| **Note Payable**                      | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
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| **Notes Payable, Current [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
| Note Payable                          | Note PayableWe had 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 had a term of five years that was due to expire May 13, 2019. We had no compensating balance requirements on short-term debt for either 2018 or 2017. At December 31, 2018 and 2017, $32 million and $24 million was drawn on the line of credit, respectively. The interest rate charged on our borrowings on this credit agreement ranged from 2.45 percent to 4.75 percent during 2018 and ranged from 1.65 percent to 2.45 percent during 2017. Effective February 4, 2019, we amended our unsecured revolving line of credit to $300 million with an accordion feature of an additional $300 million. The agreement was extended for five years, amending the expiration to February 4, 2024, with the option of two one-year extensions. Terms and conditions are similar to the former agreement except the net worth covenant has been eliminated and the debt-to-total capital maximum shall not exceed 35 percent. |

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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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