Stock-Based Compensation Plans - Narrative (Details)
|
12 Months Ended |
|
Dec. 31, 2017
USD ($)
$ / shares
shares
|
Dec. 31, 2016
USD ($)
$ / shares
shares
|
Dec. 31, 2008
stock_type
shares
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
| Total unrecognized compensation costs related to stock-based compensation | $ |
$ 11,495,449
|
$ 13,018,218
|
|
| Total unrecognized compensation costs related to stock-based compensation, period for recognition |
2 years 2 months 7 days
|
|
|
| Restricted Stock |
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
| Number of shares granted |
268,300
|
284,086
|
|
| Restricted Stock | Vesting Ratably, One Year |
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
| Vesting period |
1 year
|
1 year
|
|
| Number of shares granted |
29,430
|
23,634
|
|
| Restricted Stock | Vesting Ratably, Three Years |
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
| Vesting period |
3 years
|
3 years
|
|
| Number of shares granted |
15,192
|
59,577
|
|
| Restricted Stock | Vesting Ratably, Four Years |
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
| Vesting period |
4 years
|
4 years
|
|
| Number of shares granted |
223,678
|
200,875
|
|
| Restricted Stock | Minimum |
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
| Grant date fair value (USD per share) | $ / shares |
$ 16.75
|
$ 23.02
|
|
| Restricted Stock | Maximum |
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
| Grant date fair value (USD per share) | $ / shares |
$ 27.80
|
$ 27.79
|
|
| 2008 Stock Incentive Plan |
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
| Number of shares authorized for grant |
|
|
3,400,000
|
| Number of award types issued | stock_type |
|
|
3
|
| 2008 Stock Incentive Plan | Stock options |
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
| Maximum term |
|
|
10 years
|
| 2008 Stock Incentive Plan | Stock options | Minimum |
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
| Vesting period |
|
|
1 year
|
| 2008 Stock Incentive Plan | Stock options | Maximum |
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
| Vesting period |
|
|
5 years
|