CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Unaudited) (USD $)	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comp. Income (Loss), Net [Member]	Total
Balances at Dec. 31, 2012	$ 34,696,597	$ 543,760,439	$ 87,386,689	$ 13,227,634	$ 679,071,359
Balances (in shares) at Dec. 31, 2012	34,696,597
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of employee common stock options and related tax benefits	191,928	2,372,601	0	0	2,564,529
Exercise of employee common stock options and related tax benefits (in shares)	191,928
Common dividends paid					0
Issuance of restricted common shares, net of forfeitures	300,718	(300,718)	0	0	0
Issuance of restricted common shares, net of forfeitures (in shares)	300,718
Restricted shares withheld for taxes	(55,510)	(1,218,326)	0	0	(1,273,836)
Restricted shares withheld for taxes (in shares)	(55,510)
Compensation expense for restricted shares	0	3,014,761	0	0	3,014,761
Compensation expense for stock options	0	12,470	0	0	12,470
Net income	0	0	42,405,607	0	42,405,607
Other comprehensive income (loss)	0	0	0	(13,579,028)	(13,579,028)
Ending Balances at Sep. 30, 2013	35,133,733	547,641,227	129,792,296	(351,394)	712,215,862
Balances (in shares) at Sep. 30, 2013	35,133,733
Balances at Dec. 31, 2013	35,221,941	550,212,135	142,298,199	(4,024,614)	723,707,661
Balances (in shares) at Dec. 31, 2013	35,221,941
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of employee common stock options and related tax benefits	218,108	6,347,045	0	0	6,565,153
Exercise of employee common stock options and related tax benefits (in shares)	218,108
Common dividends paid	0	0	(8,535,894)	0	(8,535,894)
Issuance of restricted common shares, net of forfeitures	281,416	(281,416)	0	0	0
Issuance of restricted common shares, net of forfeitures (in shares)	281,416
Restricted shares withheld for taxes	(66,924)	(2,177,101)	0	0	(2,244,025)
Restricted shares withheld for taxes (in shares)	(66,924)
Compensation expense for restricted shares	0	3,969,973	0	0	3,969,973
Net income	0	0	51,733,929	0	51,733,929
Other comprehensive income (loss)	0	0	0	6,737,354	6,737,354
Ending Balances at Sep. 30, 2014	$ 35,654,541	$ 558,070,636	$ 185,496,234	$ 2,712,740	$ 781,934,151
Balances (in shares) at Sep. 30, 2014	35,654,541