notes Payable, Current [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
| Note Payable                          | Note PayableWe have one line of credit through multiple commercial banks with a borrowing capacity of $225 million and an additional $50 million accordion feature. Our unsecured revolving credit facility has a term of five years that expires May 13, 2019. We had no compensating balance requirements on short-term debt for either 2017 or 2016. At December 31, 2017 and 2016, $24 million and $20 million was drawn on the line of credit, respectively. The interest rate charged on our borrowings on this credit agreement ranged from 1.65 percent to 2.45 percent during 2017 and ranged from 1.31 percent to 1.49 percent during 2016. |

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| - Definition
The entire disclosure for information about short-term and long-term debt arrangements, which includes amounts of borrowings under each line of credit, note payable, commercial paper issue, bonds indenture, debenture issue, own-share lending arrangements and any other contractual agreement to repay funds, and about the underlying arrangements, rationale for a classification as long-term, including repayment terms, interest rates, collateral provided, restrictions on use of assets and activities, whether or not in compliance with debt covenants, and other matters important to users of the financial statements, such as the effects of refinancing and noncompliance with debt covenants.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 470 -URI http://asc.fasb.org/topic&trid=2208564
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| - References
No definition available.
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