BUSINESS SEGMENT RESULTS (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Dec. 31, 2010
Investment securities	$ 2,841,624		$ 2,841,624		$ 2,470,704
Loans, net of net deferred loan fees	3,031,445		3,031,445		3,283,951
FRB and FHLB stock	26,115		26,115		27,078
Goodwill and other intangible assets	121,967		121,967		121,967
Assets of discontinued operations	20,501		20,501		21,009
Total assets	6,566,164		6,566,164		6,608,913
Deposits	5,391,263		5,391,263		5,451,830
Other borrowings	37,814		37,814		50,910
Subordinated debentures	354,095		354,095		354,057
Liabilities of discontinued operations	339,924		339,924		346,282
Stockholders' equity	286,133		286,133		263,671	307,295
Interest income	51,608	58,993	103,945	121,158
Interest expense	7,485	11,307	15,502	23,814
Net interest income (loss)	44,123	47,686	88,443	97,344
Provision for loan losses	0	23,000	2,000	33,000
Net interest income (loss) after provision for loan losses	44,123	24,686	86,443	64,344
Noninterest income	15,688	14,968	32,579	28,877
Amortization of intangible assets	0	783	0	1,566
Other noninterest expense	50,101	54,069	99,841	109,679
Income (loss) from continuing operations before (benefit) provision for income taxes	9,710	(15,198)	19,181	(18,024)
(Benefit) provision for income taxes	121	72	216	124
Net income (loss) from continuing operations, net of tax	9,589	(15,270)	18,965	(18,148)
Loss from discontinued operations, net of tax	(2,466)	(2,706)	(5,004)	(5,911)
Net income (loss)	7,123	(17,976)	13,961	(24,059)	(44,100)
Net loss attributable to noncontrolling interest in subsidiary	(385)	(927)	(445)	(862)
Net income (loss) attributable to First Banks, Inc.	7,508	(17,049)	14,406	(23,197)
Corporate Other and Intercompany Reclassifications [Member]
Investment securities	0		0		0
Loans, net of net deferred loan fees	0		0		0
FRB and FHLB stock	0		0		0
Goodwill and other intangible assets	0		0		0
Assets of discontinued operations	0		0		0
Total assets	13,542		13,542		15,398
Deposits	(3,611)		(3,611)		(2,834)
Other borrowings	0		0		0
Subordinated debentures	354,095		354,095		354,057
Liabilities of discontinued operations	0		0		0
Stockholders' equity	(434,577)		(434,577)		(416,954)
Interest income	0	3	21	54
Interest expense	3,681	3,364	7,364	6,663
Net interest income (loss)	(3,681)	(3,361)	(7,343)	(6,609)
Provision for loan losses	0	0	0	0
Net interest income (loss) after provision for loan losses	(3,681)	(3,361)	(7,343)	(6,609)
Noninterest income	109	(65)	181	(19)
Amortization of intangible assets	0	0	0	0
Other noninterest expense	368	(416)	709	(422)
Income (loss) from continuing operations before (benefit) provision for income taxes	(3,940)	(3,010)	(7,871)	(6,206)
(Benefit) provision for income taxes	472	55	435	(36)
Net income (loss) from continuing operations, net of tax	(4,412)	(3,065)	(8,306)	(6,170)
Loss from discontinued operations, net of tax	0	0	0	0
Net income (loss)	(4,412)	(3,065)	(8,306)	(6,170)
Net loss attributable to noncontrolling interest in subsidiary	0	0	0	0
Net income (loss) attributable to First Banks, Inc.	(4,412)	(3,065)	(8,306)	(6,170)
First Bank [Member]
Investment securities	2,841,624		2,841,624		2,470,704
Loans, net of net deferred loan fees	3,031,445		3,031,445		3,283,951
FRB and FHLB stock	26,115		26,115		27,078
Goodwill and other intangible assets	121,967		121,967		121,967
Assets of discontinued operations	20,501		20,501		21,009
Total assets	6,552,622		6,552,622		6,593,515
Deposits	5,394,874		5,394,874		5,454,664
Other borrowings	37,814		37,814		50,910
Subordinated debentures	0		0		0
Liabilities of discontinued operations	339,924		339,924		346,282
Stockholders' equity	720,710		720,710		680,625
Interest income	51,608	58,990	103,924	121,104
Interest expense	3,804	7,943	8,138	17,151
Net interest income (loss)	47,804	51,047	95,786	103,953
Provision for loan losses	0	23,000	2,000	33,000
Net interest income (loss) after provision for loan losses	47,804	28,047	93,786	70,953
Noninterest income	15,579	15,033	32,398	28,896
Amortization of intangible assets	0	783	0	1,566
Other noninterest expense	49,733	54,485	99,132	110,101
Income (loss) from continuing operations before (benefit) provision for income taxes	13,650	(12,188)	27,052	(11,818)
(Benefit) provision for income taxes	(351)	17	(219)	160
Net income (loss) from continuing operations, net of tax	14,001	(12,205)	27,271	(11,978)
Loss from discontinued operations, net of tax	(2,466)	(2,706)	(5,004)	(5,911)
Net income (loss)	11,535	(14,911)	22,267	(17,889)
Net loss attributable to noncontrolling interest in subsidiary	(385)	(927)	(445)	(862)
Net income (loss) attributable to First Banks, Inc.	$ 11,920	$ (13,984)	$ 22,712	$ (17,027)