BUSINESS SEGMENT RESULTS (Schedule of Business Segment Results) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Investment securities	$ 2,129,383		$ 2,129,383		$ 2,351,931
Total loans	2,979,577		2,979,577		2,857,095
FRB and FHLB stock	30,623		30,623		27,357
Total assets	5,886,815		5,886,815		5,918,983
Deposits	4,826,930		4,826,930		4,813,895
Securities sold under agreements to repurchase	42,682		42,682		43,143
Subordinated debentures	354,267		354,267		354,210
Stockholders’ equity	508,693	269,098	508,693	269,098	488,256	299,959
Interest income	41,945	42,366	126,721	130,084
Interest expense	5,098	5,952	16,010	18,076
Net interest income (loss)	36,847	36,414	110,711	112,008
(Benefit) provision for loan losses	(5,000)	0	(5,000)	0
Net interest income (loss) after (benefit) provision for loan losses	41,847	36,414	115,711	112,008
Noninterest income	14,074	15,529	42,963	49,867
Noninterest expense	46,699	45,305	133,035	133,495
Income (loss) from continuing operations before provision (benefit) for income taxes	9,222	6,638	25,639	28,380
Provision (benefit) for income taxes	3,212	(65)	9,047	(45)
Net income (loss) from continuing operations, net of tax	6,010	6,703	16,592	28,425
Loss from discontinued operations, net of tax	0	(881)	0	(6,279)
Net income (loss)	6,010	5,822	16,592	22,146
Net (loss) income attributable to noncontrolling interest in subsidiary	(32)	(38)	(86)	113
Net income (loss) attributable to First Banks, Inc.	6,042	5,860	16,678	22,033
First Bank [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Investment securities	2,129,383		2,129,383		2,351,931
Total loans	2,979,577		2,979,577		2,857,095
FRB and FHLB stock	30,623		30,623		27,357
Total assets	5,834,780		5,834,780		5,865,160
Deposits	4,843,161		4,843,161		4,815,792
Securities sold under agreements to repurchase	42,682		42,682		43,143
Subordinated debentures	0		0		0
Stockholders’ equity	875,145		875,145		931,561
Interest income	41,945	42,366	126,691	130,084
Interest expense	2,056	2,149	6,148	6,866
Net interest income (loss)	39,889	40,217	120,543	123,218
(Benefit) provision for loan losses	(5,000)	0	(5,000)	0
Net interest income (loss) after (benefit) provision for loan losses	44,889	40,217	125,543	123,218
Noninterest income	13,982	15,414	42,665	49,529
Noninterest expense	46,551	44,994	133,151	132,882
Income (loss) from continuing operations before provision (benefit) for income taxes	12,320	10,637	35,057	39,865
Provision (benefit) for income taxes	4,296	(64)	10,317	(44)
Net income (loss) from continuing operations, net of tax	8,024	10,701	24,740	39,909
Loss from discontinued operations, net of tax	0	(881)	0	(6,279)
Net income (loss)	8,024	9,820	24,740	33,630
Net (loss) income attributable to noncontrolling interest in subsidiary	(32)	(38)	(86)	113
Net income (loss) attributable to First Banks, Inc.	8,056	9,858	24,826	33,517
Corporate, Other and Intercompany Reclassifications [Member]
Segment Reporting Information [Line Items]
Investment securities	0		0		0
Total loans	0		0		0
FRB and FHLB stock	0		0		0
Total assets	52,035		52,035		53,823
Deposits	(16,231)		(16,231)		(1,897)
Securities sold under agreements to repurchase	0		0		0
Subordinated debentures	354,267		354,267		354,210
Stockholders’ equity	(366,452)		(366,452)		(443,305)
Interest income	0	0	30	0
Interest expense	3,042	3,803	9,862	11,210
Net interest income (loss)	(3,042)	(3,803)	(9,832)	(11,210)
(Benefit) provision for loan losses	0	0	0	0
Net interest income (loss) after (benefit) provision for loan losses	(3,042)	(3,803)	(9,832)	(11,210)
Noninterest income	92	115	298	338
Noninterest expense	148	311	(116)	613
Income (loss) from continuing operations before provision (benefit) for income taxes	(3,098)	(3,999)	(9,418)	(11,485)
Provision (benefit) for income taxes	(1,084)	(1)	(1,270)	(1)
Net income (loss) from continuing operations, net of tax	(2,014)	(3,998)	(8,148)	(11,484)
Loss from discontinued operations, net of tax	0	0	0	0
Net income (loss)	(2,014)	(3,998)	(8,148)	(11,484)
Net (loss) income attributable to noncontrolling interest in subsidiary	0	0	0	0
Net income (loss) attributable to First Banks, Inc.	$ (2,014)	$ (3,998)	$ (8,148)	$ (11,484)