Stock-Based Compensation Plans - Performance and Market Based Stock (Details) - Performance and Market Based Stock - USD ($) |
12 Months Ended | |
|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | ||
| Number of shares granted | 99,933 | 91,612 |
| Grant date fair value (USD per share) | $ 31.15 | $ 34.67 |
| Stock-based compensation expense | $ 2,884,502 | $ 2,240,401 |
| Compensation expense, tax benefits | $ 1,084,573 | $ 842,391 |