BUSINESS SEGMENT RESULTS (Schedule of Business Segment Results) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Balance Sheet Related Disclosures [Abstract]
Investment securities	$ 1,925,134		$ 1,925,134		$ 2,063,837
Total loans	3,435,555		3,435,555		3,149,243
FRB and FHLB stock	30,707		30,707		30,458
Total assets	5,932,297		5,932,297		5,894,791
Deposits	4,898,040		4,898,040		4,849,504
Securities sold under agreements to repurchase	31,672		31,672		64,875
Subordinated debentures	354,343		354,343		354,286
Stockholders’ equity	532,639	$ 508,693	532,639	$ 508,693	512,444	$ 488,256
Income Statement Related Disclosures [Abstract]
Interest income	45,447	41,945	133,097	126,721
Interest expense	5,509	5,098	15,817	16,010
Net interest income (loss)	39,938	36,847	117,280	110,711
(Benefit) provision for loan losses	0	(5,000)	(7,500)	(5,000)
Net interest income (loss) after (benefit) provision for loan losses	39,938	41,847	124,780	115,711
Noninterest income	17,645	13,671	58,314	41,793
Noninterest expense	46,303	46,296	153,232	131,865
Income (loss) before provision (benefit) for income taxes	11,280	9,222	29,862	25,639
Provision (benefit) for income taxes	4,440	3,212	7,609	9,047
Net income (loss)	6,840	6,010	22,253	16,592
Net income (loss) attributable to noncontrolling interest in subsidiary	10	(32)	37	(86)
Net income (loss) attributable to First Banks, Inc.	6,830	6,042	22,216	16,678
First Bank [Member] | Operating Segments [Member]
Balance Sheet Related Disclosures [Abstract]
Investment securities	1,925,134		1,925,134		2,063,837
Total loans	3,435,555		3,435,555		3,149,243
FRB and FHLB stock	30,707		30,707		30,458
Total assets	5,881,902		5,881,902		5,847,778
Deposits	4,931,182		4,931,182		4,871,140
Securities sold under agreements to repurchase	31,672		31,672		64,875
Subordinated debentures	0		0		0
Stockholders’ equity	876,009		876,009		871,301
Income Statement Related Disclosures [Abstract]
Interest income	45,447	41,945	133,097	126,691
Interest expense	2,434	2,056	6,695	6,148
Net interest income (loss)	43,013	39,889	126,402	120,543
(Benefit) provision for loan losses	0	(5,000)	(7,500)	(5,000)
Net interest income (loss) after (benefit) provision for loan losses	43,013	44,889	133,902	125,543
Noninterest income	17,552	13,579	58,039	41,495
Noninterest expense	46,242	46,148	152,828	131,981
Income (loss) before provision (benefit) for income taxes	14,323	12,320	39,113	35,057
Provision (benefit) for income taxes	5,506	4,296	10,847	10,317
Net income (loss)	8,817	8,024	28,266	24,740
Net income (loss) attributable to noncontrolling interest in subsidiary	10	(32)	37	(86)
Net income (loss) attributable to First Banks, Inc.	8,807	8,056	28,229	24,826
Corporate, Other and Intercompany Reclassifications [Member]
Balance Sheet Related Disclosures [Abstract]
Investment securities	0		0		0
Total loans	0		0		0
FRB and FHLB stock	0		0		0
Total assets	50,395		50,395		47,013
Deposits	(33,142)		(33,142)		(21,636)
Securities sold under agreements to repurchase	0		0		0
Subordinated debentures	354,343		354,343		354,286
Stockholders’ equity	(343,370)		(343,370)		$ (358,857)
Income Statement Related Disclosures [Abstract]
Interest income	0	0	0	30
Interest expense	3,075	3,042	9,122	9,862
Net interest income (loss)	(3,075)	(3,042)	(9,122)	(9,832)
(Benefit) provision for loan losses	0	0	0	0
Net interest income (loss) after (benefit) provision for loan losses	(3,075)	(3,042)	(9,122)	(9,832)
Noninterest income	93	92	275	298
Noninterest expense	61	148	404	(116)
Income (loss) before provision (benefit) for income taxes	(3,043)	(3,098)	(9,251)	(9,418)
Provision (benefit) for income taxes	(1,066)	(1,084)	(3,238)	(1,270)
Net income (loss)	(1,977)	(2,014)	(6,013)	(8,148)
Net income (loss) attributable to noncontrolling interest in subsidiary	0	0	0	0
Net income (loss) attributable to First Banks, Inc.	$ (1,977)	$ (2,014)	$ (6,013)	$ (8,148)