BUSINESS SEGMENT RESULTS (Schedule of Business Segment Results) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Investment securities	$ 2,126,844		$ 2,351,931
Total loans	2,890,981		2,857,095
FRB and FHLB stock	32,148		27,357
Total assets	5,890,559		5,918,983
Deposits	4,842,431		4,813,895
Securities sold under agreements to repurchase	36,713		43,143
Subordinated debentures	354,229		354,210
Stockholders' equity	497,306	297,066	488,256	299,959
Interest income	42,189	43,986
Interest expense	5,849	6,149
Net interest income (loss)	36,340	37,837
Provision for loan losses	0	0
Net interest income (loss) after provision for loan losses	36,340	37,837
Noninterest income	14,675	15,395
Noninterest expense	42,748	44,047
Income (loss) from continuing operations before provision (benefit) for income taxes	8,267	9,185
Provision (benefit) for income taxes	2,917	365
Net income (loss) from continuing operations, net of tax	5,350	8,820
Loss from discontinued operations, net of tax	0	(2,064)
Net income (loss)	5,350	6,756
Net (loss) income attributable to noncontrolling interest in subsidiary	(55)	46
Net income (loss) attributable to First Banks, Inc.	5,405	6,710
First Bank [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Investment securities	2,126,844		2,351,931
Total loans	2,890,981		2,857,095
FRB and FHLB stock	32,148		27,357
Total assets	5,836,010		5,865,160
Deposits	4,849,783		4,815,792
Securities sold under agreements to repurchase	36,713		43,143
Subordinated debentures	0		0
Stockholders' equity	872,845		931,561
Interest income	42,189	43,986
Interest expense	2,051	2,473
Net interest income (loss)	40,138	41,513
Provision for loan losses	0	0
Net interest income (loss) after provision for loan losses	40,138	41,513
Noninterest income	14,559	15,284
Noninterest expense	42,993	43,863
Income (loss) from continuing operations before provision (benefit) for income taxes	11,704	12,934
Provision (benefit) for income taxes	4,120	148
Net income (loss) from continuing operations, net of tax	7,584	12,786
Loss from discontinued operations, net of tax	0	(2,064)
Net income (loss)	7,584	10,722
Net (loss) income attributable to noncontrolling interest in subsidiary	(55)	46
Net income (loss) attributable to First Banks, Inc.	7,639	10,676
Corporate, Other and Intercompany Reclassifications [Member]
Segment Reporting Information [Line Items]
Investment securities	0		0
Total loans	0		0
FRB and FHLB stock	0		0
Total assets	54,549		53,823
Deposits	(7,352)		(1,897)
Securities sold under agreements to repurchase	0		0
Subordinated debentures	354,229		354,210
Stockholders' equity	(375,539)		(443,305)
Interest income	0	0
Interest expense	3,798	3,676
Net interest income (loss)	(3,798)	(3,676)
Provision for loan losses	0	0
Net interest income (loss) after provision for loan losses	(3,798)	(3,676)
Noninterest income	116	111
Noninterest expense	(245)	184
Income (loss) from continuing operations before provision (benefit) for income taxes	(3,437)	(3,749)
Provision (benefit) for income taxes	(1,203)	217
Net income (loss) from continuing operations, net of tax	(2,234)	(3,966)
Loss from discontinued operations, net of tax	0	0
Net income (loss)	(2,234)	(3,966)
Net (loss) income attributable to noncontrolling interest in subsidiary	0	0
Net income (loss) attributable to First Banks, Inc.	$ (2,234)	$ (3,966)