Shareholders' Equity Stock Options and Net Income per Share - Employee Stock Purchase Plan (Details) - USD ($) $ / shares in Units, $ in Thousands |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Schedule of Employee Stock Purchase Plan (ESPP) Disclosures [Line Items] | |||
| Share-based compensation | $ 8,103 | $ 8,334 | $ 7,486 |
| Employee Stock Purchase Plan [Member] | |||
| Schedule of Employee Stock Purchase Plan (ESPP) Disclosures [Line Items] | |||
| Number of shares authorized under the plan | 371,859 | ||
| Common stock issued under employee stock purchase plan (in shares) | 9,954 | 11,174 | 10,805 |
| Weighted average price of shares purchased (dollars per share) | $ 46.01 | $ 39.50 | $ 41.55 |
| Purchase Period for Employee Stock Purchase Plan | 6 months | ||
| Weighted Average Grant Date Fair Value | $ 9.26 | $ 6.46 | $ 5.82 |
| Share-based compensation | $ 92 | $ 72 | $ 61 |
| X | ||||||||||
- Definition Purchase period for employee stock purchase plan. No definition available.
|
| X | ||||||||||
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. No definition available.
|
| X | ||||||||||
- Definition The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method. Reference 1: http://www.xbrl.org/2003/role/presentationRef
|
| X | ||||||||||
- Definition The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). Reference 1: http://www.xbrl.org/2003/role/presentationRef
|
| X | ||||||||||
- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). Reference 1: http://www.xbrl.org/2003/role/presentationRef
|
| X | ||||||||||
- Definition The weighted average of per share prices paid for shares purchased on the open market for issuance to employees under the plan. Reference 1: http://www.xbrl.org/2003/role/presentationRef
|
| X | ||||||||||
- Definition Number of shares issued during the period as a result of an employee stock purchase plan. Reference 1: http://www.xbrl.org/2003/role/presentationRef
|
| X | ||||||||||
- Details
|