Note 10 - Fair Values of Financial Instruments - Fair Value Hierarchy of Financial Instruments (Details) - USD ($)		Sep. 30, 2021	Dec. 31, 2020
Securities available for sale		$ 559,564,000	$ 387,347,000
Mortgages Held-for-sale, Fair Value Disclosure		49,100,000	24,000,000.0
Reported Value Measurement [Member]
Securities available for sale	[1]	559,564,000	387,347,000
FHLBI stock	[2]	18,002,000	18,002,000
Estimate of Fair Value Measurement [Member]
Securities available for sale	[1]	559,564,000	387,347,000
FHLBI stock	[2]	18,002,000	18,002,000
Fair Value, Inputs, Level 1 [Member]
Securities available for sale		0	0
Fair Value, Inputs, Level 1 [Member] | Reported Value Measurement [Member]
Cash		17,709,000	16,953,000
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value Measurement [Member]
Cash		17,709,000	16,953,000
Fair Value, Inputs, Level 2 [Member] | Reported Value Measurement [Member]
Cash equivalents		807,652,000	609,053,000
Mortgages Held-for-sale, Fair Value Disclosure		47,247,000	22,888,000
Mortgage servicing rights		11,146,000	8,189,000
Accrued interest receivable		9,986,000	10,861,000
Deposits		3,868,991,000	3,411,553,000
Repurchase agreements		175,850,000	118,365,000
FHLBI advances		394,000,000	394,000,000
Subordinated debentures		48,074,000	47,563,000
Accrued interest payable		1,406,000	2,313,000
Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value Measurement [Member]
Cash equivalents		807,652,000	609,053,000
Mortgages Held-for-sale, Fair Value Disclosure		49,107,000	24,029,000
Mortgage servicing rights		14,321,000	10,006,000
Accrued interest receivable		9,986,000	10,861,000
Deposits		3,949,449,000	3,397,768,000
Repurchase agreements		175,850,000	118,365,000
FHLBI advances		406,002,000	410,881,000
Subordinated debentures		47,980,000	47,574,000
Accrued interest payable		1,406,000	2,313,000
Fair Value, Inputs, Level 3 [Member] | Reported Value Measurement [Member]
Loans, net		3,276,286,000	3,155,503,000
Fair Value, Inputs, Level 3 [Member] | Estimate of Fair Value Measurement [Member]
Loans, net		$ 3,386,378,000	$ 3,294,522,000

[1] See Note 11 for a description of the fair value hierarchy as well as a disclosure of levels for classes of financial assets and liabilities.

[2] It is not practical to determine the fair value of FHLBI stock due to transferability restrictions; therefore, fair value is estimated at carrying amount.