Condensed Financial Information (Parent Company Only) (Tables)

12 Months Ended

Dec. 31, 2023

Condensed Financial Information Disclosure [Abstract]

Schedule of Condensed Financial Statements

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

December 31,	2023		2022
Condensed balance sheets:
Cash and cash equivalents	$	275.7	$	154.8
Investment in bank subsidiary	3,062.2		2,985.8
Advances to subsidiaries, net	43.5		0.1
Other assets	147.0		240.7
Total assets	$	3,528.4	$	3,381.4
Other liabilities	$	38.8	$	45.6
Long-term debt	99.0		98.9
Subordinated debentures held by subsidiary trusts	163.1		163.1
Total liabilities	300.9		307.6
Stockholders’ equity	3,227.5		3,073.8
Total liabilities and stockholders’ equity	$	3,528.4	$	3,381.4

Years Ended December 31,	2023		2022		2021
Condensed statements of income:
Dividends from subsidiaries	$	360.0	$	360.0	$	160.0
Other interest income	0.5		0.4		—
Other income, primarily management fees from subsidiaries	51.6		51.7		41.3
Total income	412.1		412.1		201.3
Salaries and benefits	34.5		40.6		36.4
Interest expense	18.4		12.6		8.2
Acquisition related expenses	—		62.3		11.6
Other operating expenses, net	22.5		25.0		17.6
Total expenses	75.4		140.5		73.8
Earnings before income tax benefit	336.7		271.6		127.5
Income tax benefit	(4.2)		(18.9)		(7.6)
Income before undistributed earnings of subsidiaries	340.9		290.5		135.1
Undistributed (loss) earnings of subsidiaries	(83.4)		(88.3)		57.0
Net income	$	257.5	$	202.2	$	192.1

Years Ended December 31,	2023		2022		2021
Condensed statements of cash flows:
Cash flows from operating activities:
Net income	$	257.5	$	202.2	$	192.1
Adjustments to reconcile net income to cash provided by operating activities:
Undistributed losses (earnings) of subsidiaries	83.4		88.3		(57.0)
Stock-based compensation expense	4.6		9.6		8.9
Other, net	2.7		(151.2)		(3.2)
Net cash provided by operating activities	348.2		148.9		140.8
Cash flows from investing activities:
Acquisition of bank holding company, net of cash and cash equivalents received	—		(0.8)		—
Net cash used in investing activities	$	—	$	(0.8)	$	—

Years Ended December 31,	2023		2022		2021
Cash flows from financing activities:
Net increase in advances from subsidiaries	$	1.8	$	206.5	$	23.7
Proceeds from issuance of common stock, net of stock issuance costs	—		0.1		0.4
Purchase and retirement of common stock	(34.0)		(198.9)		(5.4)
Dividends paid to common stockholders	(195.1)		(182.1)		(101.6)
Net cash used in financing activities	(227.3)		(174.4)		(82.9)
Net change in cash and cash equivalents	120.9		(26.3)		57.9
Cash and cash equivalents, beginning of year	154.8		181.1		123.2
Cash and cash equivalents, end of year	$	275.7	$	154.8	$	181.1