Quarterly Financial Results (unaudited) (Details) (USD $)	3 Months Ended												12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Results (unaudited) [Abstract]
Interest income	$ 47,203,000	$ 46,441,000	$ 45,953,000	$ 45,824,000	$ 46,446,000	$ 46,888,000	$ 47,789,000	$ 47,224,000	$ 49,079,000	$ 50,650,000	$ 50,929,000	$ 52,690,000	$ 185,421,558	$ 188,346,835	$ 203,348,364
Net interest income	42,243,000	40,932,000	40,185,000	39,504,000	33,854,000	34,723,000	31,208,000	29,882,000	36,056,000	36,060,000	35,697,000	36,560,000	157,295,628	129,666,798	90,677,932
Provision for loan losses	2,488,000	1,413,000	634,000	1,034,000	5,439,000	3,632,000	6,587,000	6,139,000	5,171,000	4,789,000	30,509,000	13,226,000	5,568,830	21,797,613	53,695,454
Net income before taxes	18,012,000	16,371,000	15,545,000	12,599,000	9,207,000	9,128,000	6,660,000	3,505,000	3,098,000	2,091,000	(20,734,000)	(4,347,000)	62,527,398	28,499,680	(19,890,334)
Net income (loss)	11,730,000	11,349,000	10,440,000	8,365,000	7,760,000	26,101,000	6,372,000	3,505,000	3,796,000	2,091,000	(26,364,000)	(3,822,000)	41,883,881	43,737,367	(24,300,492)
Net income available to common stockholders	$ 11,730,000	$ 11,349,000	$ 7,785,000	$ 7,206,000	$ 5,681,000	$ 24,537,000	$ 4,844,000	$ 2,011,000	$ 2,248,000	$ 549,000	$ (27,871,000)	$ (5,368,000)	$ 38,069,841	$ 37,072,728	$ (30,442,514)
Basic net income per share available to common stockholders (in dollars per share)	$ 0.35	$ 0.33	$ 0.23	$ 0.21	$ 0.17	$ 0.74	$ 0.14	$ 0.06	$ 0.07	$ 0.02	$ (0.85)	$ (0.16)	$ 1.12	$ 1.11	$ (0.93)
Diluted net income per share per share available to common stockholders (in dollars per share)	$ 0.34	$ 0.33	$ 0.23	$ 0.21	$ 0.17	$ 0.72	$ 0.14	$ 0.06	$ 0.07	$ 0.02	$ (0.85)	$ (0.16)	$ 1.10	$ 1.09	$ (0.93)