BUSINESS SEGMENT RESULTS (Schedule of Business Segment Results) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Dec. 31, 2013
Balance Sheet Related Disclosures [Abstract]
Investment securities	$ 2,149,777		$ 2,063,837
Total loans	3,193,397		3,149,243
FRB and FHLB stock	30,348		30,458
Total assets	6,046,630		5,935,519
Deposits	4,972,089		4,849,504
Securities sold under agreements to repurchase	51,032		64,875
Subordinated debentures	354,305		354,286
Stockholders’ equity	519,371	497,306	512,444	488,256
Income Statement Related Disclosures [Abstract]
Interest income	41,666	42,189
Interest expense	5,079	5,849
Net interest income (loss)	36,587	36,340
Provision for loan losses	0	0
Net interest income (loss) after provision for loan losses	36,587	36,340
Noninterest income	18,128	14,285
Noninterest expense	50,419	42,358
Income (loss) before provision (benefit) for income taxes	4,296	8,267
Provision (benefit) for income taxes	1,613	2,917
Net income (loss)	2,683	5,350
Net income (loss) attributable to noncontrolling interest in subsidiary	9	(55)
Net income (loss) attributable to First Banks, Inc.	2,674	5,405
First Bank [Member] | Operating Segments [Member]
Balance Sheet Related Disclosures [Abstract]
Investment securities	2,149,777		2,063,837
Total loans	3,193,397		3,149,243
FRB and FHLB stock	30,348		30,458
Total assets	5,992,340		5,882,222
Deposits	4,997,353		4,871,140
Securities sold under agreements to repurchase	51,032		64,875
Subordinated debentures	0		0
Stockholders’ equity	872,736		871,301
Income Statement Related Disclosures [Abstract]
Interest income	41,666	42,189
Interest expense	2,075	2,051
Net interest income (loss)	39,591	40,138
Provision for loan losses	0	0
Net interest income (loss) after provision for loan losses	39,591	40,138
Noninterest income	18,037	14,169
Noninterest expense	50,243	42,603
Income (loss) before provision (benefit) for income taxes	7,385	11,704
Provision (benefit) for income taxes	2,694	4,120
Net income (loss)	4,691	7,584
Net income (loss) attributable to noncontrolling interest in subsidiary	9	(55)
Net income (loss) attributable to First Banks, Inc.	4,682	7,639
Corporate, Other and Intercompany Reclassifications [Member]
Balance Sheet Related Disclosures [Abstract]
Investment securities	0		0
Total loans	0		0
FRB and FHLB stock	0		0
Total assets	54,290		53,297
Deposits	(25,264)		(21,636)
Securities sold under agreements to repurchase	0		0
Subordinated debentures	354,305		354,286
Stockholders’ equity	(353,365)		(358,857)
Income Statement Related Disclosures [Abstract]
Interest income	0	0
Interest expense	3,004	3,798
Net interest income (loss)	(3,004)	(3,798)
Provision for loan losses	0	0
Net interest income (loss) after provision for loan losses	(3,004)	(3,798)
Noninterest income	91	116
Noninterest expense	176	(245)
Income (loss) before provision (benefit) for income taxes	(3,089)	(3,437)
Provision (benefit) for income taxes	(1,081)	(1,203)
Net income (loss)	(2,008)	(2,234)
Net income (loss) attributable to noncontrolling interest in subsidiary	0	0
Net income (loss) attributable to First Banks, Inc.	$ (2,008)	$ (2,234)