BUSINESS SEGMENT RESULTS (Schedule of Business Segment Results) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Balance Sheet Related Disclosures [Abstract]
Investment securities	$ 2,088,703		$ 2,088,703		$ 2,063,837
Total loans	3,344,907		3,344,907		3,149,243
FRB and FHLB stock	30,390		30,390		30,458
Total assets	6,096,718		6,096,718		5,894,791
Deposits	5,056,525		5,056,525		4,849,504
Securities sold under agreements to repurchase	45,616		45,616		64,875
Subordinated debentures	354,324		354,324		354,286
Stockholders’ equity	527,924	$ 506,228	527,924	$ 506,228	512,444	$ 488,256
Income Statement Related Disclosures [Abstract]
Interest income	45,984	42,587	87,650	84,776
Interest expense	5,229	5,063	10,308	10,912
Net interest income (loss)	40,755	37,524	77,342	73,864
(Benefit) provision for loan losses	(7,500)	0	(7,500)	0
Net interest income (loss) after (benefit) provision for loan losses	48,255	37,524	84,842	73,864
Noninterest income	22,541	13,837	40,669	28,122
Noninterest expense	56,510	43,211	106,929	85,569
Income (loss) before provision (benefit) for income taxes	14,286	8,150	18,582	16,417
Provision (benefit) for income taxes	1,556	2,918	3,169	5,835
Net income (loss)	12,730	5,232	15,413	10,582
Net income (loss) attributable to noncontrolling interest in subsidiary	18	1	27	(54)
Net income (loss) attributable to First Banks, Inc.	12,712	5,231	15,386	10,636
First Bank [Member] | Operating Segments [Member]
Balance Sheet Related Disclosures [Abstract]
Investment securities	2,088,703		2,088,703		2,063,837
Total loans	3,344,907		3,344,907		3,149,243
FRB and FHLB stock	30,390		30,390		30,458
Total assets	6,047,605		6,047,605		5,847,778
Deposits	5,078,758		5,078,758		4,871,140
Securities sold under agreements to repurchase	45,616		45,616		64,875
Subordinated debentures	0		0		0
Stockholders’ equity	883,317		883,317		871,301
Income Statement Related Disclosures [Abstract]
Interest income	45,984	42,557	87,650	84,746
Interest expense	2,186	2,041	4,261	4,092
Net interest income (loss)	43,798	40,516	83,389	80,654
(Benefit) provision for loan losses	(7,500)	0	(7,500)	0
Net interest income (loss) after (benefit) provision for loan losses	51,298	40,516	90,889	80,654
Noninterest income	22,450	13,747	40,487	27,916
Noninterest expense	56,343	43,230	106,586	85,833
Income (loss) before provision (benefit) for income taxes	17,405	11,033	24,790	22,737
Provision (benefit) for income taxes	2,647	1,901	5,341	6,021
Net income (loss)	14,758	9,132	19,449	16,716
Net income (loss) attributable to noncontrolling interest in subsidiary	18	1	27	(54)
Net income (loss) attributable to First Banks, Inc.	14,740	9,131	19,422	16,770
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	49,113		49,113		47,013
Deposits	(22,233)		(22,233)		(21,636)
Securities sold under agreements to repurchase	0		0		0
Subordinated debentures	354,324		354,324		354,286
Stockholders’ equity	(355,393)		(355,393)		$ (358,857)
Income Statement Related Disclosures [Abstract]
Interest income	0	30	0	30
Interest expense	3,043	3,022	6,047	6,820
Net interest income (loss)	(3,043)	(2,992)	(6,047)	(6,790)
(Benefit) provision for loan losses	0	0	0	0
Net interest income (loss) after (benefit) provision for loan losses	(3,043)	(2,992)	(6,047)	(6,790)
Noninterest income	91	90	182	206
Noninterest expense	167	(19)	343	(264)
Income (loss) before provision (benefit) for income taxes	(3,119)	(2,883)	(6,208)	(6,320)
Provision (benefit) for income taxes	(1,091)	1,017	(2,172)	(186)
Net income (loss)	(2,028)	(3,900)	(4,036)	(6,134)
Net income (loss) attributable to noncontrolling interest in subsidiary	0	0	0	0
Net income (loss) attributable to First Banks, Inc.	$ (2,028)	$ (3,900)	$ (4,036)	$ (6,134)