Condensed Financial Information (Parent Company Only) (Tables)

12 Months Ended

Dec. 31, 2025

Condensed Financial Information Disclosure [Abstract]

Schedule of Condensed Financial Statements

Following is condensed financial information of First Interstate BancSystem, Inc.

December 31,	2025		2024
Condensed balance sheets:
Cash and cash equivalents	$	308.3	$	273.4
Investment in bank subsidiary	3,312.1		3,160.5
Advances to subsidiaries, net	105.9		46.8
Other assets	131.4		137.9
Total assets	$	3,857.7	$	3,618.6
Other liabilities	$	138.6	$	52.4
Long-term debt	122.3		99.1
Subordinated debentures held by subsidiary trusts	149.8		163.1
Total liabilities	410.7		314.6
Stockholders’ equity	3,447.0		3,304.0
Total liabilities and stockholders’ equity	$	3,857.7	$	3,618.6

Years Ended December 31,	2025		2024		2023
Condensed statements of income:
Dividends from subsidiaries	$	369.4	$	205.0	$	360.0
Other interest income	0.4		0.4		0.5
Other income, primarily management fees from subsidiaries	42.1		54.3		51.6
Total income	411.9		259.7		412.1
Salaries and benefits	31.8		43.6		34.5
Interest expense	21.3		18.6		18.4
Other operating expenses, net	29.1		27.8		22.5
Total expenses	82.2		90.0		75.4
Earnings before income tax benefit	329.7		169.7		336.7
Income tax benefit	(9.1)		(7.2)		(4.2)
Income before undistributed earnings of subsidiaries	338.8		176.9		340.9
Undistributed (loss) earnings of subsidiaries	(36.7)		49.1		(83.4)
Net income	$	302.1	$	226.0	$	257.5

Years Ended December 31,	2025		2024		2023
Condensed statements of cash flows:
Cash flows from operating activities:
Net income	$	302.1	$	226.0	$	257.5
Adjustments to reconcile net income to cash provided by operating activities:
Undistributed losses (earnings) of subsidiaries	36.7		(49.1)		83.4
Stock-based compensation expense	13.3		11.9		4.6
Other, net	(5.8)		1.4		2.7
Net cash provided by operating activities	346.3		190.2		348.2
Cash flows from investing activities:
Net cash used in investing activities	—		—		—
Cash flows from financing activities:
Net (decrease) increase in advances from subsidiaries	(5.0)		4.6		1.8
Proceeds from issuance of long-term debt	122.4		—		—
Repayment of long-term debt	(112.6)		—		—
Purchase and retirement of common stock	(121.9)		(1.2)		(34.0)
Dividends paid to common stockholders	(194.3)		(195.9)		(195.1)
Net cash used in financing activities	(311.4)		(192.5)		(227.3)
Net change in cash and cash equivalents	34.9		(2.3)		120.9
Cash and cash equivalents, beginning of year	273.4		275.7		154.8
Cash and cash equivalents, end of year	$	308.3	$	273.4	$	275.7