Stock-Based Compensation Plans - Stock Option Awards (Details) - USD ($)
|
12 Months Ended |
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
Dec. 31, 2014 |
| Options, outstanding: |
|
|
|
|
| Options, outstanding, beginning of period (in shares) |
578,291
|
769,528
|
921,889
|
|
| Options, outstanding, grants in period (in shares) |
0
|
0
|
0
|
|
| Options, outstanding, exercises in period (in shares) |
(146,950)
|
(191,237)
|
(150,701)
|
|
| Options, outstanding, forfeited or canceled in period (in shares) |
(10)
|
0
|
(1,660)
|
|
| Options, outstanding, end of period (in shares) |
431,331
|
578,291
|
769,528
|
921,889
|
| Options vested and exercisable at end of period (in shares) |
431,331
|
|
|
|
| Options, weighted-average exercise price: |
|
|
|
|
| Options, outstanding, weighted average exercise price, beginning of period (USD per share) |
$ 10.99
|
$ 9.76
|
$ 9.32
|
|
| Options, grants in period, weighted average exercise price (USD per share) |
0.00
|
0.00
|
0.00
|
|
| Options, exercises in period, weighted average exercise price (USD per share) |
10.48
|
6.05
|
7.02
|
|
| Options, forfeited or canceled in period, weighted average exercise price (USD per share) |
10.18
|
0.00
|
12.01
|
|
| Options, outstanding, weighted average exercise price, end of period (USD per share) |
11.16
|
$ 10.99
|
$ 9.76
|
$ 9.32
|
| Options vested and exercisable at end of period (USD per share) |
$ 11.16
|
|
|
|
| Options, additional disclosures: |
|
|
|
|
| Options, oustanding, weighted average remaining contractual term |
2 years 3 months 22 days
|
3 years
|
3 years 4 months 10 days
|
4 years 1 month 28 days
|
| Options, exercisable, weighted average remaining contractual term |
2 years 3 months 22 days
|
|
|
|
| Options, oustanding, aggregate intrinsic value |
$ 7,261,634
|
$ 8,131,137
|
$ 8,178,882
|
$ 18,329,554
|
| Options, exercises in period, aggregate intrinsic value |
1,533,353
|
3,633,746
|
2,014,921
|
|
| Options, exercisable, aggregate intrinsic value |
7,261,634
|
8,131,137
|
7,997,359
|
|
| Restricted Stock |
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
| Stock-based compensation expense |
5,731,731
|
10,625,440
|
11,821,382
|
|
| Compensation expense, tax benefits |
2,155,131
|
3,995,165
|
4,610,339
|
|
| Stock Options |
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
| Stock-based compensation expense |
0
|
32,681
|
226,050
|
|
| Compensation expense, tax benefits |
$ 0
|
$ 12,288
|
$ 88,160
|
|