| **Consolidated Statement Of Equity - USD ($) $ in Thousands**               | Common Stock [Member] | Capital In Excess Of Par Value [Member] | Retained Earnings [Member] | Treasury Stock [Member] | Accumulated Other Comprehensive Income [Member] | Total       |
|-----------------------------------------------------------------------------|-----------------------|-----------------------------------------|----------------------------|-------------------------|-------------------------------------------------|-------------|
| Balance at Dec. 31, 2017                                                    | $ 90,350              | $ 807,135                               | $ 1,132,556                | $ (73,280)              | $ 860                                           | $ 1,957,621 |
| Net income                                                                  |                  |                                    | 50,839                     |                    |                                            | 50,839      |
| Dividends declared                                                          |                  |                                    | (36,386)                   |                    |                                            | (36,386)    |
| Issuance of common stock under dividend reinvestment plan                   | 6                     | 355                                     |                       |                    |                                            | 361         |
| Repurchase of stock                                                         |                  |                                    |                       | (2,491)                 |                                            | (2,491)     |
| Equity compensation plan                                                    | 91                    | (91)                                    |                       |                    |                                            |        |
| Exercise of stock options                                                   | 31                    | 979                                     |                       |                    |                                            | 1,010       |
| Stock-based compensation                                                    |                  | 1,443                                   | (41)                       |                    |                                            | 1,402       |
| Cumulative effect of change in accounting principle - financial instruments |                  |                                    | 860                        |                    | $ (860)                                         |        |
| Other                                                                       |                  | (197)                                   |                       |                    |                                            | (197)       |
| Balance at Mar. 31, 2018                                                    | 90,478                | 809,624                                 | 1,147,828                  | (75,771)                |                                            | 1,972,159   |
| Balance at Dec. 31, 2018                                                    | 90,576                | 820,378                                 | 1,174,245                  | (75,835)                |                                            | 2,009,364   |
| Net income                                                                  |                  |                                    | 16,924                     |                    |                                            | 16,924      |
| Dividends declared                                                          |                  |                                    | (39,014)                   |                    |                                            | (39,014)    |
| Issuance of common stock under dividend reinvestment plan                   | 59                    | 3,976                                   |                       |                    |                                            | 4,035       |
| Repurchase of stock                                                         |                  |                                    |                       | (1,857)                 |                                            | (1,857)     |
| Equity compensation plan                                                    | 67                    | (67)                                    |                       |                    |                                            |        |
| Exercise of stock options                                                   | 39                    | 1,136                                   |                       |                    |                                            | 1,175       |
| Stock-based compensation                                                    |                  | 1,929                                   | 42                         |                    |                                            | 1,971       |
| Other                                                                       |                  | (13)                                    |                       |                    |                                            | (13)        |
| Balance at Mar. 31, 2019                                                    | $ 90,741              | $ 827,339                               | $ 1,152,197                | $ (77,692)              |                                            | $ 1,992,585 |

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Amount of other increase (decrease) in additional paid in capital (APIC).
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Amount of the cumulative effect on retained earnings net of related income tax effect.
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Amount of paid and unpaid common stock dividends declared with the form of settlement in cash.
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| - Definition
The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.
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| - Definition
Value, after forfeitures, of stock or other type of equity granted of any equity-based compensation plan other than an employee stock ownership plan (ESOP).
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| - Definition
Value of stock issued during the period from a dividend reinvestment plan (DRIP). A dividend reinvestment plan allows the holder of the stock to reinvest dividends paid to them by the entity on new issues of stock by the entity.
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Value of stock issued as a result of the exercise of stock options.
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.
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| - Definition
Equity impact of the cost of common and preferred stock that were repurchased during the period. Recorded using the cost method.
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| - Definition
Treasury Stock Reissued at Lower than Repurchase Price, Share-based Compensation
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