PARENT ONLY BALANCE SHEETS, STATEMENTS OF OPERATIONS AND STATEMENTS OF CASH FLOWS - Parent Company Statement of Cash Flows (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net income	$ 3,331	$ 4,657	$ 4,934	$ 3,917	$ 3,337	$ 4,047	$ 3,820	$ 2,402	$ 16,839	$ 13,606	$ 8,202
Adjustments to reconcile net loss to net cash provided by operating activities:
Change in the fair value of equity securities									(341)	267	0
Net change in:
Other assets									(2,015)	995	(639)
Accrued other liabilities									990	(2,582)	(2,274)
Net cash provided by operating activities									18,567	16,412	9,457
Cash flows from investing activities
Distributions from investments									162	39	24
Purchases of equity securities									(7,040)	(4,265)	(4,844)
Proceeds from redemption or sale of equity securities									2,986	1,299	2,000
Net cash used in investing activities									(103,063)	(182,401)	(98,348)
Cash flows from financing activities
Cash dividends paid on common stock									(2,167)	(1,468)	(722)
Proceeds from common stock offering, net of issuance costs									28,525	0	32,509
Payments to repurchase common stock									(8,326)	(3,368)	(506)
Proceeds from stock options and warrants exercised									287	1,036	1,172
Proceeds from subordinated debt, net of costs									24,558	0	18,133
Net cash provided by (used in) financing activities									112,051	152,708	89,864
Net increase (decrease) in cash and cash equivalents									27,555	(13,281)	973
Cash and cash equivalents, beginning of period				17,140				30,421	17,140	30,421	29,448
Cash and cash equivalents, end of period	44,695				17,140				44,695	17,140	30,421
Cash payments for:
Interest on borrowings									24,396	16,139	10,201
Investar Holding Corporation
Cash flows from operating activities
Net income									16,839	13,606
Adjustments to reconcile net loss to net cash provided by operating activities:
Equity in undistributed earnings of bank subsidiary									(11,941)	(11,644)
Change in the fair value of equity securities									(327)	265
Amortization of debt issuance costs and purchase accounting adjustments									105	100
Net change in:
Due from bank subsidiary									(261)	307
Other assets									25	(13)
Deferred tax asset									62	(99)
Accrued other liabilities									2,776	890
Net cash provided by operating activities									7,278	3,412
Cash flows from investing activities
Capital contributed to bank subsidiary									(23,250)	0
Repayment of investment in and advances to subsidiary									8,000	0
Distributions from investments									82	0
Purchases of equity securities									(144)	(2,189)
Proceeds from redemption or sale of equity securities									88	1,047
Net cash used in investing activities									(15,224)	(1,142)
Cash flows from financing activities
Cash dividends paid on common stock									(2,167)	(1,468)
Proceeds from common stock offering, net of issuance costs									28,525	0
Payments to repurchase common stock									(8,326)	(3,368)
Proceeds from stock options and warrants exercised									287	1,036
Proceeds from subordinated debt, net of costs									24,558	0
Net cash provided by (used in) financing activities									42,877	(3,800)
Net increase (decrease) in cash and cash equivalents									34,931	(1,530)
Cash and cash equivalents, beginning of period				$ 604				$ 2,134	604	2,134
Cash and cash equivalents, end of period	$ 35,535				$ 604				35,535	604	$ 2,134
Cash payments for:
Interest on borrowings									$ 1,513	$ 1,484