Quarterly Financial Information (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended			15 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Sep. 30, 2012
Quarterly Financial Data [Abstract]
Total interest income		$ 56,151	$ 63,810	$ 71,578	$ 73,131	$ 72,642	$ 66,852	$ 64,072	$ 264,670	$ 274,687	$ 238,711	$ 71,121
Total interest expense		33,358	33,448	36,961	38,923	39,277	37,640	37,053	142,690	153,382	142,668	39,412
Net interest income		22,793	30,362	34,617	34,208	33,365	29,212	27,019	121,980	121,305	96,043	31,709
Provision for loan losses		(4,354)	(137)	1,220	(420)	482	(160)	(1,281)	(3,691)	(610)	(1,893)	349
Net interest income after provision for loan losses		18,439	30,225	35,837	33,788	33,847	29,052	25,738	118,289	120,695	94,150	32,058
Guarantee and commitment fees		6,568	6,401	6,064	5,930	5,966	6,320	6,387	24,963	24,821	24,091	6,148
Gain (loss) on derivatives instruments and hedging activity net pretax		3,505	1,558	(31,292)	6,400	(10,277)	(17,806)	4,005	(19,829)	(92,645)	(17,159)	(68,567)
Trading Gains (Losses)		2,735	(441)	(3,086)	1,099	3,809	1,968	1,311	307	3,455	5,270	(3,633)
Gains on sale of available-for-sale investment securities		(10)	0	0	28	0	38	157	18	269	266	74
Gains (Losses) on Sales of Other Real Estate		629	(13)	262	0	254	627	97	878	974	10	(4)
Lower of cost or fair value adjustment on loans held for sale		(5,943)	0	0	0	0	(156)	(808)	(5,943)	8,887	(8,748)	9,851
Other income		890	959	771	721	1,102	1,124	3,898	3,341	6,850	1,244	726
Non-interest income/(loss)		8,374	8,464	(27,281)	14,178	854	(7,885)	15,047	3,735	(47,389)	4,974	(55,405)
Noninterest Expense		6,109	7,748	9,209	7,842	7,240	7,191	6,678	30,908	28,659	32,627	7,550
Income/(loss) before income taxes		20,704	30,941	(653)	40,124	27,461	13,976	34,107	91,116	44,647	66,497	(30,897)
Income tax expense/(benefit)		4,837	8,294	(2,629)	11,654	7,872	2,539	9,517	22,156	5,797	13,797	(14,131)
Net income		15,867	22,647	1,976	28,470	19,589	11,437	24,590	68,960	38,850	52,700	(16,766)
Net Income (Loss) Attributable to Noncontrolling Interest - preferred stock dividends		(5,546)	(5,547)	(5,547)	(5,547)	(5,546)	(5,547)	(5,547)	(22,187)	(22,187)	(20,707)	(5,547)
Net income attributable to Farmer Mac		10,321	17,100	(3,571)	22,923	14,043	5,890	19,043	46,773	16,663	31,993	(22,313)
Preferred Stock Dividends, Income Statement Impact		(720)	(719)	(720)	(720)	(720)	(720)	(720)	(2,879)	(2,879)	(4,129)	(719)
Net income attributable to common stockholders		$ 9,601	$ 16,381	$ (4,291)	$ 22,203	$ 13,323	$ 5,170	$ 18,323	$ 43,894	$ 13,784	$ 22,080	$ (23,032)
Basic earnings per common share (dollars per share)		$ 0.91	$ 1.56	$ (0.41)	$ 2.14	$ 1.29	$ 0.50	$ 1.78	$ 4.19	$ 1.33	$ 2.16	$ (2.22)
Diluted Earnings per common share		$ 0.87	$ 1.49	$ (0.41)	$ 2.04	$ 1.23	$ 0.48	$ 1.72	$ 3.98	$ 1.28	$ 2.08	$ (2.22)
Common stock dividends per common share	$ 0.12	$ 0.10	$ 0.10	$ 0.10	$ 0.10	$ 0.05	$ 0.05	$ 0.05	$ 0.40	$ 0.20	$ 0.20	$ 0.05