Condensed Financial Information of Parent Company (Details 2) (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, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash flows from operating activities:
Net income	$ 6,220,022	$ 6,839,384	$ 2,023,067	$ 8,010,147	$ (878,920)	$ 819,926	$ (1,693,989)	$ 2,205,936	$ 23,092,620	$ 452,953	$ 11,467,179
Adjustments:
Net adjustments									1,872,005	20,628,358	10,537,624
Net cash provided by operating activities									24,964,625	21,081,311	22,004,803
Cash flows from investing activities:
Net sale (purchase) of short-term investments									16,635,183	314,583	16,052,089
Net increase in investment in affiliates									(100,000)	(20,570,000)
Net cash used in investing activities									(4,757,930)	(31,109,363)	(43,190,044)
Cash flows from financing activities:
Cash dividends paid									(12,208,509)	(11,874,367)	(11,405,268)
Issuance of common stock									2,983,399	1,460,543	1,199,422
Payments on line of credit									(6,000,000)	(3,617,371)
Borrowings under line of credit									3,500,000	22,500,000	34,955,088
Purchase of treasury stock									(1,925,673)	(1,537,587)	(145,687)
Net cash (used in) provided by financing activities									(13,650,783)	6,931,218	24,603,555
Net change in cash									6,555,912	(3,096,834)	3,418,314
Parent Company [Member]
Cash flows from operating activities:
Net income									23,093,000	453,000	11,467,000
Adjustments:
Equity in undistributed net (income) loss of subsidiaries									(15,425,000)	13,914,000	(1,117,000)
Other									(2,624,000)	(396,000)	547,000
Net adjustments									(18,049,000)	13,518,000	(570,000)
Net cash provided by operating activities									5,044,000	13,971,000	10,897,000
Cash flows from investing activities:
Net sale (purchase) of short-term investments									8,932,000	6,437,000	(249,000)
Net purchase of property and equipment									(147,000)	(380,000)	(492,000)
Net increase in investment in affiliates									(100,000)	(27,777,000)	(35,088,000)
Other									44,000	43,000	20,000
Net cash used in investing activities									8,729,000	(21,677,000)	(35,809,000)
Cash flows from financing activities:
Cash dividends paid									(12,208,000)	(11,874,000)	(11,405,000)
Issuance of common stock									2,983,000	1,461,000	1,199,000
Payments on line of credit									(6,000,000)	(3,000,000)
Borrowings under line of credit									3,500,000	22,500,000	35,000,000
Purchase of treasury stock									(1,927,000)	(1,538,000)	(146,000)
Net cash (used in) provided by financing activities									(13,652,000)	7,549,000	24,648,000
Net change in cash									121,000	(157,000)	(264,000)
Cash at beginning of year				684,000				841,000	684,000	841,000	1,105,000
Cash at end of year	$ 805,000				$ 684,000				$ 805,000	$ 684,000	$ 841,000