First Interstate BancSystem (FIBK) 10-K 1 Mar 21 2020 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2020	Jan. 31, 2021	Jun. 30, 2020
Entity Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2020
Document Transition Report	false
Entity File Number	001-34653
Entity Registrant Name	FIRST INTERSTATE BANCSYSTEM, INC.
Entity Central Index Key	0000860413
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Incorporation, State or Country Code	MT
Entity Tax Identification Number	81-0331430
Entity Address, Address Line One	401 North 31st Street
Entity Address, City or Town	Billings,
Entity Address, State or Province	MT
Entity Address, Postal Zip Code	59116
City Area Code	406
Local Phone Number	255-5390
Title of 12(b) Security	Class A common stock, no par value
Trading Symbol	FIBK
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 1,379,587,569
Class A Common Stock
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		40,942,784
Class B Common Stock
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding		21,130,474

Cover

Cover	12 Months Ended
	Dec. 31, 2020
Cover [Abstract]
Documents Incorporated by Reference	The registrant intends to file a definitive Proxy Statement for the Annual Meeting of Shareholders scheduled to be held May 26, 2021. The information required by Part III of this Form 10-K is incorporated by reference to such Proxy Statement.

Earnings per Common Share - Tab

Earnings per Common Share - Tables	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Computation of Basic and Diluted Earnings per Share	The following table sets forth the computation of basic and diluted earnings per common share: Year Ended December 31, 2020 2019 2018 Net income, basic and diluted $ 161.2 $ 181.0 $ 160.2 Weighted average common shares outstanding for basic earnings per share computation 63,611,891 63,645,029 57,778,857 Dilutive effects of stock-based compensation 117,579 239,839 438,266 Weighted average common shares outstanding for diluted earnings per common share computation 63,729,470 63,884,868 58,217,123 Basic earnings per common share $ 2.53 $ 2.84 $ 2.77 Diluted earnings per common share 2.53 2.83 2.75

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets
Cash and due from banks	$ 261,400	$ 241,500
Federal funds sold	2,015,300	835,200
Interest bearing deposits in banks	100	100
Total cash and cash equivalents	2,276,800	1,076,800
Investment securities:
Available-for-sale	4,008,700	2,960,000
Held-to-maturity (estimated fair values of $94.5 and $400.7 at December 31, 2019 and 2018, respectively)	51,600	92,300
Total investment securities	4,060,300	3,052,300
Mortgage loans held for sale, at fair value	74,000	100,900
Loans and Leases Receivable, Allowance	9,807,500	8,930,700
Less allowance for loan losses	144,300	73,000
Net loans	9,663,200	8,857,700
Goodwill	621,600	621,600
Company-owned life insurance	296,400	293,800
Premises and equipment, net of accumulated depreciation	312,300	306,000
Core deposit intangibles, net of accumulated amortization	51,200	62,100
Accrued interest receivable	51,100	46,700
Mortgage servicing rights, net of accumulated amortization and impairment reserve	24,000	30,200
Other real estate owned (“OREO”)	2,500	8,500
Other assets	215,300	187,600
Total assets	17,648,700	14,644,200
Deposits:
Non-interest bearing	4,633,500	3,426,500
Interest bearing	9,583,500	8,237,000
Total deposits	14,217,000	11,663,500
Securities sold under repurchase agreements	1,091,400	697,600
Accounts payable and accrued expenses	144,400	129,600
Accrued interest payable	5,800	12,100
Deferred tax liability, net	27,200	26,700
Long-term debt	112,400	13,900
Allowance for credit losses on off-balance sheet credit exposures	3,700	0
Subordinated debentures held by subsidiary trusts	87,000	86,900
Total liabilities	$ 15,688,900	12,630,300
Preferred Stock, Shares Authorized	100,000
Stockholders’ equity:
Nonvoting noncumulative preferred stock without par value; authorized 100,000 shares; no shares issued or outstanding as of December 31, 2019 and 2018	$ 0	0
Common stock	941,100	1,049,300
Retained earnings	962,100	953,600
Accumulated other comprehensive income (loss), net	56,600	11,000
Total stockholders’ equity	1,959,800	2,013,900
Total liabilities and stockholders’ equity	$ 17,648,700	$ 14,644,200

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Assets:
Held-to-maturity investment securities, estimated fair value	$ 55	$ 94.5

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Interest income:
Interest and fees on loans	$ 453.4	$ 470.9	$ 404.3
Interest and dividends on investment securities:
Taxable	63.4	62.3	55.4
Exempt from federal taxes	2.7	2	2.4
Interest on deposits in banks	4.1	18.8	11.3
Total interest income	523.6	554	473.4
Interest expense:
Interest on deposits	18.1	49.3	32.6
Interest on securities sold under repurchase agreements	0.9	3.9	2.7
Interest on other borrowed funds	0	0	0.2
Interest on long-term debt	4.6	1.3	1.3
Interest on subordinated debentures held by subsidiary trusts	3	4.5	4.1
Total interest expense	26.6	59	40.9
Net interest income	497	495	432.5
Provision for credit losses	56.9	13.9	8.6
Net interest income after provision for credit losses	440.1	481.1	423.9
Non-interest income:
Fees and Commissions, Mortgage Banking and Servicing	47.3	33.2	29.7
Investment securities gains (losses), net	0.3	0.1	(0.1)
Other income	14.5	15.9	15.1
Total non-interest income	156.7	142.6	138.8
Non-interest expense:
Salaries and wages	173.7	155.3	146.4
Employee benefits	49.4	51.5	47.9
Information Technology and Data Processing	32.8	32.3	28.7
Occupancy, Net	28.5	28.3	25.4
Equipment Expense	15.5	13.2	12.7
OREO expense, net of income	(0.5)	(2.2)	0.3
Professional fees	10.9	11.6	10.5
FDIC insurance premiums	5.9	3.5	5.6
Core deposit intangibles amortization	10.9	11.2	7.9
Other expenses	60.4	63.6	58.6
Acquisition related expenses	0	20.3	12.4
Total non-interest expense	387.5	388.6	356.4
Income before income tax expense	209.3	235.1	206.3
Income tax expense	48.1	54.1	46.1
Net income	$ 161.2	$ 181	$ 160.2
Basic earnings per common share (in dollars per share)	$ 2.53	$ 2.84	$ 2.77
Diluted earnings per common share (in dollars per share)	$ 2.53	$ 2.83	$ 2.75
Credit and Debit Card [Member]
Non-interest income:
Revenue from Contract with Customer, Excluding Assessed Tax	$ 41.1	$ 41.5	$ 43.3
Financial Service, Other [Member]
Non-interest income:
Revenue from Contract with Customer, Excluding Assessed Tax	12.1	7	5.8
Investment Advisory, Management and Administrative Service [Member]
Non-interest income:
Investment Banking, Advisory, Brokerage, and Underwriting Fees and Commissions	23.8	23.8	23.2
Deposit Account [Member]
Non-interest income:
Revenue from Contract with Customer, Excluding Assessed Tax	$ 17.6	$ 21.1	$ 21.8

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 161.2	$ 181	$ 160.2
Investment securities available-for-sale:
Change in net unrealized gains (losses) during the period	61.8	54.9	(13.9)
Reclassification adjustment for net (gains) losses included in income	(0.3)	(0.1)	0.1
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale	0	(6)	0
Change in unamortized loss on available-for-sale investment securities transferred into held-to-maturity	0	0	1.6
Change in net unrealized loss on derivatives	0.2	0	0
Defined benefit post-retirement benefit plans:
Change in net actuarial loss	(0.5)	(0.8)	(0.6)
Other comprehensive income (loss), before tax	61.2	48	(12.8)
Deferred tax (expense) benefit related to other comprehensive income (loss)	(15.6)	(12.4)	3.3
Other comprehensive income (loss), net of tax	45.6	35.6	(9.5)
Comprehensive income, net of tax	$ 206.8	$ 216.6	$ 150.7

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Millions	Total	Common Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Restricted Stock [Member]
Common Stock, Value, Issued	$ 0	$ 0	$ 0	$ 0
Dividends, Common Stock, Cash	(2,013.9)	(1,049.3)	(953.6)	(11)
Net income	181	0	181	0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	$ 35.6	0	0	35.6
Stock Repurchased and Retired During Period, Shares	43,560
Common shares purchased and retired	$ (2.5)	(2.5)	0	0
Stock issued during period, new issues (in shares)	4,356,973
Stock issued during period, RSU (in shares)	212,587
Stock Issued During Period, Value, New Issues	$ 176.1	176.1	0	0
Non-vested common shares forfeited or canceled	0	0	0	0
Stock compensation expense	1	1	0	0
Stock compensation expense	8	8	0	0	$ 8
APIC, Share-based Payment Arrangement, Option, Increase for Cost Recognition	(79.2)	0	(79.2)	0
Accumulated Other Comprehensive Income (Loss), Net of Tax	11
Stockholders' Equity Attributable to Parent	2,013.9			11
Common Stock, Value, Issued	0	0	0	0
Dividends, Common Stock, Cash	(1,959.8)	(941.1)	(962.1)	(56.6)
Net income	161.2	0	161.2	0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	$ 45.6	0	0	45.6
Stock Repurchased and Retired During Period, Shares	3,578,743
Common shares purchased and retired	$ (116.8)	(116.8)	0	0
Stock issued during period, new issues (in shares)	19,491
Stock issued during period, RSU (in shares)	332,085
Stock Issued During Period, Value, New Issues	$ 0	0	0	0
Non-vested common shares forfeited or canceled	0	0	0	0
Stock compensation expense	1.1	1.1	0	0
Stock compensation expense	7.5	7.5	0	0	$ 7.5
APIC, Share-based Payment Arrangement, Option, Increase for Cost Recognition	(128.6)	0	(128.6)	0
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(24.1)	0	(24.1)	0
Accumulated Other Comprehensive Income (Loss), Net of Tax	56.6
Stockholders' Equity Attributable to Parent	1,959.8			56.6
Common Stock, Value, Issued	$ 0	$ 0	$ 0	$ 0

Consolidated Statements of St_2

Consolidated Statements of Stockholders' Equity (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement of Stockholders' Equity [Abstract]
Stock Repurchased and Retired During Period, Shares	3,578,743	43,560	24,271
Stock issued during period, new issues (in shares)	19,491	4,356,973	3,848,929
Stock issued during period, RSU (in shares)	332,085	212,587	214,892
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested Options Forfeited, Number of Shares	34,912	46,198	43,079
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	111,539	143,222	161,217
Shares tendered in payment of option price and income tax withholding amounts, shares	26,124	47,971	38,450
Dividends declared (in usd per share)	$ 2	$ 1.24	$ 1.12

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net income	$ 161,200	$ 181,000	$ 160,200
Adjustments to reconcile net income from operations to net cash provided by operating activities:
Provision for credit losses	56,900	13,900	8,600
Net loss (gain) on disposal of property and equipment	300	(1,500)	(1,200)
Depreciation and amortization	45,100	38,700	27,600
Net premium amortization on investment securities	15,900	8,900	10,000
Net (gain) loss on investment securities transactions	(300)	(100)	100
Realized and unrealized net gains on mortgage banking activities	(49,300)	(30,500)	(23,000)
Net gain on sale of investments in unrelated entities	(1,000)	0	0
Net gain on sale of OREO	(900)	(3,600)	(800)
Write-downs of OREO and other assets pending disposal	100	900	100
Mortgage servicing rights impairment (recovery)	9,900	400	0
Deferred taxes	(6,600)	5,400	15,800
Net increase in cash surrender value of company-owned life insurance policies	(7,600)	(6,700)	(5,000)
Stock-based compensation expense	7,500	8,000	5,600
Originations of mortgage loans held for sale	(1,404,200)	(1,015,600)	(768,100)
Proceeds from sales of mortgage loans held for sale	1,468,400	971,200	798,400
Changes in operating assets and liabilities:
(Increase) decrease in interest receivable	(4,400)	300	(3,300)
Increase in other assets	(27,700)	(22,100)	(8,000)
(Decrease) increase in interest payable	(6,300)	(13,500)	2,200
Increase (decrease) in accounts payable and accrued expenses	11,300	(7,800)	(200)
Net cash provided by operating activities	268,300	127,300	219,000
Purchases of investment securities:
Held-to-maturity	0	0	(2,000)
Available-for-sale	(2,444,100)	(1,270,000)	(541,000)
Proceeds from maturities, pay-downs, calls and sales of investment securities:
Held-to-maturity	40,400	35,600	79,300
Available-for-sale	1,441,400	978,600	460,000
Proceeds from bank-owned life insurance settlements	5,000	3,200	0
Extensions of credit to clients, net of repayments	(901,300)	(81,400)	(221,700)
Recoveries of loans charged-off	6,700	9,700	12,000
Proceeds from sales of OREO	10,100	25,400	9,100
Proceeds from the sale of health savings accounts	0	300	0
Proceeds from sale of investments in unrelated entities	2,200	0	0
Acquisition of banks and bank holding companies, net of cash and cash equivalents received	0	298,400	28,100
Capital expenditures, net of proceeds from sales	(30,200)	(16,600)	(4,900)
Net cash used in investing activities	(1,869,800)	(16,800)	(181,100)
Cash flows from financing activities:
Net increase in deposits	2,553,500	276,200	49,500
Net increase (decrease) in securities sold under repurchase agreements	393,800	(45,200)	69,400
Net decrease in other borrowed funds	0	(4,100)	(26,100)
Repayments of long-term debt	(100)	(2,000)	(7,100)
Advances on long-term debt	98,600	100	2,800
Proceeds from issuance of common stock	1,100	1,000	1,800
Purchase and retirement of common stock	(116,800)	(2,500)	(1,000)
Dividends paid to common stockholders	(128,600)	(79,200)	(64,100)
Net cash used in financing activities	2,801,500	144,300	25,200
Net increase (decrease) in cash and cash equivalents	1,200,000	254,800	63,100
Cash and cash equivalents at beginning of period	1,076,800	822,000	758,900
Cash and cash equivalents at end of period	2,276,800	1,076,800	822,000
Supplemental disclosures of cash flow information:
Cash paid during the period for income taxes	(54,400)	(51,200)	(25,300)
Cash paid during the period for interest expense	32,900	54,700	38,700
Supplemental disclosures of noncash investing and financing activities:
Amortization of unrealized gains and losses on transfers of securities	0	0	1,600
Transfer of securities from held-to-maturity to available-for-sale	0	281,100	0
Right-of-use assets obtained in exchange for operating lease liabilities	3,600	39,600	0
Transfer of loans to other real estate owned	3,300	14,100	12,100
Capitalization of internally originated mortgage servicing rights	11,700	7,300	6,000
Supplemental schedule of noncash investing activities from acquisitions:
Investment securities available for sale	0	78,700	3,100
Loans held for sale	0	500	0
Loans	0	416,600	713,100
Premises and equipment	0	24,600	14,000
Goodwill	0	75,300	101,100
Core deposit intangible	0	16,600	15,700
Company-owned life insurance	0	15,200	9,500
Interest receivable	0	2,200	3,600
Other real estate owned	0	2,400	600
Other assets	0	6,500	6,200
Total noncash assets acquired	0	638,600	866,900
Deposits	0	706,700	696,300
Securities sold under repurchase agreements	0	30,400	0
Accounts payable and accrued expenses	0	19,900	7,700
Long-term debt	0	4,100	7,000
Other borrowed funds	0	0	6,100
Trust preferred securities	0	0	4,400
Deferred tax liability	0	100	300
Total liabilities assumed	$ 0	$ (761,200)	$ (721,800)

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Business . First Interstate BancSystem, Inc. (the “Parent Company” and collectively with its subsidiaries, the “Company”) is a financial and bank holding company that, through the branch offices of its bank subsidiary, provides a comprehensive range of banking products and services to individuals, businesses, municipalities, and other entities throughout Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming. In addition to its primary emphasis on commercial and consumer banking services, the Company also offers trust, employee benefit, investment, and insurance services through its bank subsidiary. The Company is subject to competition from other financial institutions and nonbank financial companies, and is also subject to the regulations of various government agencies and undergoes periodic examinations by those regulatory authorities. Basis of Presentation . The Company’s consolidated financial statements include the accounts of the Parent Company and its operating subsidiaries. As of December 31, 2020, the Company had one significant subsidiary, First Interstate Bank (“FIB”). All significant intercompany balances and transactions have been eliminated in consolidation. Certain reclassifications, none of which were material, have been made in the consolidated financial statements for 2019 and 2018 to conform to the 2020 presentation. These reclassifications did not change previously reported net income or stockholders’ equity. Business Combinations. The Company accounts for all business combinations using the acquisition method of accounting. Under this method of accounting, acquired assets and assumed liabilities are included with the acquirer's accounts as of the date of acquisition, with any excess of purchase price over the fair value of the net assets acquired recognized as either finite lived intangibles or capitalized as goodwill. In addition, acquisition related costs and restructuring costs are recognized as period expenses as incurred. Fair values are subject to refinement over the measurement period, not to exceed one year after the closing date. Equity Method Investments. The Company has investments in real estate joint ventures that are not consolidated because the Company does not own a majority voting interest, control the operations or receive a majority of the losses or earnings of the joint venture. These joint ventures are accounted for using the equity method of accounting whereby the Company initially records its investment at cost (or fair value at the date of acquisition) and then subsequently adjusts the carrying value for the Company’s proportionate share of distributions and earnings or losses of the joint ventures. Variable Interest Entities. The Company’s wholly-owned business trusts, FI Statutory Trust I (“Trust I”), FI Capital Trust II (“Trust II”), FI Statutory Trust III (“Trust III”), FI Capital Trust IV (“Trust IV”), FI Statutory Trust V (“Trust V”), FI Statutory Trust VI (“Trust VI”), and Northwest Bancorporation Capital Trust I (“Trust VII”) are variable interest entities for which the Company is not a primary beneficiary. Accordingly, the accounts of Trust I, Trust II, Trust III, Trust IV, Trust V, Trust VI, and Trust VII are not included in the accompanying consolidated financial statements, and are instead accounted for using the equity method of accounting. The Company has equity investments in variable interest Certified Development Entities (“CDEs”) which have received allocations under the New Markets Tax Credits Program. The underlying activities of the CDEs are community development projects designed primarily to promote community welfare, such as economic rehabilitation and development of low-income areas by providing housing, services, or jobs for residents. The maximum exposure to loss in the CDEs is the amount of equity invested and credit extended by the Company. The Company has credit protection in the form of indemnification agreements, guarantees, and collateral arrangements. As the primary beneficiary of these variable interest entities, the Company’s consolidated financial statements include the assets, liabilities, and results of operations of the CDEs. The primary activities of the CDEs are recognized in interest and fees on loans, other non-interest income and long-term debt interest expense on the Company’s statements of operations. Related cash flows are recognized in loans originated, principal collected on loans and advances or repayments of long-term debt. Assets Held in Fiduciary or Agency Capacity. The Company holds certain trust assets in a fiduciary or agency capacity. The Company also purchases and sells federal funds as an agent. These and other assets held in an agency or fiduciary capacity are not assets of the Company and, accordingly, are not included in the accompanying consolidated financial statements. Use of Estimates. The preparation of consolidated financial statements in conformity with accounting principles generally accepted in the United States of America requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the financial statements and income and expenses during the reporting period. Actual results could differ from those estimates. Material estimates that are particularly susceptible to change relate to the determination of the allowance for credit losses, the valuation of goodwill, fair valuations of investment securities and other financial instruments, and the status of loss contingencies. Cash and Cash Equivalents . For purposes of reporting cash flows, cash and cash equivalents include cash on hand, amounts due from banks, federal funds sold for one-day periods, and interest-bearing deposits in banks with original maturities of less than three months. As of December 31, 2020 and 2019, the Company had cash of $1,989.3 million and $769.3 million, respectively, on deposit with the Federal Reserve Bank. In addition, the Company maintained compensating balances with the Federal Reserve Bank of approximately zero and $46.3 million as of December 31, 2020 and 2019, respectively, to reduce service charges for check clearing services. Debt Security Investments. Investments in debt securities that the Company has the positive intent and ability to hold to maturity are classified as held-to-maturity and carried at amortized cost. Investments in debt securities that may be sold in response to or in anticipation of changes in interest rates and resulting prepayment risk, or other factors, are classified as available-for-sale and carried at fair value. The unrealized gains and losses on these securities are reported, net of applicable income taxes, as a separate component of stockholders’ equity and comprehensive income. Management determines the appropriate classification of securities at the time of purchase and at each reporting date management reassesses the appropriateness of the classification. The amortized cost of debt securities classified as held-to-maturity or available-for-sale is adjusted for accretion of discounts to maturity and amortization of premiums over the estimated average life of the security, without anticipating prepayments, except for mortgage-backed securities where prepayments are anticipated, or in the case of callable securities, through the first call date, using the effective yield method. Such amortization and accretion is included in interest income. Realized gains and losses on sales are recorded on the trade date in investment securities gains and losses and determined using the specific identification method. Accrued interest receivable on investment securities totaled $12.6 million and $9.8 million at December 31, 2020 and 2019, respectively, and was reported in the accrued interest receivable line item on the consolidated balance sheets. Allowance for Credit Losses - Held-to-Maturity Securities: Management measures expected credit losses on held-to-maturity debt securities on a collective basis by major security type. Accrued interest receivable on held-to-maturity debt securities is excluded from the estimate of credit losses. The estimate of expected credit losses considers historical credit loss information that is adjusted for current conditions and reasonable and supportable forecasts. Management classifies the held-to-maturity portfolio into the following major security types: State, county, and municipal securities. Municipal bonds issued by municipal governments within the U.S. These types of securities are primarily composed of general obligation bonds, or municipal bonds backed by the credit and taxing power of the issuing jurisdiction and revenue obligation bonds, or municipal bonds that are financed by income-producing projects and are secured by a specified source of revenue. Municipal issues shall have at least an "A-" rating by Moody's and/or Standard and Poor’s, or equivalent creditworthiness must be established prior to purchase. All non-rated or private placement securities must be analyzed and approved by the Company’s Credit Department and documented prior to purchase. Obligations of U.S. government agencies and entities. Securities held by the Company are primarily issued by The Federal Home Loan Mortgage Corporation, known as Freddie Mac, and The Federal National Mortgage Association, Fannie Mae, which are implicitly guaranteed by the U.S. government and are consistently highly rated by major rating agencies with very little risk to default. U.S. agency residential mortgage backed securities and Collateralized Mortgage Obligations. Residential mortgage backed securities held by the Company are primarily issued by U.S. government agencies and entities. These securities are either explicitly or implicitly guaranteed by the U.S. government, are consistently highly rated by major rating agencies with very little risk to default. Collateralized mortgage obligations include agency and non-agency residential securities which currently carry ratings no lower than investment grade “BBB-” and pass the federal financial institutions examinations test (Collateral Mortgage Obligation volatility test) at the time of purchase. Corporate securities. Securities held by the Company are primarily comprised of corporate bonds (both senior and subordinated-debt) issued by a firm or public entity which currently carry ratings no lower than investment grade “BBB-” or better by Moody’, Standard and Poor’s, or Kroll rating agencies. All corporate subordinated-debt securities are analyzed and approved by the Company prior to purchase. Allowance for Credit Losses - Available-For-Sale Securities: For available-for-sale debt securities in an unrealized loss position, the Company first assesses whether it intends to sell, or it is more likely than not that it will be required to sell the security before recovery of its amortized cost basis. If either of the criteria regarding intent or requirement to sell is met, the security's amortized cost basis is written down to fair value through income. For available-for-sale debt securities that do not meet the aforementioned criteria, the Company performs a qualitative assessment as to whether the decline in fair value has resulted from credit losses or other factors. In making this assessment, management considers the extent to which fair value is less than amortized cost, any changes to the rating of the security by a rating agency, and adverse conditions specifically related to the security, among other factors. If this assessment indicates that a credit loss exists, the present value of cash flows expected to be collected from the security are compared to the amortized cost basis of the security. If the present value of cash flows expected to be collected is less than the amortized cost basis, a credit loss exists and an allowance for credit losses is recorded for the credit loss, limited by the amount that the fair value is less than the amortized cost basis. Any impairment that has not been recorded through an allowance for credit losses is recognized in other comprehensive income. Changes in the allowance for credit losses are recorded as provision for (or reversal of) credit loss expense. Losses are charged against the allowance when management believes the uncollectibility of an available-for-sale security is confirmed or when either of the criteria regarding intent or requirement to sell is met. Accrued interest receivable on available-for-sale debt securities is excluded from the estimate of credit losses. Loans Held for Sale. Residential loans the Company originated with the intent to sell are classified as loans held for sale and recorded at fair value, determined individually, as of the balance sheet date. The loan’s fair value includes the servicing value of the loans as well as any accrued interest. Loans Held for Investment. Loans that management has the intent and ability to hold for the foreseeable future or until maturity or payoff are reported at amortized cost or principal balance outstanding. Amortized cost is the principal balance outstanding, net of purchase premiums and discounts and deferred loan fees and costs. Loan origination fees, net of certain direct origination costs, are deferred and recognized in interest income using the level-yield method without anticipating prepayments. Accrued interest receivable on loans held for investment totaled $37.9 million and $36.9 million at December 31, 2020 and 2019, respectively, and was reported in the accrued interest receivable line item on the consolidated balance sheets. Interest income is accrued on the unpaid principal balance of underlying loans. Interest income on mortgage and commercial loans is discontinued and placed on nonaccrual status at the time the loan is 90 days delinquent unless the loan is well secured and in process of collection. Mortgage loans that are 180 days past due and commercial loans are charged off to the extent principal or interest is deemed uncollectible. Consumer and credit card loans continue to accrue interest until they are charged off no later than 120 days past due unless the loan is in the process of collection. Past-due status is based on the contractual terms of the loan. In all cases, loans are placed on nonaccrual or charged off at an earlier date if collection of principal or interest is considered doubtful. All interest accrued but not received for loans placed on nonaccrual is reversed against interest income. Interest received on such loans is accounted for on the cash-basis or cost-recovery method, until qualifying for return to accrual. Under the cost-recovery method, interest income is not recognized until the loan balance is reduced to zero. Under the cash-basis method, interest income is recorded when the payment is received in cash. Loans are returned to accrual status when all the principal and interest amounts contractually due are brought current and when, in the opinion of management, the loans are estimated to be fully collectible as to both principal and interest. Purchased Credit Deteriorated (“PCD”) Loans The Company has purchased loans, some of which have experienced more than insignificant credit deterioration since origination. Loans that meet at least one of the following criteria are considered to have experienced more-than-insignificant credit deterioration since origination at the date of acquisition: 1) delinquent as of the acquisition date; 2) has been downgraded since origination; 3) has been placed on nonaccrual status at any point since origination; or 4) for which credit spreads have widened beyond market-level thresholds. PCD loans are recorded at the amount paid for the loan. An allowance for credit losses is determined using the same methodology as other loans held for investment. The initial allowance for credit losses determined on a collective basis is allocated to individual loans. The sum of the loan’s purchase price and allowance for credit losses becomes its initial amortized cost basis. The difference between the initial amortized cost basis and the par value of the loan is a noncredit discount or premium, which is amortized into interest income over the life of the loan. Subsequent changes to the allowance for credit losses are recorded through provision expense. Allowance for Credit Losses - Loans held for investment The allowance for credit losses is a valuation account that is deducted from the loans’ amortized cost basis to present the net amount expected to be collected on the loans. Loans are charged off against the allowance when management believes the uncollectibility of a loan balance is confirmed. When forecasting expected recoveries, the amounts should not exceed the aggregate of amounts that have previously been or are expected to be charged-off loans. The Company has elected to not forecast recoveries. Management estimates the allowance balance using relevant available information, from internal and external sources, relating to past events, current conditions, and reasonable and supportable forecasts. Historical credit loss experience provides the basis for the estimation of expected credit losses. Adjustments to historical loss information are made for differences in current loan-specific risk characteristics such as differences in underwriting standards, portfolio mix, delinquency level, or term as well as for changes in environmental and economic conditions, such as changes in unemployment rates, property values, or other relevant factors. The allowance for credit losses is measured on a collective (pool) basis when similar risk characteristics exist. The Company applies Probability of Default (PD), and Loss Given Default (LGD), methodologies for all portfolio segments. The Company uses a Transition Matrix (TM) for PD components of the methodology and a historical average for the LGD components of methodology. The PD and LGD is applied to the current principal balance as of the reporting date. The TM determines the PD by tracking the historical movement of loans between loan risk tiers over a defined period of time. The Company currently has 16 portfolio segments for which we track monthly movement between either risk ratings, or delinquency date count, or delinquency band. While the TM functions similarly across all portfolio segments, generally speaking, commercial portfolios use the Company’s risk rating scale and consumer portfolios use the delinquency band. Loans using risk ratings are scored utilizing the Company’s risk rating scale. The risk rating scale is 1-10, with 1 being the best rating, 6 being a pass but on watch, and 7-10 being various stages of criticized loans. Risk ratings 8 or greater and in a non-accrual status are considered in a defaulted state. Loans using delinquency band are measured using a 5-grade band, with 1 being current and 5 being 90 or more days past due. The LGD used as the basis for the estimate of credit losses is comprised of the Company’s historical loss experience from 2008 to the current period, based on a migration analysis of our historical loss experience, designed to account for credit deterioration. The model compares the most recent period losses to prior period defaults to calculate the LGD, which is averaged over the historical observations. Economic scenarios and forecasts along with current portfolio conditions and trends are monitored and accounted for through the Company’s qualitative framework. The Company utilizes a one-year forecast period with immediate reversion to historical loss rates. The Company segments the loan portfolio into pools based on the following risk characteristics: financial asset type, collateral type, loan characteristics, credit characteristics, outstanding loan balances, contractual terms and prepayment assumptions, vintage, industry of borrower and concentrations, and historical or expected credit loss. The Company has identified the following portfolio segments and measures the allowance for credit losses using the following methods: Portfolio segments using the Company’s risk ratings include the following: Commercial real estate non-owner-occupied loans. These loans include a mix of variable and fixed rate non-farm, non-residential real estate loans secured by non-owner-occupied properties. Commercial real estate non-owner-occupied loans are generally secured by first liens on income-producing real estate and generally mature in less than 10 years. Commercial real estate owner-occupied loans. Non-farm, non-residential real estate loans are generally secured by first liens on real estate where the owner occupant is the majority tenant of the property and generally mature in less than 10 years. Construction land acquisition and development loans. Construction land acquisition and development loans are primarily to commercial builders for residential lot development and the construction of single-family residences and commercial real estate properties. Construction loans are generally underwritten pursuant to pre-approved permanent financing. During the construction phase the borrower pays interest only. Construction land acquisition and development loans generally mature in three years or less. Residential construction loans. Residential construction loans are primarily to commercial builders or owner occupants for the construction of single-family residences. Construction loans are generally underwritten pursuant to credit worthiness or pre-qualification for permanent financing. During the construction phase the borrower pays interest only. Residential construction loans generally mature in one to two years. Commercial construction loans. Commercial construction loans are primarily to commercial builders for commercial real estate properties. Construction loans are generally underwritten pursuant to credit worthiness or pre-qualification for permanent financing. During the construction phase the borrower pays interest only. Commercial construction loans generally mature in two years or less. Agricultural real estate loans. These include loans secured by farmland or ranchland consisting of short, intermediate, and long-term structures to experienced agriculturalists who have demonstrated management capabilities, established production and historical financial performance. Agricultural real estate loans generally mature in ten years or less. Commercial and floor plan loans. The Company provides a mix of variable and fixed rate commercial loans in addition to loans to finance dealership floor inventories. The loans are typically made to small and medium-sized manufacturing, wholesale, retail, and service businesses for working capital needs and business expansions. Commercial loans generally include lines of credit, business credit cards, and loans with maturities of five years or less and outstanding balances tend to be cyclical in nature. The loans are generally made with business operations as the primary source of repayment, and are typically collateralized by inventory, accounts receivable, equipment, and/or personal guarantees. Commercial and floor plan loans generally mature in seven years or less. Commercial purpose secured by 1-4 family loans. These include loans for commercial purposes secured by 1-4 family residential property. Commercial purpose loans secured by 1-4 family generally mature in seven years or less. Agricultural loans. Agricultural loans generally consist of short and medium-term loans and lines of credit that are primarily used for crops, livestock, equipment, and general operations. Agricultural loans are ordinarily secured by assets such as livestock or equipment and are repaid from the operations of the farm or ranch. Agricultural loans generally have maturities of seven years or less, with operating lines for one production season. Portfolio segments utilizing the delinquency bands include the following: Consumer indirect loans. These include loan contracts advanced for the purchase of automobiles, boats, and other consumer goods from the consumer product dealer networks within the market areas we serve. Indirect dealer loans are generally secured by automobiles, recreational vehicles, boats, and other types of personal property and are made on an installment basis. Consumer indirect line loans generally mature in seven years or less. Consumer direct and advance line loans. These loans are originated for a variety of purposes including the purchase of automobiles, boats and other consumer goods, home improvements, medical expenses, vehicle repairs, debt consolidation, and planned expenses. Consumer direct and advance line loans generally mature in seven years or less. Consumer credit card loans. These are lines of credit offered to clients in our market areas that are generally floating rate loans and include both unsecured and secured lines. Consumer credit card loans generally do not have stated maturities but are reviewed periodically and are unconditionally cancellable. Consumer home equity and home equity lines of credit (“HELOC”). These include home equity loans and lines of credit that are secured by residential property. Consumer home equity loans generally mature in 15 years or less and HELOC loans generally mature in 25 years or less. Residential 1-4 family and multi-family lending. These are loans to finance the purchase or refinance of residential property which are typically secured by first liens, inclusive of 1-4 family as well as 5+ residential properties. Residential 1-4 family loans generally mature within 15 years but can be up to 30 years. Multi-family loans generally mature in 10 years or less. Commercial real estate multi-family loans. Commercial real estate multi-family loans are generally secured by first liens on income-producing rental real estate consisting of 5 or more residential dwelling units and generally mature in less than 10 years. For CECL related segmentation, multi-family loans are modeled with residential 1-4 family but are reported under Commercial Real Estate. Commercial credit card loans. These are lines of credit for commercial purposes that are generally floating rate loans and include both unsecured and secured lines. For CECL related segmentation, commercial credit card loans are modeled separately but are reported under Commercial. Commercial credit card loans generally do not have stated maturities but are reviewed periodically and are unconditionally cancellable. Agricultural credit card loans. Lines of credit for agricultural purposes that are generally floating rate loans and are unsecured or secured. For CECL related segmentation, agricultural credit card loans are modeled separately but are reported under Commercial. Agricultural credit card loans generally do not have stated maturities but are reviewed periodically and are unconditionally cancellable. Contractual Term . Expected credit losses are estimated over the contractual term of the loans, adjusted for expected prepayments when appropriate. The contractual term excludes expected extensions, renewals, and modifications unless either management has a reasonable expectation at the reporting date that a troubled debt restructuring will be executed with an individual borrower or the extension or renewal options are included in the original or modified contract at the reporting date and are not unconditionally cancellable by the Company. A loan for which the terms have been modified resulting in a concession, and for which the borrower is experiencing financial difficulties, is considered to be a troubled debt restructuring. The allowance for credit loss on a troubled debt restructuring is measured using the same method as all other loans held for investment, except when the value of a concession cannot be measured using a method other than the discounted cash flow method. When the value of a concession is measured using the discounted cash flow method, the allowance for credit loss is determined by discounting the expected future cash flows at the original interest rate of the loan. Allowance for Credit Losses on Off-Balance Sheet Credit Exposures. The Company estimates expected credit losses over the contractual period in which the Company is exposed to credit risk via a contractual obligation to extend credit unless that obligation is unconditionally cancellable by the Company. Management considers our unused credit card lines and federal fund lines, extended to others, to be considered unconditionally cancellable. Credit card receivables are run through the transition matrices and their unused lines are excluded from the final loss calculation because they are unconditionally cancellable. The allowance for credit losses on off-balance sheet credit exposures is adjusted as a provision for credit loss expense. The estimate considers the likelihood that funding will occur and an estimate of expected credit losses on commitments expected to be funded over the estimated life. The Company has identified commitments to extend credit and standby letters of credit determined not to be unconditionally cancellable as categories with off-balance sheet credit exposures and uses the commitment balance, expected loss rate, and utilization rate as primary assumptions to develop the allowance for credit losses on those exposures. The loss rate expectation is the same for both the unfunded and funded portions of the credit exposure. The utilization rate represents management’s best estimate of the probability that the unfunded portion of the commitment will be funded given existing economic conditions. Goodwill . The excess purchase price over the fair value of net assets from acquisitions, or goodwill, is evaluated for impairment at least annually and on an interim basis if an event or circumstance indicates that it is likely impairment has occurred. Goodwill impairment is determined by comparing the fair value of a reporting unit to its carrying amount. In any given year the Company may elect to perform a qualitative assessment to determine whether it is more likely than not that the fair value of a reporting unit is in excess of its carrying value. If it is not more likely than not that the fair value of the reporting unit is in excess of the carrying value, or if the Company elects to bypass the qualitative assessment, a quantitative impairment test is performed. In performing a quantitative test for impairment, the fair value of net assets is estimated based on analyses of the Company’s market value, discounted cash flows and peer values. The determination of goodwill impairment is sensitive to market-based economics and other key assumptions used in determining or allocating fair value. Variability in the market and changes in assumptions or subjective measurements used to allocate fair value are reasonably possible and may have a material impact on our consolidated financial statements or results of operations. Core Deposit Intangibles. Core deposit intangibles represent the intangible value of depositor relationships resulting from deposit liabilities assumed, as a result of acquisitions, and are amortized using an accelerated method based on the estimated weighted average useful lives of the related deposits, which is generally ten years. Mortgage Servicing Rights . The Company recognizes the rights to service mortgage loans for others, whether acquired or internally originated. Mortgage servicing rights are initially recorded at fair value based on comparable market data and are amortized in proportion to and over the period of estimated net servicing income. Mortgage servicing rights are evaluated quarterly for impairment by discounting the expected future cash flows, taking into consideration the estimated level of prepayments based on current industry expectations and the predominant risk characteristics of the underlying loans including loan type, note rate and loan term. Impairment adjustments, if any, are recorded through a valuation allowance. Premises and Equipment . Buildings, furniture, and equipment are stated at cost less accumulated depreciation. Depreciation expense is computed using straight-line methods over estimated useful lives of 5 to 45 years for buildings and improvements and 3 to 15 years for furniture and equipment. Leasehold improvements and assets acquired under a financing lease are amortized over the shorter of their estimated useful lives or the terms of the related leases. Land is recorded at cost. Costs incurred for maintenance and repairs are expensed as incurred. We have leased branches and of

Acquisitions

Acquisitions	12 Months Ended
	Dec. 31, 2020
Business Combinations [Abstract]
Acquisitions	ACQUISITIONS Community 1 st Bank. On October 11, 2018, the Company entered into a definitive agreement to acquire all the outstanding stock of CMYF, a community bank headquartered in Post Falls, Idaho with three banking offices in North Idaho. The acquisition was completed on April 8, 2019, and conversion of the data processing systems occurred on June 7, 2019. Consideration for the acquisition was $18.8 million, consisting of the issuance of 463,134 shares of the Company’s Class A common stock valued at $40.64 per share, the closing price of the Company’s Class A common stock as quoted on the NASDAQ stock market on the acquisition date. Holders of each share of CMYF common stock received 0.3784 shares of First Interstate Class A common stock for each share of CMYF common stock. Previously unvested CMYF restricted stock awards outstanding immediately prior to the close of the transaction vested and were considered issued and outstanding at acquisition close and included in consideration. All CMYF stock options outstanding vested and were settled by CMYF prior to the close of the transaction. The assets and liabilities of CMYF were recorded in the Company’s consolidated financial statements at their estimated fair values as of the acquisition date. The excess value of the consideration paid over the fair value of assets acquired and liabilities assumed was recorded as goodwill. The purchase price allocation resulted in goodwill of $2.3 million, which is not deductible for income tax purposes. Goodwill resulting from the acquisition was allocated to the Company’s one operating segment, community banking, and consists largely of the synergies and economies of scale expected from combining the operations of CMYF and the Company. The Company recorded net assets acquired of approximately $16.5 million consisting of approximately $129.1 million in assets, inclusive of $78.8 million of loans, of which $0.7 million were classified as credit impaired, and assumed approximately $112.6 million of liabilities, inclusive of $110.1 million of deposits. All amounts reported were finalized during the fourth quarter of 2019. Core deposit intangible assets of $3.0 million are being amortized using an accelerated method over the estimated useful lives of the related deposits of 10 years. Unaudited pro forma consolidated revenues and net income as if the CMYF acquisition had occurred as of January 1, 2019, are not presented because the effect of this acquisition was not considered significant. The accompanying consolidated statements of income include the results of operations of the acquired entity from the April 8, 2019 acquisition date. Although legally merged with FIB, the acquired entity continued to do business as CMYF until June 7, 2019, at which point CMYF’s operations were integrated with the Company’s operations. Idaho Independent Bank. On October 11, 2018, the Company also entered into a definitive agreement to acquire all the outstanding stock of IIBK, a community bank headquartered in Coeur d’Alene, Idaho with 11 banking offices across Idaho. The acquisition was completed on April 8, 2019, and the Company converted data processing systems occurred on June 7, 2019. Consideration for the acquisition was $157.3 million, consisting of the issuance of 3,871,422 shares of the Company’s Class A common stock valued at $40.64 per share, the closing price of the Company’s Class A common stock as quoted on the NASDAQ stock market on the acquisition date. Holders of each share of IIBK common stock received 0.50 shares of First Interstate Class A common stock for each share of IIBK common stock. Previously unvested IIBK restricted stock awards outstanding immediately prior to the close of the transaction vested and were considered issued and outstanding at acquisition close and were included in consideration. All IIBK stock options outstanding vested and were settled by IIBK prior to the close of the transaction. The assets and liabilities of IIBK were recorded in the Company’s consolidated financial statements at their estimated fair values as of the acquisition date. The excess value of the consideration paid over the fair value of assets acquired and liabilities assumed is recorded as goodwill. The purchase price allocation resulted in goodwill of $73.0 million, which is not deductible for income tax purposes. Goodwill resulting from the acquisition was allocated to the Company’s one operating segment, community banking, and consists largely of the synergies and economies of scale expected from combining the operations of IIBK and the Company. The following table summarizes the consideration paid, fair values of the IIBK assets acquired and liabilities assumed, and the resulting goodwill. All amounts reported were finalized during the fourth quarter of 2019. As Recorded Fair Value As Recorded As of April 8, 2019 by IIBK Adjustments by the Company Assets acquired: Cash and cash equivalents $ 270.7 $ — $ 270.7 Investment securities 62.7 0.5 (1) 63.2 Loans held for investment 347.6 (9.8) (2) 337.8 Mortgage loans held for sale 0.5 — 0.5 Allowance for loan loss (6.3) 6.3 (3) — Premises and equipment 16.5 4.8 (4) 21.3 Other real estate owned (“OREO”) 0.4 2.0 (5) 2.4 Company owned life insurance 15.2 — 15.2 Core deposit intangible assets — 13.6 (6) 13.6 Deferred tax assets, net 3.2 (2.6) (7) 0.6 Other assets 8.6 (0.7) (8) 7.9 Total assets acquired 719.1 14.1 733.2 Liabilities assumed: Deposits 596.5 0.1 (9) 596.6 Accounts payable and accrued expense 15.2 2.6 (10) 17.8 Other borrowed funds 4.0 0.1 (11) 4.1 Securities sold under repurchase agreements 30.4 — 30.4 Total liabilities assumed 646.1 2.8 648.9 Net assets acquired $ 73.0 $ 11.3 $ 84.3 Consideration paid: Class A common stock $ 157.3 Total consideration paid $ 157.3 Goodwill $ 73.0 Explanation of fair value adjustments and the removal of previously recorded fair value marks recorded by IIBK: (1) Write up of the book value of investments to their estimated fair values on the date of acquisition based upon quotes obtained from an independent third-party pricing service. (2) Write down of the book value of loans to their estimated fair values. The fair value of the loans was estimated using cash flow projections based on the remaining maturity and repricing terms, adjusted for estimated future credit losses and prepayments and discounted to present value using a risk-adjusted market rate for similar loans. The fair value of collateral dependent loans acquired with deteriorated credit quality was estimated based on the Company’s analysis of the fair value of each loan’s underlying collateral, discounted using market-derived rates of return with consideration given to the period of time and costs associated with foreclosure and disposition of the collateral. (3) Adjustment to remove the IIBK allowance for loan losses at acquisition date, as the credit risk is included in the fair value adjustment for loans receivable described in (2) above. (4) Write up of the book value of premises and equipment to their estimated fair values on the date of acquisition based upon broker’s opinion of value. (5) Adjustment to the book value of other real estate owned to their estimated fair values on the date of acquisition based on appraisal value. (6) Adjustment represents the value of the core deposit base assumed in the acquisition based upon valuation from an independent accounting and advisory firm. (7) Adjustment consists of the write-off of pre-existing deferred tax assets and purchase accounting adjustments as a result of the acquisition. (8) Adjustment consists of reductions to the fair value of other items. (9) Increase in book value of time deposits to their estimated fair values based upon interest rates of similar time deposits with similar terms on the date of acquisition based upon valuation from an independent accounting and advisory firm. (10) Adjustment to the liability for the nonqualified retirement plan. (11) Adjustment of the book value of debt to the estimated fair values on the date of acquisition based upon interest rates in the market. Core deposit intangible assets of $13.6 million are being amortized using an accelerated method over the estimated useful lives of the related deposits of 10 years. Effective January 1, 2020, the Company began accounting for PCD loans pursuant to ASC Topic 326. As such, the following disclosures are no longer applicable for the current period and are only presented for periods prior to the adoption of ASC Topic 326. Prior to the adoption of ASC 326, the Company acquired certain loans that were subject to Accounting Standards Codification (“ASC”) Topic 310-30 “Loans and Debt Securities Acquired with Deteriorated Credit Quality.” ASC Topic 310-30 provides recognition, measurement, and disclosure guidance for purchased loans acquired in business combinations, for which there was, at acquisition, evidence of more than insignificant deterioration of credit quality since origination. The Company has purchased such loans through business combinations. For loans that meet the criteria stipulated in ASC Topic 310-30, the excess of all cash flows expected at acquisition over the initial fair value of the loans acquired (“accretable yield” is amortized to interest income over the expected remaining lives of the underlying loans using the effective interest method. The accretable yield will fluctuate due to changes in (i) estimated lives of underlying credit-impaired loans, (ii) assumptions regarding future principal and interest amounts collected, and (iii) indices used to fair value variable rate loans. Information regarding IIBK loans acquired deemed credit impaired as of the April 8, 2019 acquisition date are as follows: Contractually required principal and interest payments $ 24.1 Contractual cash flows not expected to be collected (“non-accretable discount”) 3.9 Cash flows expected to be collected 20.2 Interest component of cash flows expected to be collected (“accretable discount”) 3.4 Fair value of acquired credit-impaired loans $ 16.8 Information regarding IIBK acquired loans not deemed credit-impaired at the April 8, 2019 acquisition date are as follows: Contractually required principal and interest payments $ 398.7 Contractual cash flows not expected to be collected 15.2 Fair value at acquisition $ 321.5 Unaudited pro forma consolidated revenues and net income as if the IIBK acquisition had occurred as of January 1, 2019, are not presented because the effect of this acquisition was not considered significant. The accompanying consolidated statements of income include the results of operations of the acquired entity from the April 8, 2019 acquisition date. Although IIBK legally merged with FIB, the acquired entity continued to do business as IIBK until June 7, 2019, at which point IIBK’s operations were integrated with the Company’s operations. Acquisition related expenses There were no material third party acquisition related costs recorded by the Company in 2020 compared to $20.3 million and $12.4 million of third party acquisition related costs recorded in 2019 and 2018, respectively. These costs are incorporated in non-interest expense in the Company’s consolidated statements of income.

Goodwill and Core Deposit Intan

Goodwill and Core Deposit Intangibles	12 Months Ended
	Dec. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Core Deposit Intangibles	GOODWILL AND CORE DEPOSIT INTANGIBLES Goodwill Year Ended December 31, 2020 2019 Net carrying value at beginning of period $ 621.6 $ 546.7 Acquisitions and measurement period adjustments — 74.9 Net carrying value at end of period $ 621.6 $ 621.6 The Company performed its annual goodwill impairment qualitative assessment as of July 1, 2020, 2019, and 2018 and determined the Company’s goodwill was not considered impaired. In addition, there were no events or circumstances that occurred during the second half of 2020 that would more-likely-than-not reduce the fair value of a reporting unit below its carrying value, the Company did not perform interim testing as of December 31, 2020. Core deposit intangibles (“CDI”) The following table sets forth activity for identifiable core deposit intangibles subject to amortization: Year Ended December 31, 2020 2019 Gross CDI, beginning of period $ 106.0 $ 89.7 Established through acquisitions — 16.6 Reductions due to sale of accounts — (0.3) Accumulated amortization (54.8) (43.9) Net CDI, end of period $ 51.2 $ 62.1 Amortization expense of CDI assets was $10.9 million, $11.2 million and $7.9 million for the fiscal years ended December 31, 2020, 2019 and 2018, respectively. CDI are evaluated for impairment if events and circumstances indicate a possible impairment. The CDI are amortized using an accelerated method based on the estimated weighted average useful lives of the related deposits, which is generally ten years. The following table provides estimated future CDI amortization expense: Years ending December 31, 2021 $ 9.9 2022 9.0 2023 8.2 2024 7.3 2025 6.5 Thereafter 10.3 Total $ 51.2

Investment Securities

Investment Securities	12 Months Ended
	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Investment Securities	INVESTMENT SECURITIES The amortized cost and approximate fair values of investment securities are summarized as follows: December 31, 2020 Amortized Gross Gross Estimated Available-for-Sale State, county, and municipal securities $ 462.1 $ 4.8 $ (1.0) $ 465.9 Obligations of U.S. government agencies 332.9 1.0 (2.0) 331.9 U.S. agency residential mortgage-backed securities & 2,830.8 69.3 (2.5) 2,897.6 Private mortgage-backed securities 10.9 0.1 (0.1) 10.9 Corporate Securities 295.8 6.5 (0.1) 302.2 Other investments 0.2 — — 0.2 Total $ 3,932.7 $ 81.7 $ (5.7) $ 4,008.7 December 31, 2020 Amortized Gross Gross Estimated Held-to Maturity State, county, and municipal securities $ 46.6 $ 3.2 $ — $ 49.8 U.S agency residential mortgage-backed securities & 1.0 0.1 — 1.1 Corporate securities 3.9 0.1 — 4.0 Other investments 0.1 — — 0.1 Total $ 51.6 $ 3.4 $ — $ 55.0 December 31, 2019 Amortized Gross Gross Estimated Available-for-Sale U.S. Treasury notes $ 9.0 $ — $ — $ 9.0 State, county, and municipal securities 80.1 0.8 — 80.9 Obligations of U.S. government agencies 367.5 0.1 (0.8) 366.8 U.S. agency residential mortgage-backed securities & 2,303.6 19.6 (6.0) 2,317.2 Private mortgage-backed securities 47.6 — (0.4) 47.2 Corporate Securities 134.5 1.2 — 135.7 Other investments 3.2 — — 3.2 Total $ 2,945.5 $ 21.7 $ (7.2) $ 2,960.0 December 31, 2019 Amortized Gross Gross Estimated Held-to Maturity State, county, and municipal securities $ 57.3 $ 2.1 $ — $ 59.4 Obligations of U.S. government agencies 19.8 — — 19.8 U.S. agency residential mortgage-backed securities & 1.2 — — 1.2 Corporate securities 13.9 0.1 — 14.0 Other investments 0.1 — — 0.1 Total $ 92.3 $ 2.2 $ — $ 94.5 There were no material gross realized gains and no material gross realized losses on the disposition of available-for-sale securities during 2020, 2019, or 2018. As of December 31, 2020, the Company had general obligation securities with amortized costs of $38.6 million included in state, county, and municipal securities, of which $26.0 million, or 67.4% were issued by political subdivisions or agencies within the states of Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming. The following tables show the gross unrealized losses and fair values of investment securities, aggregated by investment category, and the length of time individual investment securities have been in a continuous unrealized loss position, as of December 31, 2020 and 2019. There were no held-to-maturity securities in a continuous unrealized loss position as of December 31, 2020. Less than 12 Months 12 Months or More Total December 31, 2020 Fair Gross Fair Gross Fair Gross Available-for-Sale State, county, and municipal securities $ 148.1 $ (1.0) $ — $ — $ 148.1 $ (1.0) Obligations of U.S. government agencies 235.6 (2.0) — — 235.6 (2.0) U.S. agency residential mortgage-backed 434.0 (2.4) 12.3 (0.1) 446.3 (2.5) Private mortgage-backed securities — — 4.3 (0.1) 4.3 (0.1) Corporate securities 20.9 (0.1) — — 20.9 (0.1) Total $ 838.6 $ (5.5) $ 16.6 $ (0.2) $ 855.2 $ (5.7) Less than 12 Months 12 Months or More Total December 31, 2019 Fair Gross Fair Gross Fair Gross Available-for-Sale Obligations of U.S. government agencies $ 185.3 $ (0.8) $ — $ — $ 185.3 $ (0.8) U.S. agency residential mortgage-backed 740.1 (4.6) 155.9 (1.4) 896.0 (6.0) Private mortgage-backed securities — — 46.6 (0.4) 46.6 (0.4) Total $ 925.4 $ (5.4) $ 202.5 $ (1.8) $ 1,127.9 $ (7.2) The available-for-sale securities portfolio contains securities that are guaranteed by a sovereign entity or are generally considered to have non-credit related risks, such as interest rate risk or prepayment and liquidity factors. The Company considers whether the securities are issued by the federal government or its agencies and whether downgrades by bond rating agencies have occurred. The unrealized losses are due to changes in interest rates and other market conditions. The Company had 181 and 338 individual investment securities that were in an unrealized loss position as of December 31, 2020 and 2019, respectively, related primarily to fluctuations in the current interest rates. As of December 31, 2020, the Company had the intent and ability to hold these investment securities for a period of time sufficient to allow for an anticipated recovery. Furthermore, the Company does not have the intent to sell any of the available-for-sale securities in the above table and it is more likely than not that the Company will not have to sell any securities before a recovery in cost. There were no material allowances for credit loss as of December 31, 2020 and no impairment losses were recorded during 2019 or 2018 for investment securities. Maturities of securities do not reflect rate repricing opportunities present in adjustable-rate mortgage-backed securities. As of December 31, 2020 and 2019, the Company had variable rate mortgage-backed securities with amortized costs of $220.7 million and $298.1 million, respectively, classified as available-for-sale in the table below. Maturities of investment securities as of December 31, 2020 are shown below. Maturities of mortgage-backed securities have been adjusted to reflect shorter maturities based upon estimated prepayments of principal. All other investment securities maturities are shown at contractual maturity dates. Available-for-Sale Held-to-Maturity December 31, 2020 Amortized Estimated Amortized Estimated Within one year $ 687.6 $ 991.4 $ 8.3 $ 8.4 After one year but within five years 1,587.7 1,362.6 27.8 29.0 After five years but within ten years 736.2 996.3 14.1 16.2 After ten years 921.2 658.4 1.4 1.4 Total $ 3,932.7 $ 4,008.7 $ 51.6 $ 55.0 As of December 31, 2020, the Company had investment securities callable within one year with amortized costs and estimated fair values of $282.4 million and $281.7 million, respectively. These investment securities are primarily classified as available-for-sale and included in the after five years but within ten years category in the table above. As of December 31, 2020, the Company had no callable structured notes. There were no significant concentrations of investments at December 31, 2020, (greater than 10 percent of stockholders’ equity) in any individual security issuer, except for U.S. government or agency-backed securities. As of December 31, 2020 and 2019, the Company recorded amortized costs of $2,323.0 million and $2,132.0 million, respectively, for investment securities pledged to secure public deposits and securities sold under repurchase agreements and had approximate fair values as of December 31, 2020 and 2019 of $2,383.6 million and $2,144.9 million, respectively. All securities sold under repurchase agreements are with clients and mature on the next banking day. The Company retains possession of the underlying securities sold under repurchase agreements.

Loans

Loans	12 Months Ended
	Dec. 31, 2020
Loans and Leases Receivable, Net Amount [Abstract]
Loans	LOANS HELD FOR INVESTMENT The following table presents loans by segment as of the dates indicated: 2020 2019 Real estate loans: Commercial* $ 3,743.2 $ 3,487.8 Construction loans: Land acquisition & development 265.0 302.1 Residential 250.9 244.1 Commercial 523.5 431.5 Total construction loans 1,039.4 977.7 Residential* 1,396.3 1,246.1 Agricultural 220.6 226.6 Total real estate loans 6,399.5 5,938.2 Consumer loans: Indirect 805.1 784.6 Direct and advance lines 150.6 179.0 Credit card 70.2 81.6 Total consumer loans 1,025.9 1,045.2 Commercial* 2,153.9 1,673.7 Agricultural 247.6 279.1 Other, including overdrafts 1.6 — Loans held for investment 9,828.5 8,936.2 Deferred loan fees and costs (21.0) (5.5) Loans held for investment, net of deferred fees and costs 9,807.5 8,930.7 Allowance for credit losses (144.3) (73.0) Net loans held for investment $ 9,663.2 $ 8,857.7 *In conjunction with the adoption of ASC 326, the Company reclassified certain 2019 amounts in our multi-family loan portfolio from residential real estate to commercial real estate. Additionally, we reclassified certain 2019 amounts in our commercial 1-4 family and commercial home equity lines of credit from commercial real estate to commercial to conform to the 2020 presentation. These reclassifications did not change previously reported loan totals. Allowance for Credit Losses The following tables represent, by loan portfolio segment, the activity in the allowance for credit losses for loans held for investment: December 31, 2020 Beginning Balance Impact of Adopting ASC 326 Provision for Credit Loss Expense Loans Charged-Off Recoveries Collected Ending Balance Allowance for credit losses (1) Real estate: Commercial real estate: Non-owner occupied $ 8.8 $ 4.9 $ 11.7 $ — $ 0.1 $ 25.5 Owner occupied 10.0 3.5 5.0 (0.4) 0.2 18.3 Multi-family 0.7 6.9 3.4 — — 11.0 Total commercial real estate 19.5 15.3 20.1 (0.4) 0.3 54.8 Construction: Land acquisition & development 1.9 (0.1) (0.4) (0.5) 0.4 1.3 Residential construction 1.5 (0.9) 1.0 — — 1.6 Commercial construction 2.7 1.3 3.3 — — 7.3 Total construction 6.1 0.3 3.9 (0.5) 0.4 10.2 Residential real estate: Residential 1-4 family 1.8 10.6 (1.1) — 0.1 11.4 Home equity and HELOC 1.0 0.5 (0.4) — 0.3 1.4 Total residential real estate 2.8 11.1 (1.5) — 0.4 12.8 Agricultural real estate 0.5 1.8 0.4 — — 2.7 Total real estate 28.9 28.5 22.9 (0.9) 1.1 80.5 Consumer: Indirect 4.5 8.8 5.4 (4.1) 2.1 16.7 Direct and advance lines 2.9 3.0 1.6 (3.9) 1.0 4.6 Credit card 2.5 0.3 1.8 (2.8) 0.8 2.6 Total consumer 9.9 12.1 8.8 (10.8) 3.9 23.9 Commercial: Commercial and floor plans 25.5 (5.1) 20.4 (8.0) 1.4 34.2 Commercial purpose secured by 1-4 family 5.9 (3.8) 2.5 (0.1) 0.2 4.7 Credit card 1.2 (1.1) 1.1 (1.0) 0.1 0.3 Total commercial 32.6 (10.0) 24.0 (9.1) 1.7 39.2 Agricultural: Agricultural 1.6 (0.6) (0.2) (0.1) — 0.7 Total agricultural 1.6 (0.6) (0.2) (0.1) — 0.7 Total allowance for credit losses $ 73.0 $ 30.0 $ 55.5 $ (20.9) $ 6.7 $ 144.3 (1) Amounts presented are exclusive of the allowance for credit losses related to unfunded commitments which are included in “Note 19 - Financial Instruments with Off-Balance Sheet Risk” included in this report. The following tables represent activity in the allowance for credit losses for loans held for investment under historical GAAP: Year Ended December 31, 2019 Real Estate Consumer Commercial Agriculture Other Total Allowance for loan losses: Beginning balance $ 31.0 $ 8.7 $ 31.3 $ 2.0 $ — $ 73.0 Provision charged (credited) to operating (1.3) 10.6 4.5 0.1 — 13.9 Less loans charged-off (3.5) (13.0) (6.6) (0.5) — (23.6) Add back recoveries of loans previously charged-off 2.7 3.6 3.4 — — 9.7 Ending balance $ 28.9 $ 9.9 $ 32.6 $ 1.6 $ — $ 73.0 Individually evaluated for impairment $ 1.7 $ — $ 1.7 $ 0.2 $ — $ 3.6 Collectively evaluated for impairment 27.2 9.9 30.9 1.4 — 69.4 Ending balance $ 28.9 $ 9.9 $ 32.6 $ 1.6 $ — $ 73.0 Total loans: Individually evaluated for impairment $ 41.1 $ — $ 17.3 $ 6.3 $ — $ 64.7 Collectively evaluated for impairment 5,897.1 1,045.2 1,656.4 272.8 — 8,871.5 Loans held for investment $ 5,938.2 $ 1,045.2 $ 1,673.7 $ 279.1 $ — $ 8,936.2 Year Ended December 31, 2018 Real Estate Consumer Commercial Agriculture Other Total Allowance for loan losses: Beginning balance $ 31.7 $ 8.7 $ 30.5 $ 1.2 $ — $ 72.1 Provision charged (credited) to operating (0.7) 6.8 1.9 0.6 — 8.6 Less loans charged-off (3.7) (11.3) (4.7) — — (19.7) Add back recoveries of loans previously charged-off 3.7 4.5 3.6 0.2 — 12.0 Ending balance $ 31.0 $ 8.7 $ 31.3 $ 2.0 $ — $ 73.0 Individually evaluated for impairment $ 1.3 $ — $ 5.2 $ 0.3 $ — $ 6.8 Collectively evaluated for impairment 29.7 8.7 26.1 1.7 — 66.2 Ending balance $ 31.0 $ 8.7 $ 31.3 $ 2.0 $ — $ 73.0 Total loans: Individually evaluated for impairment $ 41.8 $ — $ 19.9 $ 3.1 $ — $ 64.8 Collectively evaluated for impairment 5,546.1 1,070.2 1,540.4 251.7 1.6 8,410.0 Loans held for investment $ 5,587.9 $ 1,070.2 $ 1,560.3 $ 254.8 $ 1.6 $ 8,474.8 Collateral-Dependent Financial Loans A collateral-dependent financial loan relies solely on the operation or sale of the collateral for repayment. In evaluating the overall risk associated with a loan, the Company considers character, overall financial condition and resources, and payment record of the borrower; the prospects for support from any financially responsible guarantors; and the nature and degree of protection provided by the cash flow and value of any underlying collateral. The loan may become collateral-dependent where the borrower is experiencing financial difficulty and as sources of repayment become inadequate over time and that repayment is expected to be provided substantially through the operation or sale of the collateral. The following tables present the amortized cost basis of collateral-dependent loans by class of loans as of December 31, 2020. Collateral Type As of December 31, 2020 Business Assets Real Property Other Total Real estate $ 1.3 $ 6.5 $ 1.1 $ 8.9 Commercial 6.1 1.3 0.4 7.8 Agricultural — 0.8 — 0.8 Total collateral-dependent $ 7.4 $ 8.6 $ 1.5 $ 17.5 Impaired Loans Prior to the adoption of ASC 326 on January 1, 2020, loans were reported as impaired when, based on then current information and events, it was probable we would be unable to collect all amounts due in accordance with the original contractual terms of the loan agreement, including scheduled principal and interest payments. The amount of the impairment was measured using cash flows discounted at the loan’s effective interest rate, except when it was determined that the primary source of repayment for the loan was the operation or liquidation of the underlying collateral. In such cases, the current fair value of the collateral, reduced by anticipated selling costs, was used to measure impairment. The Company considered impaired loans to include all loans, except consumer loans, that were risk rated as doubtful or for which interest accrual had been discontinued or that would have been renegotiated in a troubled debt restructuring. Interest payments received on impaired loans were applied based on whether they were on accrual or non-accrual status. Interest income recognized by the Company on impaired loans primarily related to loans modified in troubled debt restructurings that remained on accrual status. Interest payments received on non-accrual impaired loans were applied to principal. Interest income was subsequently recognized only to the extent cash payments were received in excess of the principal due. The following tables present information on the Company’s recorded investment of impaired loans as of the dates indicated: December 31, 2019 Unpaid Recorded Recorded Total Related Real estate: Commercial $ 29.2 $ 12.9 $ 10.8 $ 23.7 $ 0.7 Construction: Land acquisition & development 9.2 0.4 2.6 3.0 0.5 Residential 0.1 — — — — Commercial 1.0 0.5 — 0.5 0.1 Total construction loans 10.3 0.9 2.6 3.5 0.6 Residential 6.9 3.9 1.8 5.7 0.2 Agricultural 8.6 5.2 3.0 8.2 0.2 Total real estate loans 55.0 22.9 18.2 41.1 1.7 Commercial 25.5 12.0 5.3 17.3 1.7 Agricultural 6.9 2.3 4.0 6.3 0.2 Total $ 87.4 $ 37.2 $ 27.5 $ 64.7 $ 3.6 December 31, 2018 Unpaid Recorded Recorded Total Related Real estate: Commercial $ 22.2 $ 8.6 $ 7.7 $ 16.3 $ 0.7 Construction: Land acquisition & development 10.0 0.4 3.5 3.9 0.2 Residential 1.1 0.6 0.4 1.0 0.1 Commercial 0.7 0.2 — 0.2 — Total construction loans 11.8 1.2 3.9 5.1 0.3 Residential 8.8 5.7 2.0 7.7 0.3 Agricultural 12.9 12.5 0.2 12.7 — Total real estate loans 55.7 28.0 13.8 41.8 1.3 Commercial 24.1 5.5 14.4 19.9 5.2 Agricultural 3.2 2.5 0.6 3.1 0.3 Total $ 83.0 $ 36.0 $ 28.8 $ 64.8 $ 6.8 The following table presents the average recorded investment in and income recognized on impaired loans for the periods indicated: Year Ended December 31, 2019 2018 Average Recorded Investment Income Recognized Average Recorded Investment Income Recognized Real estate $ 41.4 $ 0.1 $ 50.0 $ 0.1 Commercial 18.7 0.2 21.9 0.2 Agricultural 4.6 — 2.1 — Total $ 64.7 $ 0.3 $ 74.0 $ 0.3 Loans are considered past due if the required principal and interest payments have not been received as of the date such payments were due. Loans classified in the following table as greater than 90 days past due are still accruing interest. The following tables present the contractual aging of the Company’s recorded amortized cost basis in loans by portfolio as of the dates indicated. Total Loans 30 - 59 60 - 89 > 90 30 or More Days Days Days Days Current Non-accrual Total As of December 31, 2020 Past Due Past Due Past Due Past Due Loans Loans (1) Loans Real estate Commercial $ 7.6 $ 1.2 $ 4.0 $ 12.8 $ 3,720.8 $ 9.6 $ 3,743.2 Construction: Land acquisition & development 2.5 1.1 0.1 3.7 260.6 0.7 265.0 Residential 1.5 0.4 — 1.9 247.9 1.1 250.9 Commercial 12.2 — — 12.2 511.2 0.1 523.5 Total construction loans 16.2 1.5 0.1 17.8 1,019.7 1.9 1,039.4 Residential 4.7 1.6 0.5 6.8 1,384.9 4.6 1,396.3 Agricultural 2.0 — — 2.0 212.4 6.2 220.6 Total real estate loans 30.5 4.3 4.6 39.4 6,337.8 22.3 6,399.5 Consumer: Indirect consumer 6.4 2.0 0.5 8.9 794.3 1.9 805.1 Other consumer 0.8 0.2 0.2 1.2 149.0 0.4 150.6 Credit card 0.6 0.4 0.6 1.6 68.6 — 70.2 Total consumer loans 7.8 2.6 1.3 11.7 1,011.9 2.3 1,025.9 Commercial 6.2 1.8 1.2 9.2 2,132.9 11.8 2,153.9 Agricultural 0.4 0.6 1.4 2.4 242.1 3.1 247.6 Other, including overdrafts — — — — 1.6 — 1.6 Loans held for investment $ 44.9 $ 9.3 $ 8.5 $ 62.7 $ 9,726.3 $ 39.5 $ 9,828.5 Total Loans 30 - 59 60 - 89 > 90 30 or More Days Days Days Days Current Non-accrual Total As of December 31, 2019 Past Due Past Due Past Due Past Due Loans Loans (1) Loans Real estate Commercial $ 5.5 $ 1.1 $ 0.6 $ 7.2 $ 3,467.6 $ 13.0 $ 3,487.8 Construction: Land acquisition & development 0.7 0.8 0.3 1.8 298.9 1.4 302.1 Residential 1.5 0.8 — 2.3 241.8 — 244.1 Commercial — — — — 431.0 0.5 431.5 Total construction loans 2.2 1.6 0.3 4.1 971.7 1.9 977.7 Residential 3.8 1.4 1.1 6.3 1,235.2 4.6 1,246.1 Agricultural 0.8 0.5 — 1.3 220.1 5.2 226.6 Total real estate loans 12.3 4.6 2.0 18.9 5,894.6 24.7 5,938.2 Consumer: Indirect consumer 7.6 1.9 0.5 10.0 773.0 1.6 784.6 Other consumer 1.2 0.5 0.1 1.8 176.7 0.5 179.0 Credit card 0.8 0.5 0.8 2.1 79.5 — 81.6 Total consumer loans 9.6 2.9 1.4 13.9 1,029.2 2.1 1,045.2 Commercial 4.8 2.6 2.3 9.7 1,650.3 13.7 1,673.7 Agricultural 0.9 0.1 — 1.0 275.7 2.4 279.1 Other, including overdrafts — — — — — — — Loans held for investment $ 27.6 $ 10.2 $ 5.7 $ 43.5 $ 8,849.8 $ 42.9 $ 8,936.2 (1) As of December 31, 2020 and December 31, 2019, none of our non-accrual loans were earning interest income. No material interest income was recognized on non-accrual loans at December 31, 2020 and 2019, respectively. Additionally, no material accrued interest was reversed at December 31, 2020. Troubled Debt Restructurings Modifications of performing loans are made in the ordinary course of business and are completed on a case-by-case basis as negotiated with the borrower in connection with the ongoing loan collection processes. Loan modifications typically include interest rate changes, interest only periods of less than twelve months, short-term payment deferrals and extension of amortization periods to provide payment relief. A loan modification is considered a troubled debt restructuring if the borrower is experiencing financial difficulties and the Company, for economic or legal reasons, grants a concession to the borrower that it would not otherwise consider. Certain troubled debt restructurings are on non-accrual status at the time of restructuring and may be returned to accrual status if the borrower has sustained repayment performance in accordance with the restructuring agreement for a period of at least six months and management is reasonably assured of the borrower’s future performance. If the troubled debt restructuring meets these performance criteria, and the interest rate granted at the modification is equal to or greater than the rate that the Company was willing to accept at the time of the restructuring for a new loan with comparable risk, then the loan will return to performing status and the accrual of interest will resume. Any such loan will continue to be individually evaluated for credit deterioration and disclosed as collateral dependent loans. The Coronavirus Aid, Relief, and Economic Security Act (“CARES Act”) was signed into law on March 27, 2020 in response to the outbreak of a new strain of coronavirus (also known as, and hereinafter referred to as, “COVID-19”). Key provisions of the CARES Act include one-time payments to individuals, strengthened unemployment insurance, additional health-care funding, temporary amendments to the Internal Revenue Code, and loans and grants to certain businesses. The CARES Act was later extended by the Coronavirus Response and Relief Supplemental Appropriations Act, 2021, or RELIEF Act, which was signed into law on December 27, 2020. The CARES Act provided financial institutions with options on the treatment of troubled debt restructurings, and the Company elected to apply these options at the individual loan level. Under the CARES Act, the Company can elect: (1) to suspend the requirements under GAAP for loan modifications related to the COVID–19 pandemic that would otherwise be categorized as a troubled debt restructuring; and/or (2) to suspend any determination of a loan modified as being a troubled debt restructuring as a result of the effects of the COVID–19 pandemic, including impairment for accounting purposes. If the Company elects a suspension noted above, the suspension (a) will be effective for the term of the loan modification, but solely with respect to any modification, including a forbearance arrangement, an interest rate modification, a repayment plan, and any other similar arrangement that defers or delays the payment of principal or interest, occurring for a loan that was not more than 30 days past due as of December 31, 2019; and (b) will not apply to any adverse impact on the credit of a borrower that is not related to the COVID–19 pandemic. These suspensions currently are expected to end on January 1, 2022. The Company renegotiated loans in troubled debt restructurings in the amount of $14.5 million as of December 31, 2020, of which $11.3 million were included in non-accrual loans and $3.2 million were on accrual status. As of December 31, 2020, the Company allocated $2.9 million of allowance for credit losses to those loans and the Company had no material commitments to lend additional funds to borrowers whose existing loans have been renegotiated or are classified as non-accrual. The Company renegotiated loans in troubled debt restructurings in the amount of $24.9 million as of December 31, 2019, of which $19.4 million were included in non-accrual loans and $5.5 million were on accrual status. As of December 31, 2019, the Company allocated $0.3 million of allowance for credit losses to those loans and the Company had no material commitments to lend additional funds to borrowers whose existing loans have been renegotiated or are classified as non-accrual. The following table presents information of the Company’s troubled debt restructurings that occurred during the periods indicated: Number of Notes Type of Concession Principal Balance at Restructure December 31, 2020 Interest only period Extension of term or amortization schedule Interest rate adjustment Other (1) Commercial real estate 3 $ 0.2 $ — $ — $ 0.1 $ 0.3 Total loans restructured during period 3 $ 0.2 $ — $ — $ 0.1 $ 0.3 December 31, 2019 Commercial real estate 4 $ 0.2 $ 0.2 $ — $ 2.9 $ 3.3 Commercial 1 — — — 5.0 5.0 Agriculture 6 — — — 2.1 2.1 Total loans restructured during period 11 $ 0.2 $ 0.2 $ — $ 10.0 $ 10.4 December 31, 2018 Commercial real estate 3 $ 3.6 $ — $ — $ — $ 3.6 Agriculture real estate 1 — — — 0.2 0.2 Consumer 1 — — — 0.3 0.3 Total loans restructured during period 5 $ 3.6 $ — $ — $ 0.5 $ 4.1 (1) Other includes concessions that reduce or defer payments for a specified period of time and/or concessions that do not fit into other designated categories. For troubled debt restructurings that were on non-accrual status or otherwise deemed collateral-dependent before the modification, a specific reserve may already be recorded. In periods subsequent to modification, the Company continues to evaluate all troubled debt restructurings for possible credit deterioration and recognizes credit loss through the allowance. Additionally, these loans continue to work through the credit cycle through charge-off, pay-off, or foreclosure. Financial effects of modifications of troubled debt restructurings may include principal loan forgiveness or other charge-offs directly related to the restructuring. The Company had no charge-offs directly related to modifying troubled debt restructurings during December 31, 2020, 2019, and 2018. The Company had no material troubled debt restructurings during the previous 12 months for which there was a payment default during December 31, 2020, 2019, and 2018. The Company considers a payment default to occur on troubled debt restructurings when the loan is 90 days or more past due or is placed on non-accrual status after the modification. The terms of certain other loans were modified during the quarter ended December 31, 2020 that did not meet the definition of a troubled debt restructuring. These loans have a total recorded investment of $63.5 million as of December 31, 2020. The modification of these loans involved either a modification of the terms of a loan to borrowers who were not experiencing financial difficulties or a delay in a payment that was considered to be insignificant. In order to determine whether a borrower is experiencing financial difficulty, the Company evaluates the probability that the borrower will be in payment default on any of its debt in the foreseeable future without the modification. This evaluation is performed under the Company’s internal underwriting policy. Credit Quality Indicators As part of the on-going and continuous monitoring of the credit quality of the Company’s loan portfolio, management tracks internally assigned risk classifications of loans based on relevant information about the ability of borrowers to service their debt including, among other factors, current financial information, historical payment experience, credit documentation, public information, and current economic trends. The Company analyzes loans individually to classify the credit risk of the loans. This analysis generally includes loans with an outstanding balance greater than $1.0 million, which are generally considered non-homogeneous loans, such as commercial and commercial real estate loans. This analysis is performed no less than on an annual basis, dependent upon the size of exposure and the financial reporting frequency to which the borrower is contractually obligated. Homogeneous loans, including small business loans are typically managed by payment performance. The Company risk rates its loans internally in accordance with a Uniform Classification System developed jointly by the various bank regulatory agencies to internally risk rate loans. The Uniform Classification System defines three broad categories of criticized assets, which the Company uses as credit quality indicators in addition to the 6 Pass ratings in its 10-point rating scale: Special Mention — includes loans that exhibit a potential weakness in financial condition, loan structure, or documentation that warrants management’s close attention. If not promptly corrected, the potential weaknesses may result in deterioration of the repayment prospects for the loan or of the institution’s credit position at some future date. Substandard — includes loans that are inadequately protected by the current net worth and paying capacity of the borrower which have a well-defined weakness or weaknesses that jeopardize the liquidation of the debt. Although the primary source of repayment for a substandard loan may not currently be sufficient, collateral or other sources of repayment are sufficient to satisfy the debt. Continuance of a substandard loan is not warranted unless positive steps are taken to improve the worthiness of the credit. Doubtful — includes loans that exhibit pronounced weaknesses on the basis of currently existing facts, conditions, and values to a point where collection or liquidation for full repayment is highly questionable and improbable. Doubtful loans are required to be placed on non-accrual status and are assigned specific loss exposure. Loans not meeting the criteria above that are analyzed individually as part of the above described process are considered to be pass-rated loans. The Company evaluates the credit quality and loan performance for the allowance for credit loan losses of the following segments based on the aforementioned risk scale: December 31, 2020 Term Loans Amortized Cost Basis by Origination Year Risk by Collateral 2020 2019 2018 2017 2016 Prior Revolving Loans Amortized Cost Basis Total Commercial real estate non-owner occupied: Pass $ 495.9 $ 304.9 $ 216.0 $ 105.3 $ 139.7 $ 336.5 $ 13.8 $ 1,612.1 Special mention 0.3 2.3 0.9 0.1 6.4 13.7 — 23.7 Substandard 15.7 2.7 1.0 4.1 1.1 13.9 — 38.5 Doubtful — — 0.2 — — — — 0.2 Total $ 511.9 $ 309.9 $ 218.1 $ 109.5 $ 147.2 $ 364.1 $ 13.8 $ 1,674.5 Commercial real estate owner occupied: Pass $ 416.3 $ 312.5 $ 211.2 $ 122.4 $ 153.7 $ 357.9 $ 8.9 $ 1,582.9 Special mention 7.1 9.6 4.8 3.1 18.6 20.0 — 63.2 Substandard 8.9 6.5 11.5 5.0 15.1 11.1 0.3 58.4 Doubtful 0.2 — — 0.2 — 0.1 — 0.5 Total $ 432.5 $ 328.6 $ 227.5 $ 130.7 $ 187.4 $ 389.1 $ 9.2 $ 1,705.0 Commercial multi-family: Pass $ 132.5 $ 58.9 $ 23.5 $ 41.6 $ 25.8 $ 80.5 $ 0.8 $ 363.6 Special mention — — — — — 0.1 — 0.1 Total $ 132.5 $ 58.9 $ 23.5 $ 41.6 $ 25.8 $ 80.6 $ 0.8 $ 363.7 Land, acquisition and development: Pass $ 104.6 $ 58.8 $ 26.4 $ 30.7 $ 7.6 $ 26.3 $ 5.8 $ 260.2 Special mention 0.2 0.1 — 0.9 — 1.2 0.5 2.9 Substandard 0.3 — 1.2 0.1 — 0.1 0.1 1.8 Doubtful — — — — — 0.1 — 0.1 Total $ 105.1 $ 58.9 $ 27.6 $ 31.7 $ 7.6 $ 27.7 $ 6.4 $ 265.0 Residential construction: Pass $ 80.4 $ 64.7 $ 16.7 $ 5.6 $ — $ 0.1 $ 82.1 $ 249.6 Substandard 0.2 — — — — — 1.1 1.3 Total $ 80.6 $ 64.7 $ 16.7 $ 5.6 $ — $ 0.1 $ 83.2 $ 250.9 Commercial construction: Pass $ 236.1 $ 195.4 $ 61.2 $ 11.9 $ 6.0 $ (0.3) $ 11.6 $ 521.9 Special mention — — 1.5 — — — — 1.5 Substandard — — — — — 0.1 — 0.1 Total $ 236.1 $ 195.4 $ 62.7 $ 11.9 $ 6.0 $ (0.2) $ 11.6 $ 523.5 Agricultural real estate: Pass $ 50.0 $ 45.3 $ 28.2 $ 17.2 $ 12.7 $ 27.2 $ 5.3 $ 185.9 Special mention 2.8 6.7 1.0 1.5 0.9 1.0 0.5 14.4 Substandard 1.4 5.9 3.4 0.9 3.4 4.4 0.6 20.0 Doubtful — 0.3 — — — — — 0.3 Total $ 54.2 $ 58.2 $ 32.6 $ 19.6 $ 17.0 $ 32.6 $ 6.4 $ 220.6 Commercial and floor plans: Pass $ 1,029.4 $ 153.0 $ 136.0 $ 69.8 $ 43.0 $ 92.3 $ 233.1 $ 1,756.6 Special mention 5.6 1.0 1.9 7.0 3.9 0.5 2.4 22.3 Substandard 8.8 1.8 4.3 0.4 4.1 1.1 11.5 32.0 Doubtful 0.3 0.4 — — 0.1 2.6 0.2 3.6 Total $ 1,044.1 $ 156.2 $ 142.2 $ 77.2 $ 51.1 $ 96.5 $ 247.2 $ 1,814.5 December 31, 2020 Term Loans Amortized Cost Basis by Origination Year Risk by Collateral 2020 2019 2018 2017 2016 Prior Revolving Loans Amortized Cost Basis Total Commercial purpose secured by 1-4 family: Pass $ 82.3 $ 53.7 $ 33.7 $ 20.7 $ 15.5 $ 34.9 $ 17.5 $ 258.3 Special mention 0.5 0.5 0.3 0.1 0.5 0.8 0.8 3.5 Substandard 2.4 1.0 4.8 0.3 1.4 1.0 0.1 11.0 Total $ 85.2 $ 55.2 $ 38.8 $ 21.1 $ 17.4 $ 36.7 $ 18.4 $ 272.8 Agricultural: Pass $ 47.4 $ 18.1 $ 10.7 $ 4.2 $ 3.0 $ 1.3 $ 130.9 $ 215.6 Special mention 1.5 0.7 0.4 — 0.1 0.3 13.4 16.4 Substandard 3.7 1.5 4.2 0.6 0.1 0.4 3.6 14.1 Total $ 52.6 $ 20.3 $ 15.3 $ 4.8 $ 3.2 $ 2.0 $ 147.9 $ 246.1 The Company evaluates the credit quality, loan performance, and the allowance for credit loan losses of its residential and consumer loan portfolios, based primarily on the aging status of the loan and payment activity. Accordingly, loans on nonaccrual status, loans past due 90 days or more and still accruing interest, and loans modified under troubled debt restructurings are considered to be nonperforming for purposes of credit quality evaluation. The following tables present the recorded investment of our other loan portfolios based on the credit risk profile of loans that are performing and loans that are nonperforming as of the periods indicated: December 31, 2020 Term Loans Amortized Cost Basis by Origination Year Risk by Collateral 2020 2019 2018 2017 2016 Prior Revolving Loans Amortized Cost Basis Total Residential 1-4 family: Performing $ 491.1 $ 113.9 $ 57.2 $ 47.8 $ 65.7 $ 234.6 $ — $ 1,010.3 Nonperforming 0.1 0.7 — — — 1.2 — 2.0 Total $ 491.2 $ 114.6 $ 57.2 $ 47.8 $ 65.7 $ 235.8 $ — $ 1,012.3 Consumer home equity and HELOC: Performing $ 12.0 $ 7.1 $ 7.1 $ 9.7 $ 4.7 $ 14.4 $ 328.1 $ 383.1 Nonperforming 0.1 0.2 — — 0.1 0.4 0.1 0.9 Total $ 12.1 $ 7.3 $ 7.1 $ 9.7 $ 4.8 $ 14.8 $ 328.2 $ 384.0 Consumer indirect: Performing $ 334.5 $ 187.9 $ 117.9 $ 73.8 $ 47.6 $ 42.6 $ — $ 804.3 Nonperforming 0.1 0.2 0.1 0.2 0.1 0.1 — 0.8 Total $ 334.6 $ 188.1 $ 118.0 $ 74.0 $ 47.7 $ 42.7 $ — $ 805.1 Consumer direct and advance line: Performing $ 47.1 $ 29.4 $ 28.1 $ 11.9 $ 5.3 $ 8.6 $ 19.9 $ 150.3 Nonperforming 0.1 — 0.1 — — — 0.1 0.3 Total $ 47.2 $ 29.4 $ 28.2 $ 11.9 $ 5.3 $ 8.6 $ 20.0 $ 150.6 The Company considers the performance of the loan portfolio and its impact on the allowance for credit loan losses. For certain credit card loan classes, the Company also evaluates credit quality based on the aging status of the loan, which was previously presented, and by payment activity. The following table presents the recorded investment in credit card loans based on payment activity: As of December 31, 2020 Consumer Commercial Agricultural Total Credit Card: Performing $ 69.6 $ 66.3 $ 1.5 $ 137.4 Nonperforming 0.6 0.3 — 0.9 Total $ 70.2 $ 66.6 $ 1.5 $ 138.3 The following presents the recorded investment in the Company’s loans by risk grades and loan class as of the date shown below: As of December 31, 2019 Pass Other Assets Substandard Doubtful Total Total Loans Real estate: Commercial $ 3,305.0 $ 84.7 $ 97.3 $ 0.8 $ 182.8 $ 3,487.8 Construction: Land acquisition & development 295.4 3.8 1.9 1.0 6.7 302.1 Residential 241.0 0.9 2.2 — 3.1 244.1 Commercial 428.3 1.7 1.5 — 3.2 431.5 Total construction loans 964.7 6.4 5.6 1.0 13.0 977.7 Residential 1,235.4 2.6 7.8 0.3 10.7 1,246.1 Agricultural 185.7 14.3 26.6 — 40.9 226.6 Total real estate loans 5,690.8 108.0 137.3 2.1 247.4 5,938.2 Consumer: Indirect consumer 781.5 0.2 2.9 — 3.1 784.6 Direct consumer 177.7 0.4 0.8 0.1 1.3 179.0 Credit card 81.6 — — — — 81.6 Total consumer loans 1,040.8 0.6 3.7 0.1 4.4 1,045.2 Commercial 1,569.4 40.4 60.3 3.6 104.3 1,673.7 Agricultural 247.8 8.5 22.7 0.1 31.3 279.1 Total $ 8,548.8 $ 157.5 $ 224.0 $ 5.9 $ 387.4 $ 8,936.2 There were no material purchases of portfolio loans and no material sales of loans held for investment during the three and nine months ended December 31, 2020 or 2019. Purchased Credit Deteriorated Loans The Company has purchased loans acquired in business combinations, for which there was, at acquisition, evidence of more than insignificant deterioration of credit quality since origination. See “Note 2 - Acquisitions” included in this report, for additional details.

Premises and Equipment

Premises and Equipment	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Premises and Equipment	PREMISES AND EQUIPMENT Premises and equipment and related accumulated depreciation are as follows: December 31, 2020 2019 Land $ 52.9 $ 50.8 Buildings and improvements 349.5 330.1 Furniture and equipment 88.9 94.2 Total premises and equipment 491.3 475.1 Less accumulated depreciation (179.0) (169.1) Premises and equipment, net $ 312.3 $ 306.0 Depreciation expense was $25.8 million, $23.3 million, and $16.2 million for the years ended December 31, 2020, 2019, and 2018, respectively. The Parent Company and a FIB branch office lease premises from an affiliated entity. See Note 18—Commitments and Contingencies.

Company-Owned Life Insurance

Company-Owned Life Insurance	12 Months Ended
	Dec. 31, 2020
Investments, All Other Investments [Abstract]
Company-Owned Life Insurance	COMPANY-OWNED LIFE INSURANCE Company-owned life insurance consists of the following: December 31, 2020 2019 Key executive, principal shareholder $ 3.1 $ 4.4 Key executive split dollar 7.0 7.0 Group life 286.3 282.4 Total $ 296.4 $ 293.8 The Company maintains key executive life insurance policies on certain principal shareholders. Under these policies, the Company receives benefits payable upon the death of the insured. The net cash surrender value of key executive, principal shareholder insurance policies was $3.1 million and $4.4 million at December 31, 2020 and 2019, respectively. The Company also has life insurance policies covering selected other key officers. The net cash surrender value of these policies was $7.0 million at December 31, 2020 and 2019, respectively. Under these policies, the Company receives benefits payable upon death of the insured. An endorsement split dollar agreement has been executed with the selected key officers whereby a portion of the policy death benefit is payable to their designated beneficiaries. The endorsement split dollar agreement will provide post-retirement coverage for those selected key officers meeting specified retirement qualifications. The Company expenses the earned portion of the post-employment benefit through the vesting period.

Other Real Estate Owned

Other Real Estate Owned	12 Months Ended
	Dec. 31, 2020
Repossessed Assets [Abstract]
Other Real Estate Owned	OTHER REAL ESTATE OWNED Information with respect to the Company’s other real estate owned follows: Year Ended December 31, 2020 2019 2018 Balance at beginning of year $ 8.5 $ 14.4 $ 10.1 OREO acquired through acquisitions — 2.4 0.6 Additions 3.3 14.1 12.1 Capitalized improvements — 0.3 — Valuation adjustments (0.1) (0.9) (0.1) Dispositions (9.2) (21.8) (8.3) Balance at end of year $ 2.5 $ 8.5 $ 14.4 Write-downs of $0.1 million, $0.9 million, and $0.1 million during 2020, 2019, and 2018, respectively, were adjustments based on internal evaluations and other sources, including management estimates of the current fair value of properties, and adjustments directly related to receipt of updated appraisals. The carrying value of foreclosed residential real estate properties included in other real estate owned was zero as of December 31, 2020 and $2.3 million as of December 31, 2019. The Company had recorded investments in consumer mortgage loans secured by residential real estate for which formal foreclosure proceedings were in process of foreclosure of $0.2 million and $0.4 million as of December 31, 2020 and December 31, 2019, respectively.

Derivatives and Hedging Activit

Derivatives and Hedging Activities	12 Months Ended
	Dec. 31, 2020
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivatives and Hedging Activities	DERIVATIVES AND HEDGING ACTIVITIES For asset and liability management purposes, the Company enters into interest rate swap contracts to hedge against changes in forecasted cash flows due to interest rate exposures. Interest rate swaps are contracts in which a series of interest payments are exchanged over a prescribed period. The notional amount upon which the interest payments are based is not exchanged. The amount exchanged is determined by reference to the notional amount and the other terms of the individual agreements. The swap agreements are derivative instruments and convert a portion of the Company’s forecasted variable rate debt to a fixed rate (i.e., cash flow hedge) over the payment term of the interest rate swap. The gain or loss on cash flow hedging instruments is initially reported as a component of other comprehensive income and subsequently reclassified into earnings in the same period during which the transaction affects earnings. The Company does not enter into interest rate swap agreements for trading or speculative purposes. Changes in the fair value of derivatives designated and that qualify as cash flow hedges is recorded in Accumulated Other Comprehensive Income and is subsequently reclassified into earnings in the period that the hedged forecasted transaction affects earnings. On May 1, 2020, the Company entered into three interest rate swap contracts that were designated as cash flow hedges. The contracts included a notional amount of $46.4 million, $36.1 million, and $5.1 million. The Company pays a fixed interest rate of 0.40%, 0.34%, and 0.40%, respectively, and the counterparty pays to the Company a variable interest rate equal to the three-month LIBOR under the terms of the interest rate swap contracts. No cash was exchanged until the effective date, which began on May 1, 2020 and ends on April 1, 2022, March 15, 2022, and March 30, 2022, respectively. The Company designated the interest payments related to the trust preferred securities as the cash flow hedge. The hedge was fully effective during the current period. The Company expects the hedge to remain highly effective during the remaining term of the interest rate swap. The Company also enters into certain interest rate swap contracts that are not designated as hedging instruments. These derivative contracts relate to transactions in which the Company enters into an interest rate swap with a client while at the same time entering into an offsetting interest rate swap with a third-party financial institution. Because the Company acts as an intermediary for the client, changes in the fair value of the underlying derivative contracts for the most part offset each other and do not significantly impact the Company’s results of operations. In the normal course of business, the Company enters into interest rate lock commitments to finance residential mortgage loans that are not designated as accounting hedges. These commitments, which contain fixed expiration dates, offer the borrower an interest rate guarantee, provided the loan meets underwriting guidelines and closes within the timeframe established by the Company. Interest rate risk arises on these commitments and subsequently closed loans if interest rates change between the time of the interest rate lock and the delivery of the loan to the investor. Loan commitments related to residential mortgage loans intended to be sold are considered derivatives and are marked to market through earnings. In addition to the effects of the change in market interest rate, the fair value measurement of the derivative also contemplates the expected cash flows to be received from the counterparty from the future sale of the loan. The Company sells residential mortgage loans on either a best efforts or mandatory delivery basis. The Company mitigates the effect of the interest rate risk inherent in providing interest rate lock commitments by entering into forward loan sales contracts. During the interest rate lock commitment period, these forward loan sales contracts are marked to market through earnings and are not designated as accounting hedges. Exclusive of the fair value component associated with the projected cash flows from the loan delivery to the investor, the changes in fair value related to movements in market rates of the interest rate lock commitments and the forward loan sales contracts generally move in opposite directions, and the net impact of changes in these valuations on net income during the loan commitment period is generally inconsequential. When the loan is funded to the borrower, the interest rate lock commitment derivative expires, and the Company records a loan held for sale. The forward loan sales contract acts as a hedge against the variability in cash to be received from the loan sale. The changes in measurement of the estimated fair values of the interest rate lock commitments and forward loan sales contracts are included in mortgage banking revenues in the accompanying consolidated statements of income. The notional amounts and estimated fair values of the Company’s derivatives are presented in the following table. Fair value estimates are obtained from third parties and are based on pricing models. December 31, 2020 December 31, 2019 Notional Amount Estimated Notional Amount Estimated Derivative Assets (included in other assets on the consolidated balance sheets) Non-hedging interest rate derivatives: Interest rate swap contracts $ 799.7 $ 52.0 $ 503.2 $ 21.9 Interest rate lock commitments 101.9 3.3 67.8 1.3 Total derivative assets $ 901.6 $ 55.3 $ 571.0 $ 23.2 Derivative Liabilities (included in accounts payable and accrued expenses on the consolidated balance sheets) Derivatives designated as hedges: Interest rate swap contracts $ 87.6 $ 0.2 $ — $ — Non-hedging interest rate derivatives: Interest rate swap contracts 799.7 $ 52.0 503.2 $ 21.9 Forward loan sales contracts 126.8 1.1 128.0 0.3 Total derivative liabilities $ 1,014.1 $ 53.3 $ 631.2 $ 22.2 There were no material effects of derivative instruments in cash flow hedging relationships on the consolidated statements of income at December 31, 2020. Derivative assets and liabilities are recorded at fair value on the balance sheet and do not take into account the effects of master netting arrangements. Master netting arrangements allow the Company to settle all contracts held with a single counterparty on a net basis and to offset net contract position with related collateral where applicable. The following table illustrates the potential effect of the Company’s master netting arrangements, by type of financial instrument, on the Company’s consolidated balance sheets as of the periods indicated: December 31, 2020 Gross Amounts Recognized Gross Amounts Offset in the Balance Sheet Net Amounts in the Balance Sheet Financial Instruments Fair Value of Financial Collateral in the Balance Sheet Net Amount Financial Assets Interest rate swap contracts $ 52.0 $ — $ 52.0 $ — $ 17.2 $ 34.8 Mortgage related derivatives 3.3 — 3.3 — — 3.3 Total derivatives 55.3 — 55.3 — 17.2 38.1 Total assets $ 55.3 $ — $ 55.3 $ — $ 17.2 $ 38.1 Financial Liabilities Interest rate swap contracts $ 52.2 $ — $ 52.2 $ — $ — $ 52.2 Mortgage related derivatives 1.1 — 1.1 — — 1.1 Total derivatives 53.3 — 53.3 — — 53.3 Repurchase agreements 1,091.4 1,091.4 — 1,091.4 — Total liabilities $ 1,144.7 $ — $ 1,144.7 $ — $ 1,091.4 $ 53.3 December 31, 2019 Gross Amounts Recognized Gross Amounts Offset in the Balance Sheet Net Amounts in the Balance Sheet Financial Instruments Fair Value of Financial Collateral in the Balance Sheet Net Amount Financial Assets Interest rate swap contracts $ 21.9 $ — $ 21.9 $ 0.1 $ 18.0 $ 3.8 Mortgage related derivatives 1.3 — 1.3 — — 1.3 Total derivatives 23.2 — 23.2 0.1 18.0 5.1 Total assets $ 23.2 $ — $ 23.2 $ 0.1 $ 18.0 $ 5.1 Financial Liabilities Interest rate swap contracts $ 21.9 $ — $ 21.9 $ 0.1 $ — $ 21.8 Mortgage related derivatives 0.3 — 0.3 — — 0.3 Total derivatives 22.2 — 22.2 0.1 — 22.1 Repurchase agreements 697.6 — 697.6 — 697.6 — Total liabilities $ 719.8 $ — $ 719.8 $ 0.1 $ 697.6 $ 22.1 The following table presents the pre-tax gains or losses related to derivative contracts that were recorded in accumulated other comprehensive income and other non-interest income in the Company’s statements of income: As of or For The Year Ended December 31, 2020 2019 2018 Derivatives designated as hedges: Amount of loss recognized in other comprehensive income (effective portion) $ (0.1) $ — $ — Reclassification adjustment for derivative net gain included in income (0.1) — — Non-hedging interest rate derivatives: Amount of gain recognized in other non-interest income — — 0.3 Amount of net fee income recognized in other non-interest income 7.7 2.5 1.3 Amount of net gains (losses) recognized in mortgage banking revenues $ 1.2 $ 0.3 $ (0.5)

Mortgage Servicing Rights

Mortgage Servicing Rights	12 Months Ended
	Dec. 31, 2020
Transfers and Servicing [Abstract]
Mortgage Servicing Rights	MORTGAGE SERVICING RIGHTS Information with respect to the Company’s mortgage servicing rights follows: Year Ended December 31, 2020 2019 2018 Balance at beginning of year $ 30.6 $ 27.7 $ 24.8 Originations of mortgage servicing rights 11.7 7.3 6.0 Amortization expense (8.0) (4.4) (3.1) Balance at end of year 34.3 30.6 27.7 Less valuation reserve (10.3) (0.4) — Balance at end of year, net of valuation reserve $ 24.0 $ 30.2 $ 27.7 Principal balance of serviced loans underlying mortgage servicing rights $ 3,585.5 $ 3,710.1 $ 3,698.2 Mortgage servicing rights as a percentage of serviced loans 0.67 % 0.81 % 0.75 %

Deposits

Deposits	12 Months Ended
	Dec. 31, 2020
Deposits [Abstract]
Deposits	DEPOSITS Deposits are summarized as follows: December 31, 2020 2019 Non-interest bearing demand $ 4,633.5 $ 3,426.5 Interest bearing: Demand 4,118.9 3,195.4 Savings 4,405.9 3,591.6 Time, $100 and over 419.3 651.1 Time, other 639.4 798.9 Total interest bearing 9,583.5 8,237.0 Total deposits $ 14,217.0 $ 11,663.5 Other time deposits include zero and $2.9 million brokered deposits as of December 31, 2020 and 2019, respectively, and deposits obtained through the Company’s participation in the Certificate of Deposit Account Registry Service (“CDARS”). CDARS deposits totaled $97.3 million and $117.7 million as of December 31, 2020 and 2019, respectively. As of December 31, 2020 and 2019, the Company had time deposits of $193.0 million and $278.4 million, respectively, that met or exceeded the FDIC insurance limit of $250,000. Maturities of time deposits at December 31, 2020 are as follows: Time, $100 Total Time Due within 3 months or less $ 65.8 $ 271.1 Due after 3 months and within 6 months 77.2 190.8 Due after 6 months and within 12 months 173.0 355.5 Due within 2022 81.7 171.1 Due within 2023 7.3 30.0 Due within 2024 4.0 15.2 Due within 2025 and thereafter 10.3 25.0 Total $ 419.3 $ 1,058.7

Long-Term Debt and Other Borrow

Long-Term Debt and Other Borrowed Funds	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Long-Term Debt and Other Borrowed Funds	LONG-TERM DEBT AND OTHER BORROWED FUNDS A summary of long-term debt follows: December 31, 2020 2019 Parent Company: Fixed to floating subordinated notes, 5.25% fixed rate effective May 2020 through May 2025 $ 98.6 $ — Subsidiaries: 8.00% finance lease obligation with term ending October 25, 2029 1.1 1.2 2.28% note payable maturing July 29, 2022, principal due at maturity, interest payable monthly 5.0 5.0 1.00% note payable maturing December 31, 2041, interest only payable quarterly until December 31, 2025 and then principal and interest until maturity 5.1 5.1 Note payable maturing March 31, 2038, interest only payable at 1.30% monthly until March 31, 2025 and then principal and interest at 3.25% until maturity 2.0 2.0 1.30% note payable maturing June 1, 2034, interest only payable monthly until March 31, 2025 and then principal and interest until maturity 0.6 0.6 Total long-term debt $ 112.4 $ 13.9 Maturities of long-term debt at December 31, 2020 were as follows: 2021 $ 0.1 2022 5.2 2023 0.1 2024 0.1 2025 0.1 Thereafter 106.8 Total $ 112.4 The Company has available lines of credit with the FHLB of approximately $1,697.5 million, subject to collateral availability. As of December 31, 2020 and 2019, there were no long or short-term advances outstanding with the FHLB. The Company has a financing lease obligation on a banking office. Assets acquired under the financing lease, consist solely of a building and leasehold improvements, and are included in premises and equipment subject to depreciation. On May 15, 2020, the Company completed a public offering of $100.0 million fixed-to-floating rate subordinated notes due May 15, 2030 (the “Notes”). The debt is included in Tier 2 capital for the Company. The Company may elect to redeem the Notes, in whole or in part, on any early redemption date which is any interest payment date on or after May 15, 2025 at a redemption price equal to 100% of the principal amount plus any accrued and unpaid interest. The Company may also redeem the Notes, in whole but not in part, upon certain conditions as defined in the indenture agreement. Any early redemption of the Notes will be subject to regulatory approval. From and including the date of issuance to, but excluding, May 15, 2025, or earlier redemption date, the Notes bear interest at an initial fixed rate of 5.25% per annum, payable semi-annually in arrears on May 15 and November 15 of each year, which commenced on November 15, 2020. From and including May 15, 2025 to, but excluding, May 15, 2025, or earlier redemption date, the Notes will bear interest at a floating rate per annum equal to a benchmark rate, which is expected to be Three-Month Term SOFR (as defined in the Indenture Agreement), plus 518.0 basis points, payable quarterly in arrears on February 15, May 15, August 15 and November 15 of each year, commencing on August 15, 2025. Unamortized debt issuance costs of $1.4 million are being amortized to maturity. Subordinated debt is presented net of issuance costs on the consolidated balance sheet. The Notes are unsecured, subordinated obligations of the Company and: (i) rank junior to all of the Company’s existing and future senior indebtedness; (ii) rank equal in right of payment with any of the Company’s existing and future subordinated indebtedness; (iii) rank senior to the Company’s obligations relating to any junior subordinated debt securities issued to its capital trust subsidiaries; (iv) are effectively subordinated to all of the Company’s secured indebtedness to the extent of the value of the assets securing such indebtedness; and (v) are structurally subordinated to all of the existing and future liabilities and obligations of the Company’s subsidiaries, including deposit liabilities and claims of other creditors of the Company’s bank subsidiary, First Interstate Bank. Proceeds from the private placement of subordinated notes were used for general corporate purposes. Additionally, the Company borrowed or assumed through acquisitions $12.6 million as of December 31, 2020 and 2019, related to New Market Tax Credits. The long-term debt obligations consists of fixed rate note payables with various interest rates from 1.00% to 3.25% and maturities from July 29, 2022 through December 31, 2041, collateralized by the Company’s equity interest in various CDEs, which are 99.9% owned by the Company. As of December 31, 2020 and 2019, the Company had no material other borrowed funds. The Company has federal funds lines of credit with third parties amounting to $205.0 million, subject to funds availability. These lines are subject to cancellation without notice. The Company also has a line of credit with the Federal Reserve Bank for borrowings up to $478.6 million secured by a blanket pledge of indirect consumer loans, and has an unused $50.0 million revolving line of credit with U.S. Bank National Association.

Subordinated Debentures Held by

Subordinated Debentures Held by Subsidiary Trusts	12 Months Ended
	Dec. 31, 2020
Subordinated Borrowings [Abstract]
Subordinated Debentures Held by Subsidiary Trusts	SUBORDINATED DEBENTURES HELD BY SUBSIDIARY TRUSTS The Company sponsors seven wholly-owned business trusts, Trust I, Trust II, Trust III, Trust IV, Trust V, Trust VI, and Trust VII (collectively, the “Trusts”). The Trusts were formed for the exclusive purpose of issuing an aggregate of $84.4 million of 30-year floating rate mandatorily redeemable capital trust preferred securities (“Trust Preferred Securities”) to third-party investors. The Trusts also issued, in aggregate, $2.6 million of common equity securities to the Parent Company. Proceeds from the issuance of the Trust Preferred Securities and common equity securities were invested in 30-year junior subordinated deferrable interest debentures (“Subordinated Debentures”) issued by the Parent Company. A summary of Subordinated Debenture issuances follows: Principal Amount Outstanding Issuance Maturity Date 2020 2019 October 2007 January 1, 2038 $ 10.3 $ 10.3 November 2007 December 15, 2037 15.5 15.5 December 2007 December 15, 2037 20.6 20.6 December 2007 April 1, 2038 15.5 15.5 January 2008 April 1, 2038 10.3 10.3 January 2008 April 1, 2038 10.3 10.3 June 2005 June 30, 2035 4.5 4.4 Total subordinated debentures held by subsidiary trusts $ 87.0 $ 86.9 In October 2007, the Company issued $10.3 million of Subordinated Debentures to Trust II. The Subordinated Debentures bear a cumulative floating interest rate equal to LIBOR plus 2.25% per annum. As of December 31, 2020, the interest rate on the Subordinated Debentures was 2.48%. In November 2007, the Company issued $15.5 million of Subordinated Debentures to Trust I. The Subordinated Debentures bore interest at a fixed rate of 7.50% for five years after issuance until December 16, 2012, and thereafter at a variable rate equal to LIBOR plus 2.75% per annum. As of December 31, 2020, the interest rate on the Subordinated Debentures was 2.97%. In December 2007, the Company issued $20.6 million of Subordinated Debentures to Trust III. The Subordinated Debentures bore interest at a fixed rate of 6.88% for five years after issuance until December 15, 2012, and thereafter at a variable rate equal to LIBOR plus 2.40% per annum. As of December 31, 2020, the interest rate on the Subordinated Debentures was 2.62%. In December 2007, the Company issued $15.5 million of Subordinated Debentures to Trust IV. The Subordinated Debentures bear a cumulative floating interest rate equal to LIBOR plus 2.70% per annum. As of December 31, 2020 the interest rate on the Subordinated Debentures was 2.93%. In January 2008, the Company issued $10.3 million of Subordinated Debentures to Trust V. The Subordinated Debentures bore interest at a fixed rate of 6.78% for five years after issuance until April 1, 2013, and thereafter at a variable rate equal to LIBOR plus 2.75% per annum. As of December 31, 2020 the interest rate on the Subordinated Debentures was 2.98%. In January 2008, the Company issued $10.3 million of Subordinated Debentures to Trust VI. The Subordinated Debentures bear a cumulative floating interest rate equal to LIBOR plus 2.75% per annum. As of December 31, 2020, the interest rate on the Subordinated Debentures was 2.98%. In conjunction with the acquisition of Northwest in August 2018, the Company acquired Northwest Bancorporation Capital Trust I (“Trust VII”). The Northwest Trust was formed for the exclusive purpose of issuing an aggregate of $5.0 million of 30-year floating rate mandatorily redeemable capital trust preferred securities (“Northwest Trust Preferred Securities”) to third-party investors. The Trusts also issued, in aggregate, $0.2 million of common equity securities to Northwest. Proceeds from the issuance of the Trust Preferred Securities and common equity securities were invested in 30-year junior subordinated deferrable interest debentures (“Northwest Subordinated Debentures”) issued by Northwest. The Subordinated Debentures bore interest at a fixed rate of 5.95% for five years after issuance until June 30, 2010, and thereafter at a variable rate equal to LIBOR plus 1.70% per annum. As of December 31, 2020 the interest rate on the Subordinated Debentures was 1.94%. The Subordinated Debentures are unsecured with interest distributions payable quarterly. The Company may defer the payment of interest at any time provided that the deferral period does not extend past the stated maturity. During any such deferral period, distributions on the Trust Preferred Securities will also be deferred and the Company’s ability to pay dividends on its common and preferred shares is restricted. The Subordinated Debentures may be redeemed, subject to approval by the Federal Reserve Bank, at the Company’s option on or after five years from the date of issue, or at any time in the event of unfavorable changes in laws or regulations. Debt issuance costs consisting primarily of underwriting discounts and professional fees were capitalized and are being amortized through maturity to interest expense using the straight-line method, which approximates level yield. The terms of the Trust Preferred Securities are identical to those of the Subordinated Debentures. The Trust Preferred Securities are subject to mandatory redemption upon repayment of the Subordinated Debentures at their stated maturity dates or earlier redemption in an amount equal to their liquidation amount plus accumulated and unpaid distributions to the date of redemption. The Company guarantees the payment of distributions and payments for redemption or liquidation of the Trust Preferred Securities to the extent of funds held by the Trusts.

Capital Stock and Dividend Rest

Capital Stock and Dividend Restrictions	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Capital Stock and Dividend Restrictions	CAPITAL STOCK AND DIVIDEND RESTRICTIONS The Company’s authorized common stock consists of 200,000,000 shares, of which, 100,000,000 shares are designated as Class A common stock and 100,000,000 are designated as Class B common stock. The Class A common stock has one vote per share. The Class B common stock has five votes per share and is convertible to Class A common stock on a share-for-share basis at any time. The Company had 40,335,113 shares of Class A common stock and 21,760,686 shares of Class B common stock outstanding as of December 31, 2020. The Company had 43,129,085 shares of Class A common stock and 22,117,254 shares of Class B common stock outstanding as of December 31, 2019. During 2020, the Company issued 19,491 shares of its Class A common stock with an aggregate value of $0.6 million to directors for their service on the Company’s board of directors during 2020. During 2019, the Company issued 22,417 shares of its Class A common stock with an aggregate value of $0.8 million to directors for their service on the Company’s board of directors during 2019. The aggregate value of the shares issued to directors is included in stock-based compensation expense in the accompanying consolidated statements of changes in stockholders’ equity. On June 11, 2019, the Company’s board of directors adopted a stock repurchase program permitting the Company to repurchase up to 2.5 million of its outstanding shares of Class A common stock. On March 23, 2020, the Company’s board of directors suspended stock repurchases in response to the COVID-19 pandemic. Effective August 24, 2020, the Company’s board of directors lifted the temporary suspension of the Company’s stock repurchase program. On September 12, 2020, the Company’s board of directors increased the number of shares of Class A common stock authorized to be repurchased by the Company under the stock repurchase program by an additional 3.0 million shares bringing the total number of shares authorized under the program to 5.5 million shares. During 2020, the Company repurchased and retired 3,538,142 shares of Class A common stock under the stock repurchase program. All other stock repurchases during 2020 and 2019 were redemptions of vested restricted shares tendered in lieu of cash for payment of income tax withholding amounts by participants in the Company’s equity compensation plans. On April 8, 2019, the Company issued 3,871,422 and 463,134 shares of its Class A common stock with an aggregate value of $157.3 million and $18.8 million as consideration for the acquisitions of IIBK and CMYF, respectively.

Earnings per Common Share

Earnings per Common Share	12 Months Ended
	Dec. 31, 2020
Earnings Per Share [Abstract]
Earnings per Common Share	EARNINGS PER COMMON SHARE Basic earnings per common share is calculated by dividing net income by the weighted average number of common shares outstanding during the period presented, excluding unvested restricted stock. Diluted earnings per share is calculated by dividing net income by the weighted average number of common shares determined for the basic earnings per share computation plus the dilutive effects of stock-based compensation using the treasury stock method. The following table sets forth the computation of basic and diluted earnings per common share: Year Ended December 31, 2020 2019 2018 Net income, basic and diluted $ 161.2 $ 181.0 $ 160.2 Weighted average common shares outstanding for basic earnings per share computation 63,611,891 63,645,029 57,778,857 Dilutive effects of stock-based compensation 117,579 239,839 438,266 Weighted average common shares outstanding for diluted earnings per common share computation 63,729,470 63,884,868 58,217,123 Basic earnings per common share $ 2.53 $ 2.84 $ 2.77 Diluted earnings per common share 2.53 2.83 2.75

Regulatory Capital

Regulatory Capital	12 Months Ended
	Dec. 31, 2020
Banking and Thrift [Abstract]
Regulatory Capital	REGULATORY CAPITAL The Company and the Bank are subject to various regulatory capital requirements administered by federal banking regulators and the Federal Reserve. Failure to meet minimum capital requirements can initiate certain mandatory and possibly additional discretionary actions by regulators that, if undertaken, could have a direct material effect on the Company’s and the Bank’s financial statements. Under capital adequacy guidelines and the regulatory framework for prompt corrective action, the Company and the Bank must meet specific capital guidelines that involve quantitative measures of the Company’s and Bank’s assets, liabilities and certain off-balance sheet items as calculated under regulatory accounting practices. The Parent Company, like all bank holding companies, is not subject to the prompt corrective action provisions. The Company’s and the Bank’s capital amounts and classification are also subject to qualitative judgments by the regulators about components, risk weightings and other factors. Quantitative measures established by regulation to ensure capital adequacy require the Company to maintain minimum amounts and ratios of total and tier 1 capital to risk-weighted assets, and of tier 1 capital to average assets, as defined in the regulations. As of December 31, 2020, the Company exceeded all capital adequacy requirements to which it is subject. As of December 31, 2020, the most recent notification from the regulatory agencies categorized the Bank as well capitalized under the regulatory framework for prompt corrective action. To be categorized as well-capitalized, the institution must maintain minimum total risk-based, Tier 1 risk-based, and Tier 1 leverage ratios as set forth in the following table. There are no conditions or events since the most recent notification that management believes have changed the Bank's categories. As an approved mortgage seller, the Bank is required to maintain a minimum level of capital specified by the United States Department of Housing and Urban Development. At December 31, 2020 and 2019, the Bank met these requirements. The Company’s actual capital amounts and ratios and selected minimum regulatory thresholds and prompt corrective action provisions as of December 31, 2020 and 2019 are presented in the following tables: Actual Minimum Required for Capital Adequacy Purposes For Capital Adequacy Purposes Plus Capital Conservation Buffer Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements (1) December 31, 2020 Amount Ratio Amount Ratio Amount Ratio Amount Ratio Total risk-based capital: Consolidated $ 1,575.7 14.19 $ 888.3 8.00 % $ 1,165.8 10.50 % $ 1,110.3 10.00 % FIB 1,426.8 12.89 885.6 8.00 1,162.3 10.50 1,107.0 10.00 Tier 1 risk-based capital: Consolidated 1,369.0 12.33 666.2 6.00 943.8 8.50 888.3 8.00 FIB 1,320.1 11.93 664.2 6.00 940.9 8.50 885.6 8.00 Common equity tier 1 risk-based capital: Consolidated 1,284.9 11.57 499.6 4.50 777.2 7.00 721.7 6.50 FIB 1,320.1 11.93 498.1 4.50 774.9 7.00 719.5 6.50 Leverage capital ratio: Consolidated 1,369.0 8.16 671.0 4.00 671.0 4.00 838.7 5.00 FIB 1,320.1 7.88 669.7 4.00 669.7 4.00 837.2 5.00 (1) The ratios for the well capitalized requirement are only applicable to FIB. However, the Company manages its capital position as if the requirement applies to the consolidated entity and has presented the ratios as if they also applied on a consolidated basis. In connection with the adoption of CECL, or ASC 326, the Company recognized an after-tax cumulative effect reduction to retained earnings totaling $24.1 million. In March 2020, the Office of the Comptroller of Currency, the Board of Governors of the Federal Reserve System, and the FDIC issued an interim final rule that allows banking organizations to mitigate the effects of ASC 326 on their regulatory capital computations. This interim rule is in addition to the three-year transition period already in place under the capital transition rule previously issued in February 2019. Banking organizations can elect to mitigate the estimated cumulative regulatory capital effects for an additional two years. This rule allows an institution to defer transitioning the impact of ASC 326 into its regulatory capital calculation, including ratios, over an extended period. Additionally, the interim rule extends the transition period whereby an institution can defer the impact from ASC 326 on the current period, determined based on the difference between the new ASC 326 allowance for credit losses and the allowance for loan losses under the incurred loss method from previous GAAP, for up to two years. The total impact related to ASC 326 would then be transitioned into regulatory capital and the associated ratios over a three-year transition period, beginning after the initial two-year deferral period, for a total transition period of five years. The Company has elected to opt into the transition election and is adopting transition relief over the permissible five-year period. Actual Minimum Required for Capital Adequacy Purposes For Capital Adequacy Purposes Plus Capital Conservation Buffer Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements (1) December 31, 2019 Amount Ratio Amount Ratio Amount Ratio Amount Ratio Total risk-based capital: Consolidated $ 1,495.3 14.10 % $ 848.5 8.00 % $ 1,113.6 10.50 % $ 1,060.6 10.00 % FIB 1,321.4 12.50 845.8 8.00 1,110.1 10.50 1,057.2 10.00 Tier 1 risk-based capital: Consolidated 1,422.3 13.41 636.3 6.00 901.5 8.50 848.5 8.00 FIB 1,248.4 11.81 634.3 6.00 898.6 8.50 845.8 8.00 Common equity tier 1 risk-based capital: Consolidated 1,338.2 12.62 477.3 4.50 742.4 7.00 689.4 6.50 FIB 1,248.4 11.81 475.7 4.50 740.0 7.00 687.2 6.50 Leverage capital ratio: Consolidated 1,422.3 10.13 561.6 4.00 561.6 4.00 702.0 5.00 FIB 1,248.4 8.91 560.4 4.00 560.4 4.00 700.4 5.00 (1) The ratios for the well capitalized requirement are only applicable to FIB. However, the Company manages its capital position as if the requirement applies to the consolidated entity and has presented the ratios as if they also applied on a consolidated basis.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	COMMITMENTS AND CONTINGENCIES The Company had commitments under construction contracts of $4.6 million as of December 31, 2020. The Parent Company and the Billings office of FIB are the anchor tenants in a building owned by an entity in which FIB has a 50.0% ownership interest. The Company leases certain premises and equipment from third parties under operating leases. Total rental expense to third parties was $5.1 million, $4.8 million, and $3.2 million, in 2020, 2019, and 2018, respectively. The total future minimum rental commitments, exclusive of maintenance and operating costs, required under operating leases that have initial or remaining noncancelable lease terms in excess of one year at December 31, 2020, are as follows: Third Related Total For the year ending December 31: 2021 $ 5.6 $ 1.4 $ 7.0 2022 5.2 1.4 6.6 2023 4.0 1.1 5.1 2024 3.5 1.0 4.5 2025 3.1 1.0 4.1 Thereafter 13.4 2.0 15.4 Total $ 34.8 $ 7.9 $ 42.7 Residential mortgage loans sold to investors in the secondary market are sold with varying recourse provisions. Essentially all the loan sales agreements require the repurchase of a mortgage loan by the seller in situations such as breach of representation, warranty, or covenant; untimely document delivery; false or misleading statements; failure to obtain certain certificates or insurance; or unmarketability. Certain loan sales agreements contain repurchase requirements based on payment-related defects that are defined in terms of the number of days or months since the purchase, the sequence number of the payment, and/or the number of days of payment delinquency. Based on the specific terms stated in the agreements, the Company had $0.6 million and $0.9 million of sold residential mortgage loans with recourse provisions still in effect as of December 31, 2020 and 2019, respectively. The Company did not repurchase any significant amount of loans from secondary market investors under the terms of loan sales agreements during the years ended December 31, 2020, 2019 and 2018. In the opinion of management, the risk of recourse and the subsequent requirement of loan repurchase to the Company is not significant, and accordingly no liabilities have been established related to such. In addition, the Company made various representations and warranties associated with the sale of loans. The Company has not incurred significant losses resulting from these provisions. A substantial portion of the Company’s clients’ ability to honor their contracts is dependent on the economy in Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming. The Company’s loan portfolio is diversified and assigned to risk classifications by industry concentrations and the current economic conditions. These industry concentrations of credit are taken into consideration by management in determining the allowance for credit losses. In the normal course of business, the Company is involved in various other claims and litigation. In the opinion of management, following consultation with legal counsel, the ultimate liability or disposition thereof is not expected to have a material adverse effect on the consolidated financial condition, results of operations, or liquidity of the Company.

Financial Instruments with Off-

Financial Instruments with Off-Balance Sheet Risk	12 Months Ended
	Dec. 31, 2020
Financial Instruments with Off-Balance Sheet Risk [Abstract]
Financial Instruments with Off-Balance Sheet Risk	FINANCIAL INSTRUMENTS WITH OFF-BALANCE SHEET RISK The Company is a party to financial instruments with off-balance sheet risk in the normal course of business to meet the financing needs of its clients. These financial instruments include commitments to extend credit and standby letters of credit. These instruments involve, to varying degrees, elements of credit and interest rate risk in excess of amounts recorded in the consolidated balance sheets. Commitments to extend credit are agreements to lend to a client as long as there is no violation of any condition established in the commitment contract. Since many of the commitments are expected to expire without being drawn upon, the total commitment amounts do not necessarily represent future cash requirements. Standby letters of credit are conditional commitments issued by the Company to guarantee the performance of a client to a third party. The credit risk involved in issuing letters of credit is essentially the same as that involved in extending loan facilities to clients. The Company’s policy for obtaining collateral, and the nature of such collateral, is essentially the same as that involved in making commitments to extend credit. The estimated fair value of the obligation undertaken by the Company in issuing standby letters of credit is included in accounts payable and accrued expenses in the Company’s consolidated balance sheets. The following table presents our financial instruments with off-balance sheet risk, as well as the activity in the allowance for off-balance sheet credit losses related to those financial instruments not considered unconditionally cancellable: December 31, December 31, Beginning balance $ — $ — Initial impact of adopting ASC 326 2.3 — Provision for credit loss expense 1.4 — Ending balance of allowance for off-balance sheet credit losses $ 3.7 $ — December 31, December 31, Unused credit card lines* $ 682.8 $ 671.8 Commitments to extend credit* 2,280.0 2,067.0 Standby letter of credit 59.0 42.7 * In conjunction with the adoption of ASC 326, the Company presented the December 31, 2019 balances to conform to the December 30, 2020 presentation.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Income Taxes	INCOME TAXES Income tax expense consists of the following: Year ended December 31, 2020 2019 2018 Current: Federal $ 42.4 $ 40.5 $ 23.1 State 12.3 8.2 7.2 Total current 54.7 48.7 30.3 Deferred: Federal (5.7) 3.7 12.3 State (0.9) 1.7 3.5 Total deferred (6.6) 5.4 15.8 Total income tax expense $ 48.1 $ 54.1 $ 46.1 Total income tax provision differs from the amount of income tax determined by applying the statutory federal income tax rate of 21% for the periods presented to income before income taxes due to the following: Year ended December 31, 2020 2019 2018 Tax expense at the statutory tax rate $ 44.0 $ 49.4 $ 43.3 Increase (decrease) in tax resulting from: Tax-exempt income (2.1) (2.8) (2.8) State income tax, net of federal income tax benefit 9.0 9.9 8.5 Benefit of stock-based compensation plans (0.4) (1.2) (1.1) Federal tax credits (2.3) (2.0) (2.6) Other, net (0.1) 0.8 0.8 Tax expense at effective tax rate $ 48.1 $ 54.1 $ 46.1 The tax effects of temporary differences between the financial statement carrying amounts and tax bases of assets and liabilities that give rise to significant portions of the net deferred tax asset (liability) relate to the following: December 31, 2020 2019 Deferred tax assets: Loans, principally due to allowance for credit losses $ 36.5 $ 18.1 Loan discount 4.0 7.2 Deferred compensation 19.5 18.1 Non-performing loan interest 1.2 1.0 Other real estate owned write-downs and carrying costs — 0.1 Tax credit carryforwards — 0.2 Net operating loss carryforwards (1) 2.0 2.6 Lease liabilities 9.5 9.9 Other 3.2 0.9 Deferred tax assets 75.9 58.1 Deferred tax liabilities: Fixed assets, principally differences in bases and depreciation (10.9) (8.8) Deferred loan costs (4.4) (2.8) Investment securities, unrealized gains (19.2) (3.9) Investment in joint venture partnership, principally due to differences in depreciation of partnership assets (0.9) (0.7) Right of use assets (9.2) (9.7) Prepaid amounts (0.5) (0.5) Government agency stock dividends (1.2) (1.2) Goodwill and core deposit intangibles (50.3) (49.2) Mortgage servicing rights (5.6) (7.2) Other (0.9) (0.8) Deferred tax liabilities (103.1) (84.8) Net deferred tax assets (liabilities) $ (27.2) $ (26.7) (1) As of December 31, 2020, we had remaining federal net operating loss carryforwards of $3.4 million from acquired companies, which is available to offset federal taxable income and state net operating loss carryforwards in amounts which vary by state. The federal net operating losses will expire beginning in 2030 and ending in 2036 and the state net operating losses will expire beginning in 2023 and ending in 2034. The use of these carryforwards is subject to annual limitations. The Company had current net income tax receivables of $8.1 million and $6.1 million at December 31, 2020 and 2019, respectively.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Stock-Based Compensation	STOCK-BASED COMPENSATION The Company has equity awards outstanding under two stock-based compensation plans; the 2015 Equity Incentive Plan (the “2015 Plan”) and the 2006 Equity Compensation Plan, as amended and restated (the “2006 Plan”). These plans were primarily established to enhance the Company’s ability to attract, retain and motivate employees. The Company’s Board of Directors or, upon delegation, the Compensation Committee of the Board of Directors (“Compensation Committee”) has exclusive authority to select employees, advisors and others, including directors, to receive awards and to establish the terms and conditions of each award made pursuant to the Company’s stock-based compensation plans. The 2015 Plan, approved by the Company’s shareholders in May 2015, was established to provide the Company with flexibility to select from various equity-based performance compensation methods, and to be able to address changing accounting and tax rules and corporate governance practices by optimally utilizing performance based compensation. The 2015 Plan did not increase the number of shares of common stock available for awards under the 2006 Plan. The 2006 Plan, approved by the Company’s shareholders in May 2006 and May 2014, was established to consolidate into one plan the benefits available under all other than existing share-based award plans. The 2006 Plan continues with respect to awards made prior to June 2015. All shares of common stock available for future grant under the 2006 Plan were transferred into the 2015 Plan. At December 31, 2020, there were 1,141,186 common shares available for future grant under the 2015 Plan. Stock Options. All options granted have an exercise price equal to fair market value, which is currently defined as the closing sales price for the stock as quoted on the NASDAQ Stock Market for the last market trading day preceding the date that the Company’s Board of Directors awards the benefit. Options may be subject to vesting as determined by the Company’s Board of Directors or Compensation Committee, and can be exercised for periods of up to ten years from the date of grant. No stock option awards were granted in 2020 or 2019. All outstanding stock option awards were fully vested as of December 31, 2016. As such, there was no compensation expense or related income tax benefits recognized related to stock option awards in 2020 or 2019. Compensation expense related to stock option awards and the related income tax benefits for the year ended December 31, 2016 were not considered material. The following table summarizes Class A and Class B stock option activity under the Company’s active stock option plans: Year Ended December 31, 2020 Number of Weighted-Average Weighted-Average Outstanding options, beginning of year 221,197 $ 15.33 Granted — — Exercised (139,795) 15.80 Forfeited (2,084) 16.35 Expired — — Outstanding options, end of year 79,318 $ 14.49 0.83 Outstanding options exercisable, end of year 79,318 $ 14.49 0.83 The total intrinsic value of fully-vested stock options outstanding as of December 31, 2020 was $2.1 million. The total intrinsic value of options exercised was $3.1 million, $4.9 million, and $4.9 million during the years ended December 31, 2020, 2019, and 2018, respectively. The actual tax benefit realized for the tax deduction from option exercises totaled $0.5 million, $0.9 million, and $0.9 million for the years ended December 31, 2020, 2019 and 2018, respectively. The Company received cash of $1.1 million, $1.0 million, and $1.8 million from stock option exercises during the years ended December 31, 2020, 2019, and 2018, respectively. The Company redeemed common stock with aggregate values of $1.0 million, $2.0 million, and $1.6 million tendered in payment for stock option exercises during the years ended December 31, 2020, 2019, and 2018, respectively. Restricted Stock Awards. Common stock issued under the Company’s restricted stock plan may not be sold or otherwise transferred until restrictions have lapsed or performance objectives have been obtained. During the vesting periods, participants have voting rights and receive dividends on all time restricted shares and vesting performance restricted shares. Upon termination of employment, common shares upon which restrictions have not lapsed must be returned to the Company. All restricted share awards are classified as equity awards. The fair value of equity-classified restricted stock awards is amortized as compensation expense on a straight-line basis over the period restrictions lapse or performance goals are met. Compensation expense related to restricted stock awards of $7.5 million, $8.0 million and $5.6 million was included in benefits on the Company’s consolidated statements of income for the years ended December 31, 2020, 2019 and 2018, respectively. Related income tax expense of $0.1 million was recognized for the year ended December 31, 2020 and related income tax benefit of $0.4 million and $0.2 million were recognized for the years ended December 31, 2019 and 2018, respectively. The following table presents information regarding the Company’s restricted stock: As of December 31, 2020 Number of Weighted-Average Restricted stock, beginning of year 363,022 $ 41.47 Granted 332,085 28.56 Vested (135,366) 40.11 Forfeited (34,912) 35.50 Canceled — — Restricted stock, end of year 524,829 $ 33.65 During 2020, the Company issued 332,085 restricted common shares. The 2020 restricted share awards included 1,352 additional shares related to the 2017 performance restricted stock grants and 196,278 performance restricted shares, of which 98,139 vest in varying percentages upon achievement of defined return on equity performance goals, and 98,139 vest in varying percentages upon achievement of defined total return to shareholder goals. Vesting of the 2020 performance restricted shares is also contingent on employment as of March 15, 2023. Additionally, 134,455 time-restricted shares were issued during 2020 that vest one-third on each annual anniversary of the grant date through February 15, 2023, contingent on continued employment through the vesting date. As of December 31, 2020, there was $9.5 million of unrecognized compensation cost related to non-vested, restricted stock awards expected to be recognized over a period of 1.33 years.

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2020
Compensation Related Costs [Abstract]
Employee Benefit Plans	EMPLOYEE BENEFIT PLANS Profit Sharing Plan. The Company had a noncontributory profit sharing plan which was terminated on January 1, 2020. All employees, other than temporary employees, working 20 hours or more per week were eligible to participate in the profit sharing plan. The Company’s Board of Directors authorized all contributions to the profit sharing plan. Participants became 100% vested upon the completion of two years of vesting service. Accrued contribution expense for this plan of $2.1 million and $2.4 million in 2019 and 2018, respectively, is included in employee benefits expense in the Company’s consolidated statements of income. Savings Plan. In addition, the Company has a contributory employee savings plan. All employees are eligible to participate in the plan. Employee participation in the plan is at the option of the employee. The Company contributed 100% of the first 6% and 5% of the participating employee’s eligible compensation in 2020 and 2019, respectively. Contribution expense for this plan of $8.9 million, $7.0 million, and $6.3 million in 2020, 2019, and 2018, respectively, is included in employee benefits expense in the Company’s consolidated statements of income. Post-Retirement Healthcare Plan. The Company sponsors a contributory defined benefit healthcare plan (the “Plan”) for active employees and employees and directors retiring from the Company at the age of at least 55 years and with at least 15 years of continuous service. Retired Plan participants contribute the full cost of benefits based on the average per capita cost of benefit coverage for both active employees and retired Plan participants. In 2016, the Company amended the Plan to discontinue offering healthcare benefits to future retirees beginning July 1, 2016, with current retirees as of July 1, 2016 continuing in the Plan. The Plan’s unfunded benefit obligation of $0.3 million and $0.4 million as of December 31, 2020 and 2019, respectively, is included in accounts payable and accrued expenses in the Company’s consolidated balance sheets. Net periodic benefit costs of $0.5 million, $0.7 million and $0.8 million for the years ended December 31, 2020, 2019 and 2018, respectively, are included in employee benefits expense in the Company’s consolidated statements of income.

Other Comprehensive Income

Other Comprehensive Income	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Other Comprehensive Income	OTHER COMPREHENSIVE INCOME The gross amounts of each component of other comprehensive income and the related tax effects for the periods indicated are as follows: Year Ended December 31, 2020 Before Tax Amount Tax Expense (Benefit) Net of Tax Amount Investment securities available-for sale: Change in net unrealized gains during period $ 61.8 $ 15.8 $ 46.0 Reclassification adjustment for net gains included in net income (0.3) (0.1) (0.2) Change in net unrealized loss on derivatives 0.2 — 0.2 Defined benefits post-retirement benefit plan: Change in net actuarial gains (0.5) (0.1) (0.4) Total other comprehensive income $ 61.2 $ 15.6 $ 45.6 Year Ended December 31, 2019 Before Tax Amount Tax Expense (Benefit) Net of Tax Amount Investment securities available-for sale: Change in net unrealized gains during period $ 54.9 $ 14.1 $ 40.8 Reclassification adjustment for net gains included in net income (0.1) — (0.1) Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale (6.0) (1.6) (4.4) Defined benefits post-retirement benefit plan: Change in net actuarial gains (0.8) (0.1) (0.7) Total other comprehensive income $ 48.0 $ 12.4 $ 35.6 Year Ended December 31, 2018 Before Tax Amount Tax Expense (Benefit) Net of Tax Amount Investment securities available-for sale: Change in net unrealized loss during period $ (13.9) $ (3.6) $ (10.3) Reclassification adjustment for net loss included in net income 0.1 — 0.1 Change in unamortized loss on available-for-sale securities transferred into held-to-maturity 1.6 0.4 1.2 Defined benefits post-retirement benefit plan: Change in net actuarial loss (0.6) (0.1) (0.5) Total other comprehensive loss $ (12.8) $ (3.3) $ (9.5) The components of accumulated other comprehensive income, net of income taxes, are as follows: Year ended December 31, 2020 2019 Net unrealized gain on investment securities available-for-sale $ 56.8 $ 10.6 Net unrealized loss on derivatives (0.2) — Net actuarial gain on defined benefit post-retirement benefit plans — 0.4 Net accumulated other comprehensive income (loss) $ 56.6 $ 11.0

Condensed Financial Information

Condensed Financial Information (Parent Company Only)	12 Months Ended
	Dec. 31, 2020
Condensed Financial Information Disclosure [Abstract]
Condensed Financial Information (Parent Company Only)	CONDENSED FINANCIAL INFORMATION (PARENT COMPANY ONLY) Following is condensed financial information of First Interstate BancSystem, Inc. December 31, 2020 2019 Condensed balance sheets: Cash and cash equivalents $ 123.2 $ 147.1 Investment in bank subsidiary 1,986.6 1,899.3 Advances to subsidiaries, net 41.0 48.9 Other assets 61.3 61.7 Total assets $ 2,212.1 $ 2,157.0 Other liabilities $ 66.7 $ 56.2 Long-term debt 98.6 — Subordinated debentures held by subsidiary trusts 87.0 86.9 Total liabilities 252.3 143.1 Stockholders’ equity 1,959.8 2,013.9 Total liabilities and stockholders’ equity $ 2,212.1 $ 2,157.0 Years Ended December 31, 2020 2019 2018 Condensed statements of income: Dividends from subsidiaries $ 130.0 $ 178.0 $ 148.5 Other interest income 0.1 0.3 0.1 Other income, primarily management fees from subsidiaries 28.7 25.9 17.0 Total income 158.8 204.2 165.6 Salaries and benefits 31.5 34.2 25.3 Interest expense 6.6 4.7 4.5 Acquisition expenses — 17.0 8.1 Other operating expenses, net 15.6 14.8 14.5 Total expenses 53.7 70.7 52.4 Earnings before income tax benefit 105.1 133.5 113.2 Income tax benefit (6.1) (11.9) (9.3) Income before undistributed earnings of subsidiaries 111.2 145.4 122.5 Undistributed earnings of subsidiaries 50.0 35.6 37.7 Net income $ 161.2 $ 181.0 $ 160.2 Years Ended December 31, 2020 2019 2018 Condensed statements of cash flows: Cash flows from operating activities: Net income $ 161.2 $ 181.0 $ 160.2 Adjustments to reconcile net income to cash provided by operating activities: Undistributed earnings of subsidiaries (50.0) (35.6) (37.7) Stock-based compensation expense 7.5 8.0 5.6 Other, net (13.6) 8.1 16.4 Net cash provided by operating activities 105.1 161.5 144.5 Cash flows from investing activities: Acquisition of bank holding company, net of cash and cash equivalents received — — (14.7) Net cash used in investing activities — — (14.7) Years Ended December 31, 2020 2019 2018 Cash flows from financing activities: Net (decrease) increase in advances from subsidiaries 16.7 (6.6) (9.9) Proceeds from issuance of long-term debt 98.6 — — Repayment of long-term debt — — (26.0) Proceeds from issuance of common stock, net of stock issuance costs 1.1 1.0 1.8 Purchase and retirement of common stock (116.8) (2.5) (1.0) Dividends paid to common stockholders (128.6) (79.2) (64.1) Net cash used in financing activities (129.0) (87.3) (99.2) Net change in cash and cash equivalents (23.9) 74.2 30.6 Cash and cash equivalents, beginning of year 147.1 72.9 42.3 Cash and cash equivalents, end of year $ 123.2 $ 147.1 $ 72.9

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurements	FAIR VALUE MEASUREMENTS Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. There is a fair value hierarchy which requires an entity to maximize the use of observable inputs and minimize the use of unobservable inputs when measuring fair value. The three levels of inputs that may be used to measure fair value are as follows: • Level 1 - Quoted prices in active markets for identical assets or liabilities • Level 2 - Observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities • Level 3 - Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of assets or liabilities The methodologies used by the Company in determining the fair values of each class of financial instruments are based primarily on the use of independent, market-based data to reflect a value that would be reasonably expected in an orderly transaction between market participants at the measurement date, and therefore are classified within Level 2 of the valuation hierarchy. There have been no significant changes in the valuation techniques during the periods ended December 31, 2020 and 2019. The Company’s policy is to recognize transfers between levels as of the end of the reporting period. Transfers in and out of Level 1, Level 2 and Level 3 are recognized on the actual transfer date. There were no transfers between fair value hierarchy levels during the years ended December 31, 2020 and 2019. Further details on the methods used to estimate the fair value of each class of financial instruments above are discussed below: Investment Debt Securities Available-for-Sale . The Company obtains fair value measurements for investment securities from an independent pricing service. The fair value measurements consider observable data that may include dealer quotes, market spreads, cash flows, the U.S. Treasury yield curve, live trading levels, trade execution data, market consensus prepayment speeds, credit information, and the investment’s terms and conditions, among other things. Vendors chosen by the Company are widely recognized vendors whose evaluations support the pricing functions of financial institutions, investment and mutual funds, and portfolio managers. If needed, a broker may be utilized to determine the reported fair value of investment securities. Loans Held for Sale. Fair value measurements for loans held for sale are obtained from an independent pricing service. The fair value measurements consider observable data that may include binding contracts or quotes or bids from third party investors as well as loan level pricing adjustments. Interest Rate Swap Contracts. Fair values for derivative interest rate swap contracts are based upon the estimated amounts to settle the contracts considering current interest rates and are calculated using discounted cash flows that are observable or that can be corroborated by observable market data. The inputs used to determine fair value include the three-month LIBOR forward curve to estimate variable rate cash inflows and the federal funds effective swap rate to estimate the discount rate. The estimated variable rate cash inflows are compared to the fixed rate outflows and such difference is discounted to a present value to estimate the fair value of the interest rate swaps. The change in the value of derivative assets attributable to basis risk, or the risk that offsetting investments in a hedging strategy will not experience price changes in entirely opposite directions from each other, was not significant in the reported periods. The Company also obtains and compares the reasonableness of the pricing from an independent third party. For purposes of potential valuation adjustments to our derivative positions, we evaluate the credit risk of our counterparties as well as ours. Accordingly, we have considered factors such as the likelihood of our default and the default of our counterparties, our net exposures and remaining contractual life, among other things, in determining if any fair value adjustments related to credit risk are required. The change in value of derivative assets and derivative liabilities attributable to credit risk was not significant during the reported periods. Interest Rate Lock Commitments. Fair value measurements for interest rate lock commitments are obtained from an independent pricing service. The fair value measurements consider observable data that may include prices available from secondary market investors taking into consideration various characteristics of the loan, including the loan amount, interest rate, value of the servicing, and loan to value ratio, among other things. Observable data is then adjusted to reflect changes in interest rates, the Company’s estimated pull-through rate, and estimated direct costs necessary to complete the commitment into a closed loan net of origination and processing fees collected from the borrower. Forward Loan Sales Contracts. The fair value measurements for forward loan sales contracts are obtained from an independent pricing service. The fair value measurements consider observable data that includes sales of similar loans. Deferred Compensation Plan Assets and Liabilities. The fair values of deferred compensation plan assets and liabilities are based primarily on the use of independent, market-based data to reflect a value that would be reasonably expected in an orderly transaction between market participants at the measurement date. These investments are in the same funds and purchased in the same amounts as the participants’ selected investments, which represent the underlying liabilities to plan participants. Deferred compensation plan liabilities are recorded at amounts due to participants, based on the fair value of participants’ selected investments. Financial assets and financial liabilities measured at fair value on a recurring basis are as follows: Fair Value Measurements at Reporting Date Using As of December 31, 2020 Balance Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Investment debt securities available-for-sale: State, county, and municipal securities $ 465.9 $ — $ 465.9 $ — Obligations of U.S. government agencies 331.9 — 331.9 — U.S. agency mortgage-backed securities & collateralized mortgage obligations 2,897.6 — 2,897.6 — Private mortgage-backed securities 10.9 — 10.9 — Corporate securities 302.2 — 302.2 — Other investments 0.2 — 0.2 — Loans held for sale 74.0 — 74.0 — Derivative assets: Interest rate swap contracts 52.0 — 52.0 — Interest rate lock commitments 3.3 — 3.3 — Derivative liabilities: Interest rate swap contracts 52.2 — 52.2 — Forward loan sales contracts 1.1 — 1.1 — Deferred compensation plan assets 19.1 — 19.1 — Deferred compensation plan liabilities 19.1 — 19.1 — Fair Value Measurements at Reporting Date Using As of December 31, 2019 Balance Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Investment debt securities available-for-sale: U.S. Treasury Notes $ 9.0 $ — $ 9.0 $ — State, county, and municipal securities 80.9 — 80.9 — Obligations of U.S. government agencies 366.8 — 366.8 — U.S. agency mortgage-backed securities & collateralized mortgage obligations 2,317.2 — 2,317.2 — Private mortgage-backed securities 47.2 — 47.2 — Corporate securities 135.7 — 135.7 — Other investments 3.2 — 3.2 — Loans held for sale 100.9 — 100.9 — Derivative assets: Interest rate swap contracts 21.9 — 21.9 — Interest rate lock commitments 1.3 — 1.3 — Derivative liabilities: Interest rate swap contracts 21.9 — 21.9 — Forward loan sales contracts 0.3 — 0.3 — Deferred compensation plan assets 18.2 — 18.2 — Deferred compensation plan liabilities 18.2 — 18.2 — Additionally, from time to time, certain assets are measured at fair value on a non-recurring basis. Adjustments to fair value generally result from the application of lower-of-cost-or-market accounting or write-downs of individual assets due to credit deterioration. The following table presents information about the Company’s assets and liabilities measured at fair value on a non-recurring basis: Fair Value Measurements at Reporting Date Using As of December 31, 2020 Total Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Total Gains (Losses) Collateral-dependent loans* $ 14.7 $ — $ — $ 14.7 $ (2.8) Long-lived assets to be disposed of by sale 5.3 — — 5.3 (0.2) Fair Value Measurements at Reporting Date Using As of December 31, 2019 Total Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Total Impaired loans* $ 27.6 $ — $ — $ 27.6 $ (13.7) Other real estate owned 2.2 — — 2.2 (1.2) Long-lived assets to be disposed of by sale 6.2 — — 6.2 (0.2) *The Company adopted ASC 326 as of January 1, 2020 which changes the methodology of impaired loans. The comparable period presents impaired loans under previously applicable GAAP. Collateral-dependent Loans. Collateral-dependent loans are reported at the fair value of the underlying collateral if repayment is expected solely from collateral. The collateral-dependent loans are reported at fair value through specific valuation allowance allocations. In addition, when it is determined that the fair value of a collateral-dependent loan is less than the recorded investment in the loan, the carrying value of the loan is adjusted to fair value through a charge to the allowance for credit losses. Collateral values are estimated using independent appraisals and management estimates of current market conditions. As of December 31, 2020, certain collateral-dependent loans with a carrying value of $17.5 million were reduced by specific valuation allowance allocations of $2.8 million resulting in a reported fair value of $14.7 million. As of December 31, 2019, certain impaired loans with a carrying value of $41.3 million were reduced by specific valuation allowance allocations of $3.6 million and partial loan charge-offs of $10.1 million resulting in a reported fair value of $27.6 million. OREO. The fair values of OREO are estimated using independent appraisals and management estimates of current market conditions. Upon initial recognition, write-downs based on the foreclosed asset’s fair value at foreclosure are reported through charges to the allowance for credit losses. Periodically, the fair value of foreclosed assets is remeasured with any subsequent write-downs charged to OREO expense in the period in which they are identified. Long-lived Assets to be Disposed of by Sale. Long-lived assets to be disposed of by sale are carried at the lower of carrying value or fair value less estimated costs to sell. The fair values of long-lived assets to be disposed of by sale are based upon observable market data and management estimates of current market conditions. As of December 31, 2020, the Company had long-lived assets to be disposed of by sale with carrying values aggregating $5.5 million, which was reduced by write-downs of $0.2 million charged to other expense, resulting in a reported fair value of $5.3 million. As of December 31, 2019, the Company had long-lived assets to be disposed of by sale with carrying values of $6.4 million, reduced by write-downs of $0.2 million, resulting in a fair value of $6.2 million. The following table presents additional quantitative information about assets measured at fair value on a non-recurring basis and for which the Company has utilized Level 3 inputs to determine fair values: As of December 31, 2020 Fair Value Valuation Technique Unobservable Inputs Range (Weighted Average) Collateral-dependent loans* $ 14.7 Appraisal Appraisal adjustment 0% - 18% (9%) Long-lived assets to be disposed of by sale 5.3 Appraisal Appraisal adjustment 0% - 0% 0% As of December 31, 2019 Fair Value Valuation Technique Unobservable Inputs Range (Weighted Average) Impaired loans* $ 27.6 Appraisal Appraisal adjustment 0% - 56% (22%) Other real estate owned 2.2 Appraisal Appraisal adjustment 8% - 65% (27%) Long-lived assets to be disposed of by sale 6.2 Appraisal Appraisal adjustment 0% - 37% (3%) *The Company adopted ASC 326 as of January 1, 2020 which changes the methodology of impaired loans. The comparable period presents impaired loans under previously applicable GAAP. The Company is required to disclose the fair value of financial instruments for which it is practical to estimate fair value. The methodologies for estimating the fair value of financial instruments that are measured at fair value on a recurring or non-recurring basis are discussed above. The methodologies for estimating the fair value of other financial instruments are discussed below. For financial instruments bearing a variable interest rate where no credit risk exists, it is presumed that recorded book values are reasonable estimates of fair value. Financial Assets. Carrying values of cash, cash equivalents, and accrued interest receivable approximate fair values due to the liquid and/or short-term nature of these instruments. Fair values for investment securities held-to-maturity are obtained from an independent pricing service, which considers observable data that may include dealer quotes, market spreads, cash flows, the U.S. Treasury yield curve, live trading levels, trade execution data, market consensus prepayment speeds, credit information, and the investment’s terms and conditions, among other things. Fair values of fixed rate loans and variable rate loans that reprice on an infrequent basis are estimated using an exit price by discounting future cash flows using current interest rates at which similar loans with similar terms would be made to borrowers of similar credit quality using an exit price notion. Carrying values of variable rate loans that reprice frequently, and with no change in credit risk, approximate the fair values of these instruments. Financial Liabilities. The fair values of demand deposits, savings accounts, securities sold under repurchase agreements, and accrued interest payable are the amounts payable on demand at the reporting date. The fair values of fixed-maturity certificates of deposit are estimated using external market rates currently offered for deposits with similar remaining maturities. The fair values of derivative liabilities are obtained from an independent pricing service, which considers observable data that may include the three-month LIBOR forward curve, the federal funds effective swap rate and cash flows, among other things. The carrying values of the interest-bearing demand notes to the United States Treasury are deemed an approximation of fair values due to the frequent repayment and repricing at market rates. The fixed and floating rate subordinated debentures, floating rate subordinated term loan, notes payable to the FHLB, fixed rate subordinated term debt, and capital lease obligation are estimated by discounting future cash flows using current rates for advances with similar characteristics. Commitments to Extend Credit and Standby Letters of Credit. The fair value of commitments to extend credit and standby letters of credit, based on fees currently charged to enter into similar agreements, is not significant. The estimated fair values of financial instruments that are reported in the Company’s consolidated balance sheets, segregated by the level of the valuation inputs within the fair value hierarchy utilized to measure fair value, are as follows: ` Fair Value Measurements at Reporting Date Using As of December 31, 2020 Carrying Amount Estimated Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Financial assets: Cash and cash equivalents $ 2,276.8 $ 2,276.8 $ 2,276.8 $ — $ — Investment debt securities available-for-sale 4,008.7 4,008.7 — 4,008.7 — Investment debt securities held-to-maturity 51.6 55.0 — 55.0 — Accrued interest receivable 51.1 51.1 — 51.1 — Mortgage servicing rights, net 24.0 24.0 — 24.0 — Loans held for sale 74.0 74.0 — 74.0 — Net loans held for investment 9,663.2 9,785.6 — 9,770.9 14.7 Derivative assets 55.3 55.3 — 55.3 — Deferred compensation plan assets 19.1 19.1 — 19.1 — Total financial assets $ 16,223.8 $ 16,349.6 $ 2,276.8 $ 14,058.1 $ 14.7 Financial liabilities: Total deposits, excluding time deposits $ 13,158.3 $ 13,158.3 $ 13,158.3 $ — $ — Time deposits 1,058.7 1,061.1 — 1,061.1 — Securities sold under repurchase agreements 1,091.4 1,091.4 — 1,091.4 — Accrued interest payable 5.8 5.8 — 5.8 — Long-term debt 112.4 116.5 — 116.5 — Subordinated debentures held by subsidiary trusts 87.0 81.3 — 81.3 — Derivative liabilities 53.3 53.3 — 53.3 — Deferred compensation plan liabilities 19.1 19.1 — 19.1 — Total financial liabilities $ 15,586.0 $ 15,586.8 $ 13,158.3 $ 2,428.5 $ — Fair Value Measurements at Reporting Date Using As of December 31, 2019 Carrying Amount Estimated Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Financial assets: Cash and cash equivalents $ 1,076.8 $ 1,076.8 $ 1,076.8 $ — $ — Investment debt securities available-for-sale 2,960.0 2,960.0 — 2,960.0 — Investment debt securities held-to-maturity 92.3 94.5 — 94.5 — Accrued interest receivable 46.7 46.7 — 46.7 — Mortgage servicing rights, net 30.2 34.8 — 34.8 — Loans held for sale 100.9 100.9 — 100.9 — Net loans held for investment 8,857.7 8,930.7 — 8,906.7 24.0 Derivative assets 23.2 23.2 — 23.2 — Deferred compensation plan assets 18.2 18.2 — 18.2 — Total financial assets $ 13,206.0 $ 13,285.8 $ 1,076.8 $ 12,185.0 $ 24.0 Financial liabilities: Total deposits, excluding time deposits $ 10,213.5 $ 10,213.5 $ 10,213.5 $ — $ — Time deposits 1,450.0 1,446.6 — 1,446.6 — Securities sold under repurchase agreements 697.6 697.6 — 697.6 — Accrued interest payable 12.1 12.1 — 12.1 — Long-term debt 13.9 10.4 — 10.4 — Subordinated debentures held by subsidiary trusts 86.9 81.3 — 81.3 — Derivative liabilities 22.2 22.2 — 22.2 — Deferred compensation plan liabilities 18.2 18.2 — 18.2 — Total financial liabilities $ 12,514.4 $ 12,501.9 $ 10,213.5 $ 2,288.4 $ —

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2020
Related Party Transactions [Abstract]
Related Party Transactions	RELATED PARTY TRANSACTIONS Certain executive officers, directors and greater than 5% shareholders of the Company and certain entities and individuals related to such persons had transactions with the Company in the ordinary course of business. These parties were deposit clients of the Bank and incurred indebtedness in the form of loans, as clients, of $22.4 million and $40.3 million at December 31, 2020 and 2019, respectively. During 2020, new loans and advances on existing loans of $17.2 million were funded and loan repayments totaled $17.0 million. In addition, $18.1 million of loans were removed due to changes in related parties during the year. All deposit and loan transactions were made on substantially the same terms, including interest rates and collateral, as those prevailing at the time for comparable transactions with persons not related to the Company and do not involve more than a normal risk of collectability or present other unfavorable features. Prior to 2020, the Company leased an aircraft from an entity wholly-owned by a member of the Scott family control group. Under the terms of the lease, the Company paid a fee for each flight hour plus certain third-party operating expenses related to the aircraft. During 2019 and 2018, the Company paid total fees and operating expenses of $22 thousand and $53 thousand respectively, for its use of the aircraft. In addition, we lease a portion of our hanger and provide pilot services to the Scott family control group’s related entity. During 2020, 2019, and 2018, the Company received payments from the related entity of $54 thousand, $30 thousand, and $25 thousand, respectively, for hangar use, pilot fees, and reimbursement of certain third-party operating expenses related to the use of the aircraft.

Authoritative Accounting Guidan

Authoritative Accounting Guidance	12 Months Ended
	Dec. 31, 2020
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Recent Authoritative Accounting Guidance	RECENT AUTHORITATIVE ACCOUNTING GUIDANCE ASU 2016-13, “Financial Instruments – Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments.” The amendments in ASU 2016-13 require a financial asset or group of financial assets measured at amortized cost basis to be presented on a company’s financial statements at the net amount expected to be collected based on historical experience, current conditions and reasonable and supportable forecasts. ASU 2016-13 requires a company’s income statement to reflect the measurement of credit losses for newly recognized financial assets as well as the expected increases or decreases of expected credit losses that have taken place during the period. The amendments in ASU 2016-13 require that the allowance for credit losses for purchased financial assets with a more-than-insignificant amount of credit deterioration since origination be measured at amortized cost basis with the initial allowance for credit losses added to the purchase price rather than being reported as a credit loss expense. ASU 2016-13 also requires that credit losses relating to available-for-sale debt securities be recorded through an allowance for credit losses. The amendments in ASU 2016-13 are effective for the Company for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. A prospective transition approach is required for debt securities for which other-than-temporary impairment was recognized before the effective date. On January 1, 2020, the Company adopted ASC 326 using the modified retrospective method for all financial assets measured at amortized cost and off-balance sheet credit exposures. The adoption of ASU 2016-13 resulted in an increase in the allowance for loan losses as a result of changing from the “incurred loss” model, which encompassed allowances for current known and inherent losses within the portfolio, to the “expected loss” model, which encompasses allowances for losses expected to be incurred over the life of the portfolio. The amendments, as applied to our debt securities, had no material impact. The amendments were applied through a cumulative-effect adjustment to retained earnings of $24.1 million as of January 1, 2020. The transition adjustment included an increase in the allowance for credit losses on loans of $30.0 million and an increase in the allowance for credit losses on off-balance sheet credit exposures of $2.3 million, net of the corresponding increases in deferred tax assets of $8.2 million. The Company adopted ASC 326 using the prospective transition approach for PCD financial assets, previously classified as purchased credit impaired, or PCI, and accounted for under ASC 310-30. In accordance with the standard, the Company did not reassess whether PCI assets met the criteria of PCD assets as of the date of adoption. The remaining discount on the PCD assets was determined to be related to noncredit factors and will be accreted into interest income on a level-yield method over the life of the loans. The Company has elected to opt into the transition election to mitigate the effects of ASC 326 on the regulatory capital ratios relative to recent legislation in relief of COVID‑19 pandemic on the economy and financial institutions in the United States. The referenced relief allows a total five-year phase in of the CECL impact on capital and relief over the next two years for the impact on the allowance for credit losses resulting from COVID‑19. Refer to “Note 17 - Regulatory Capital” for additional details of the election. ASU 2018-13, “Fair Value Measurement (Topic 820): Disclosure Framework – Changes to the Disclosure Requirements for Fair Value Measurement.” In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement: Disclosure Framework - Changes to the Disclosure Requirements for Fair Value Measurement (ASU 2018-13). The amendments in this Update removes, modifies, and adds to the disclosure requirements on fair value measurements in Topic 820, Fair Value Measurement, based on the concepts in the Concepts Statement, including the consideration of costs and benefits. The amendments in this Update are effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. The amendments on changes in unrealized gains and losses, the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements, and the narrative description of measurement uncertainty should be applied prospectively for only the most recent interim or annual period presented in the initial fiscal year of adoption. All other amendments should be applied retrospectively to all periods presented upon their effective date. The amendments in this Update became effective for the Company on January 1, 2020, and as the amendment is a revision to the disclosure requirements did not have a significant impact on the Company’s consolidated financial statements, results of operations, or liquidity. ASU 2018-14, “Compensation – Retirement Benefits – Defined Benefit Plans – General (Subtopic 715-20): Disclosure Framework – Changes to the Disclosure Requirements for Defined Benefit Plans.” In August 2018, the FASB issued ASU 2018-14, Compensation - Retirement Benefits - Defined Benefit Plans - General: Disclosure Framework - Changes to the Disclosure Requirements for Defined Benefit Plans (ASU 2018-14). The amendments in this Update remove disclosures that no longer are considered cost beneficial, clarify the specific requirements of disclosures, and add disclosure requirements identified as relevant. Although narrow in scope, the amendments are considered an important part of the Board’s efforts to improve the effectiveness of disclosures in the notes to financial statements by applying concepts in the Concepts Statement. The amendments in this Update are effective for fiscal years ending after December 15, 2020, for public business entities. Early adoption is permitted. While the Company continues to assess all potential impacts of the standard, we currently expect adoption to have an immaterial impact on our consolidated financial statements disclosures. ASU 2018-15, “Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40) - Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract.” In August 2018, the FASB issued ASU 2018-15, Intangibles-Goodwill and Other- Internal-Use Software (Subtopic 350-40). The amendments in this Update clarifies certain aspects of ASU 2015-05, “Customer’s Accounting for Fees Paid in a Cloud Computing Arrangement,” which was issued in April 2015. ASU 2018-15 aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal-use software license). The accounting for the service element of a hosting arrangement that is a service contract is not affected by the Update. The amendments in this Update are effective for fiscal years beginning after December 15, 2019, for public business entities. The amendments in this Update became effective for the Company on January 1, 2020 and did not have a significant impact on the Company’s consolidated financial statements, results of operations, or liquidity.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2020
Subsequent Events [Abstract]
Subsequent Events	SUBSEQUENT EVENTS Subsequent events have been evaluated for potential recognition and disclosure through the date financial statements were filed with the Securities and Exchange Commission. On January 28, 2021, the Company declared a quarterly dividend to common shareholders of $0.41 per share, which was paid on February 19, 2021 to shareholders of record as of February 9, 2021. As part of the U.S. government’s response to COVID-19, the CARES Act enacted a number of measures to mitigate the impact of the COVID-19 pandemic on the U.S. economy. For instance, the CARES Act created a new guaranteed, unsecured loan program under the Small Business Administration, or SBA, called the Payroll Protection Program, or the PPP, which the Company participated in, designed to provide a direct incentive for sole proprietors, independent contractors, self-employed persons, non-profits, and small businesses with less than 500 employees, allowing for narrow exceptions with businesses greater than 500 employees, to keep their workers on the payroll during the pandemic period. The RELIEF Act extended the program, beginning in January 2021, for which the Company is actively participating in assisting our customers with applications for resources through the program and as of February 19, 2021 has approved more than 3,150 applications, for approximately $296.3 million. No other events requiring recognition or disclosure were identified.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Accounting Policies [Abstract]
Revenue	Revenue Recognition. The Company recognizes revenue as it is earned based on contractual terms, as transactions occur, or as services are provided and collectability is reasonably assured. The principal source of revenue is interest income from loans and investments. The Company also earns non-interest income from various banking and financial services offered to its clients. Certain specific policies related to non-interest income include the following: Wealth management and trust fee income Wealth management and trust fee income represents monthly fees due from wealth management clients as consideration for managing the clients’ assets. Wealth management and trust services include custody of assets, investment management, fees for trust services and similar fiduciary activities. Revenue is recognized when our performance obligation is completed. The Company does not earn performance-based incentives. Optional services such as settlement, court, and regulatory fees are also available to existing trust and asset management clients. The Company’s performance obligation for these transactional-based services is generally satisfied, and related revenue recognized, at a point in time. Service charges on deposit accounts Service charges on deposit accounts represent general service fees for account maintenance and activity- or transaction-based fees and consist of transaction-based revenue, time-based revenue (service period), item-based revenue or some other individual attribute-based revenue. Revenue is recognized when our performance obligation is completed for account maintenance services or when a transaction has been completed (such as a wire transfer or check orders). Payment for such performance obligations are generally received at a point in time when the performance obligations are satisfied. Interchange and other fees Interchange and other fees primarily represent debit and credit card income comprised of interchange fees earned whenever the Company’s debit and credit cards are processed through card payment networks such as MasterCard. ATM fees are primarily generated when a Company cardholder uses a non-Company ATM or a non-Company cardholder uses a Company ATM. Merchant services income primarily represents fees charged to merchants to process their debit and credit card transactions, in addition to account management fees. Swap fee income primarily represents income associated with the execution of dealer bank swap agreements. Other service charges include revenue from processing wire transfers, bill pay service, cashier’s checks, and other services. The Company’s performance obligation for interchange and other service charges are largely satisfied, and related revenue recognized, when completion of the services are rendered at a point in time. Annuity and insurance commissions Annuity and insurance commissions primarily represent commissions received on annuity product sales. The Company acts as an intermediary between the Company’s client and the insurance carrier. The Company’s performance obligation is generally satisfied upon the issuance of the annuity policy, the carrier then remits the commission payment to the Company, and the Company recognizes the revenue at a point in time.
Consolidation Policy	Basis of Presentation . The Company’s consolidated financial statements include the accounts of the Parent Company and its operating subsidiaries. As of December 31, 2020, the Company had one significant subsidiary, First Interstate Bank (“FIB”). All significant intercompany balances and transactions have been eliminated in consolidation. Certain reclassifications, none of which were material, have been made in the consolidated financial statements for 2019 and 2018 to conform to the 2020 presentation. These reclassifications did not change previously reported net income or stockholders’ equity.
Equity Method Investments	Equity Method Investments. The Company has investments in real estate joint ventures that are not consolidated because the Company does not own a majority voting interest, control the operations or receive a majority of the losses or earnings of the joint venture. These joint ventures are accounted for using the equity method of accounting whereby the Company initially records its investment at cost (or fair value at the date of acquisition) and then subsequently adjusts the carrying value for the Company’s proportionate share of distributions and earnings or losses of the joint ventures.
Variable Interest Entities	Variable Interest Entities. The Company’s wholly-owned business trusts, FI Statutory Trust I (“Trust I”), FI Capital Trust II (“Trust II”), FI Statutory Trust III (“Trust III”), FI Capital Trust IV (“Trust IV”), FI Statutory Trust V (“Trust V”), FI Statutory Trust VI (“Trust VI”), and Northwest Bancorporation Capital Trust I (“Trust VII”) are variable interest entities for which the Company is not a primary beneficiary. Accordingly, the accounts of Trust I, Trust II, Trust III, Trust IV, Trust V, Trust VI, and Trust VII are not included in the accompanying consolidated financial statements, and are instead accounted for using the equity method of accounting.
Assets Held In Fiduciary Or Agency Capacity	Assets Held in Fiduciary or Agency Capacity. The Company holds certain trust assets in a fiduciary or agency capacity. The Company also purchases and sells federal funds as an agent. These and other assets held in an agency or fiduciary capacity are not assets of the Company and, accordingly, are not included in the accompanying consolidated financial statements.
Use of Estimates	Use of Estimates. The preparation of consolidated financial statements in conformity with accounting principles generally accepted in the United States of America requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the financial statements and income and expenses during the reporting period. Actual results could differ from those estimates. Material estimates that are particularly susceptible to change relate to the determination of the allowance for credit losses, the valuation of goodwill, fair valuations of investment securities and other financial instruments, and the status of loss contingencies.
Cash and Cash Equivalents	Cash and Cash Equivalents. For purposes of reporting cash flows, cash and cash equivalents include cash on hand, amounts due from banks, federal funds sold for one-day periods, and interest-bearing deposits in banks with original maturities of less than three months.
Goodwill and Core Deposit Intangibles	Goodwill . The excess purchase price over the fair value of net assets from acquisitions, or goodwill, is evaluated for impairment at least annually and on an interim basis if an event or circumstance indicates that it is likely impairment has occurred. Goodwill impairment is determined by comparing the fair value of a reporting unit to its carrying amount. In any given year the Company may elect to perform a qualitative assessment to determine whether it is more likely than not that the fair value of a reporting unit is in excess of its carrying value. If it is not more likely than not that the fair value of the reporting unit is in excess of the carrying value, or if the Company elects to bypass the qualitative assessment, a quantitative impairment test is performed. In performing a quantitative test for impairment, the fair value of net assets is estimated based on analyses of the Company’s market value, discounted cash flows and peer values. The determination of goodwill impairment is sensitive to market-based economics and other key assumptions used in determining or allocating fair value. Variability in the market and changes in assumptions or subjective measurements used to allocate fair value are reasonably possible and may have a material impact on our consolidated financial statements or results of operations. Core Deposit Intangibles. Core deposit intangibles represent the intangible value of depositor relationships resulting from deposit liabilities assumed, as a result of acquisitions, and are amortized using an accelerated method based on the estimated weighted average useful lives of the related deposits, which is generally ten years.
Mortgage Servicing Rights	Mortgage Servicing Rights . The Company recognizes the rights to service mortgage loans for others, whether acquired or internally originated. Mortgage servicing rights are initially recorded at fair value based on comparable market data and are amortized in proportion to and over the period of estimated net servicing income. Mortgage servicing rights are evaluated quarterly for impairment by discounting the expected future cash flows, taking into consideration the estimated level of prepayments based on current industry expectations and the predominant risk characteristics of the underlying loans including loan type, note rate and loan term. Impairment adjustments, if any, are recorded through a valuation allowance.
Premises and Equipment	Premises and Equipment. Buildings, furniture, and equipment are stated at cost less accumulated depreciation. Depreciation expense is computed using straight-line methods over estimated useful lives of 5 to 45 years for buildings and improvements and 3 to 15 years for furniture and equipment. Leasehold improvements and assets acquired under a financing lease are amortized over the shorter of their estimated useful lives or the terms of the related leases. Land is recorded at cost.
Company-Owned Life Insurance	Company-Owned Life Insurance . Key executive and group life insurance policies are recorded at their cash surrender value. Separate account group life insurance policies are subject to a stable value contract that offsets the impact of interest rate fluctuations on the market value of the policies and are recorded at the stabilized investment value. Increases in the cash surrender or stabilized investment value of insurance policies, as well as insurance proceeds received, are recorded as other non-interest income, and are not subject to income taxes.
Deferred Compensation Plan	Deferred Compensation Plan. The Company has a deferred compensation plan for the benefit of certain highly compensated officers and directors of the Company. The plan allows for discretionary employer contributions in excess of tax limits applicable to the Company’s 401(k) plan and the deferral of salary, short-term incentives, or director fees subject to certain limitations. Deferred compensation plan assets and liabilities are included in the Company’s consolidated balance sheets at fair value.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets. Long-lived assets, including premises and equipment and certain identifiable intangibles, are reviewed for impairment whenever events or changes in circumstances indicate the carrying amount of an asset may not be recoverable.
Other Real Estate Owned	Other Real Estate Owned. Real estate acquired in satisfaction of loans is initially carried at current fair value less estimated selling costs. Any excess of loan carrying value over the fair value of the real estate acquired is recorded as a charge to the allowance for credit losses. Subsequent declines in fair value less estimated selling costs are included in OREO expense. Subsequent increases in fair value less estimated selling costs are recorded as a reduction in OREO expense to the extent of recognized losses. Operating expenses, net of related income, and gains or losses on sales are included in OR
Restricted Equity Securities	Restricted Equity Securities. The Company, as a member of the Federal Reserve Bank and the Federal Home Loan Bank (“FHLB”), is required to maintain investments in each of the organization’s capectively. No ready market exists for these restricted equity securities, and they have no quoted market values. Restricted equity securities are periodically reviewed for impairment based on ultimate recovery of par value. The determination of whether a decline affects the ultimate recovery of par value is influenced by the significance of the decline compared to the cost basis of the restricted equity securities, length of time a decline has persisted, impact of legislative and regulatory changes on the issuing organizations, and the liquidity positions of the issuing orga
Derivatives and Hedging Activities	or 2018. Derivatives and Hedging Activities. For asset and liability management purposes, the Company enters into interest rate swap contracts to hedge against changes in forecasted cash flows due to interest rate exposures. Interest rate swaps are contracts in which a series of interest payments are exchanged over a prescribed period. The notional amount upon which the interest payments are based is not exchanged. The Company formally assesses, both at the hedge's inception and on an ongoing basis, whether the derivatives that are used in hedging transactions are highly effective in offsetting changes in cash flows of hedged items. The swap agreements are derivative instruments and convert a portion of the Company’s forecasted variable rate debt to a fixed rate (i.e., cash flow hedge) over the payment term of the interest rate swap. The gain or loss on cash flow hedging instruments is initially reported as a component of other comprehensive income and subsequently reclassified into earnings in the same period during which the transaction affects earnings. When it is determined that a derivative is not highly effective as a hedge or that it has ceased to be a highly effective hedge, the Company discontinues hedge accounting prospectively when (a) it is determined that the derivative is no longer effective in offsetting changes in the cash flows of a hedged item (including forecasted transactions); (b) the derivative expires or is sold, terminated, or exercised; (c) the derivative is dedesignated as a hedge instrument, because it is unlikely that a forecasted transaction will occur; or (d) management determines that designation of the derivative as a hedge instrument is no longer appropriate. When hedge accounting is discontinued because it is probable that a forecasted transaction will not occur, the derivative will continue to be carried on the balance sheet at its fair value, and gains and losses that were accumulated in other comprehensive income will be recognized immediately in earnings. In all other situations in which hedge accounting is discontinued, the derivative will be carried at its fair value on the balance sheet, with subsequent changes in its fair value recognized in current-period earnings. The Company also enters into certain interest rate swap contracts that are not designated as hedging instruments. These derivative contracts relate to transactions in which the Company enters into an interest rate swap with a client while at the same time entering into an offsetting interest rate swap with a third-party financial institution. Because the Company acts as an intermediary for the client, changes in the fair value of the underlying derivative contracts for the most part offset each other and do not significantly impact the Company’s results of operations. In the normal course of business, the Company enters into interest rate lock commitments to finance residential mortgage loans that are not designated as accounting hedges. These commitments, which contain fixed expiration dates, offer the borrower an interest rate guarantee provided the loan meets underwriting guidelines and closes within the timeframe established by the Company. Interest rate risk arises on these commitments and subsequently closed loans if interest rates change between the time of the interest rate lock and the delivery of the loan to the investor. Loan commitments related to residential mortgage loans intended to be sold are considered derivatives and are marked to market through earnings. In addition to the effects of the change in market interest rate, the fair value measurement of the derivative also contemplates the expected cash flows to be received from the counterparty from the future sale of the loan. The Company sells residential mortgage loans on either a best efforts or mandatory delivery basis. The Company mitigates the effect of the interest rate risk inherent in providing interest rate lock commitments by entering into forward loan sales contracts. During the interest rate lock commitment period, these forward loan sales contracts are marked to market through earnings and are not designated as accounting hedges. Exclusive of the fair value component associated with the projected cash flows from the loan delivery to the investor, the changes in fair value related to movements in market rates of the interest rate lock commitments and the forward loan sales contracts generally move in opposite directions, and the net impact of changes in these valuations on net income during the loan commitment period is generally inconsequential. When the loan is funded to the borrower, the interest rate lock commitment derivative expires and the Company records a loan held for sale. The forward loan sales contract acts as a hedge against the variability in cash to be received from the loan sale.
Earnings Per Common Share	f income. Software. Capitalized software, stated at cost less accumulated amortization, includes purchased software, capitalizable application development costs associated with internally developed software, and cloud computing arrangements, including capitalizable implementation costs associated with hosting arrangements that are service contracts. Capitalized software is included in premises and equipment, net of accumulated depreciation on the Consolidated Balance Sheets. Amortization expense, generally computed on the straight-line method, is charged to furniture and equipment in the Consolidated Statements of Income over the estimated useful life of the software, generally three to five years, or the term of the hosting arrangement for implementation costs related to service contracts. Cloud computing arrangements include software as a service (SaaS), platform as a service (PaaS), infrastructure as a service (IaaS) and other similar hosting arrangements. The Company primarily utilizes SaaS and PaaS arrangements. Capitalized implementation costs of hosting arrangements that are service contracts were $6.0 million and $9.2 million at December 31, 2020 and 2019, respectively. Earnings Per Common Share . Basic and diluted earnings per common share are calculated using a two-class method. Under the two-class method, basic earnings per common share is calculated by dividing net income available to common shareholders by the weighted average number of common shares outstanding during the period, excluding outstanding participating securities. Participating securities include non-vested performance restricted stock awards granted and all non-vested time restricted stock awards.
Income Taxes	k method. Income Taxes . The Parent Company and its subsidiaries have elected to be included in a consolidated federal income tax return. For state income tax purposes, the combined taxable income of the Parent Company and its subsidiaries is apportioned among the states in which operations take place. Federal and state income taxes attributable to the subsidiaries, computed on a separate return basis, are paid to or received from the Parent Company. The Company accounts for income taxes using the liability method. Under the liability method, deferred tax assets and liabilities are determined based on enacted income tax rates which will be in effect when the differences between the financial statement carrying values and tax bases of existing assets and liabilities are expected to be reported in taxable income.
Comprehensive Income	Comprehensive Income. Comprehensive income includes net income, as well as other changes in stockholders’ equity that result from transactions and economic events other than those with shareholders. In addition to net income, the Company’s comprehensive income includes the after tax effect of changes in unrealized gains and losses on available-for-sale investment securities and derivatives designated as cash flow hedges, changes in the unamortized gain or loss on available-for-sale investment securities transferred to held-to-maturity and changes in net actuarial gains and losses on defined benefit post-retirement benefits plans.
Segment Reporting	s plans.Segment Reporting. An operating segment is defined as a component of a business for which separate financial information is available that is evaluated regularly by the chief operating decision maker in deciding how to allocate resources and evaluate performance. The "Segment Reporting" topic of the FASB ASC requires that public companies report certain information about operating segments. It also requires that public companies report certain information about their products and services, the geographic areas in which they operate, and their major clients. The Company is a holding company for a regional community bank, which offers a wide array of products and services to its clients. The Company has one reporting unit and one operating segment, community banking, which encompasses commercial and consumer banking services offered to individuals, businesses, municipalities and other entities.
Advertising Costs	tities. Advertising Costs. Advertising costs are expensed as incurred.
Transfers of Financial Assets	Transfers of Financial Assets. Transfers of financial assets are accounted for as sales when control over the assets has been surrendered. Control over transferred assets is deemed to be surrendered when the assets have been isolated from the Company; the transferee obtains the right, free of conditions that constrain it from taking advantage of that right, to pledge or exchange the transferred assets; and, the Company does not maintain effective control over the transferred assets through an agreement to repurchase them before their maturity.
Stock-Based Compensation	Stock-Based Compensation. Compensation cost for all stock-based awards is measured at fair value on the date of grant and is recognized over the requisite service period for awards expected to vest. The impact of forfeitures of stock-based payment awards on compensation expense is recognized as forfeitures occur.
Fair Value Measurements	Fair Value Measurements. In general, fair value measurements are based upon quoted market prices, where available. If quoted market prices are not available, fair value measurements are estimated using relevant market information and other assumptions. Fair value estimates involve uncertainties and require some degree of judgment regarding interest rates, credit risk, prepayments and other factors. The use of different assumptions or estimation techniques may have a significant effect on the fair value amounts reported.

Acquisitions (Tables)

Acquisitions (Tables)	12 Months Ended
	Dec. 31, 2020
Business Combinations [Abstract]
Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed	The following table summarizes the consideration paid, fair values of the IIBK assets acquired and liabilities assumed, and the resulting goodwill. All amounts reported were finalized during the fourth quarter of 2019. As Recorded Fair Value As Recorded As of April 8, 2019 by IIBK Adjustments by the Company Assets acquired: Cash and cash equivalents $ 270.7 $ — $ 270.7 Investment securities 62.7 0.5 (1) 63.2 Loans held for investment 347.6 (9.8) (2) 337.8 Mortgage loans held for sale 0.5 — 0.5 Allowance for loan loss (6.3) 6.3 (3) — Premises and equipment 16.5 4.8 (4) 21.3 Other real estate owned (“OREO”) 0.4 2.0 (5) 2.4 Company owned life insurance 15.2 — 15.2 Core deposit intangible assets — 13.6 (6) 13.6 Deferred tax assets, net 3.2 (2.6) (7) 0.6 Other assets 8.6 (0.7) (8) 7.9 Total assets acquired 719.1 14.1 733.2 Liabilities assumed: Deposits 596.5 0.1 (9) 596.6 Accounts payable and accrued expense 15.2 2.6 (10) 17.8 Other borrowed funds 4.0 0.1 (11) 4.1 Securities sold under repurchase agreements 30.4 — 30.4 Total liabilities assumed 646.1 2.8 648.9 Net assets acquired $ 73.0 $ 11.3 $ 84.3 Consideration paid: Class A common stock $ 157.3 Total consideration paid $ 157.3 Goodwill $ 73.0 Explanation of fair value adjustments and the removal of previously recorded fair value marks recorded by IIBK: (1) Write up of the book value of investments to their estimated fair values on the date of acquisition based upon quotes obtained from an independent third-party pricing service. (2) Write down of the book value of loans to their estimated fair values. The fair value of the loans was estimated using cash flow projections based on the remaining maturity and repricing terms, adjusted for estimated future credit losses and prepayments and discounted to present value using a risk-adjusted market rate for similar loans. The fair value of collateral dependent loans acquired with deteriorated credit quality was estimated based on the Company’s analysis of the fair value of each loan’s underlying collateral, discounted using market-derived rates of return with consideration given to the period of time and costs associated with foreclosure and disposition of the collateral. (3) Adjustment to remove the IIBK allowance for loan losses at acquisition date, as the credit risk is included in the fair value adjustment for loans receivable described in (2) above. (4) Write up of the book value of premises and equipment to their estimated fair values on the date of acquisition based upon broker’s opinion of value. (5) Adjustment to the book value of other real estate owned to their estimated fair values on the date of acquisition based on appraisal value. (6) Adjustment represents the value of the core deposit base assumed in the acquisition based upon valuation from an independent accounting and advisory firm. (7) Adjustment consists of the write-off of pre-existing deferred tax assets and purchase accounting adjustments as a result of the acquisition. (8) Adjustment consists of reductions to the fair value of other items. (9) Increase in book value of time deposits to their estimated fair values based upon interest rates of similar time deposits with similar terms on the date of acquisition based upon valuation from an independent accounting and advisory firm. (10) Adjustment to the liability for the nonqualified retirement plan. (11) Adjustment of the book value of debt to the estimated fair values on the date of acquisition based upon interest rates in the market.
Acquisition Related Expenses	These costs are incorporated in non-interest expense in the Company’s consolidated statements of income.
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities Acquired During Period	Information regarding IIBK loans acquired deemed credit impaired as of the April 8, 2019 acquisition date are as follows: Contractually required principal and interest payments $ 24.1 Contractual cash flows not expected to be collected (“non-accretable discount”) 3.9 Cash flows expected to be collected 20.2 Interest component of cash flows expected to be collected (“accretable discount”) 3.4 Fair value of acquired credit-impaired loans $ 16.8 Information regarding IIBK acquired loans not deemed credit-impaired at the April 8, 2019 acquisition date are as follows: Contractually required principal and interest payments $ 398.7 Contractual cash flows not expected to be collected 15.2 Fair value at acquisition $ 321.5

Goodwill and Core Deposit Int_2

Goodwill and Core Deposit Intangibles (Tables)	12 Months Ended
	Dec. 31, 2020
Goodwill and Intangible Assets Disclosure [Abstract]
Change in Carrying Amount of Goodwill	Year Ended December 31, 2020 2019 Net carrying value at beginning of period $ 621.6 $ 546.7 Acquisitions and measurement period adjustments — 74.9 Net carrying value at end of period $ 621.6 $ 621.6
Schedule of Activity of Identifiable Core Deposit Intangibles	Year Ended December 31, 2020 2019 Gross CDI, beginning of period $ 106.0 $ 89.7 Established through acquisitions — 16.6 Reductions due to sale of accounts — (0.3) Accumulated amortization (54.8) (43.9) Net CDI, end of period $ 51.2 $ 62.1
Schedule of Finite-Lived Intangible Assets, Future Amortization Expense	The following table provides estimated future CDI amortization expense: Years ending December 31, 2021 $ 9.9 2022 9.0 2023 8.2 2024 7.3 2025 6.5 Thereafter 10.3 Total $ 51.2

Investment Securities (Tables)

Investment Securities (Tables)	12 Months Ended
	Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]
Amortized Cost and Approximate Fair Values of Investment Securities	The amortized cost and approximate fair values of investment securities are summarized as follows: December 31, 2020 Amortized Gross Gross Estimated Available-for-Sale State, county, and municipal securities $ 462.1 $ 4.8 $ (1.0) $ 465.9 Obligations of U.S. government agencies 332.9 1.0 (2.0) 331.9 U.S. agency residential mortgage-backed securities & 2,830.8 69.3 (2.5) 2,897.6 Private mortgage-backed securities 10.9 0.1 (0.1) 10.9 Corporate Securities 295.8 6.5 (0.1) 302.2 Other investments 0.2 — — 0.2 Total $ 3,932.7 $ 81.7 $ (5.7) $ 4,008.7 December 31, 2020 Amortized Gross Gross Estimated Held-to Maturity State, county, and municipal securities $ 46.6 $ 3.2 $ — $ 49.8 U.S agency residential mortgage-backed securities & 1.0 0.1 — 1.1 Corporate securities 3.9 0.1 — 4.0 Other investments 0.1 — — 0.1 Total $ 51.6 $ 3.4 $ — $ 55.0 December 31, 2019 Amortized Gross Gross Estimated Available-for-Sale U.S. Treasury notes $ 9.0 $ — $ — $ 9.0 State, county, and municipal securities 80.1 0.8 — 80.9 Obligations of U.S. government agencies 367.5 0.1 (0.8) 366.8 U.S. agency residential mortgage-backed securities & 2,303.6 19.6 (6.0) 2,317.2 Private mortgage-backed securities 47.6 — (0.4) 47.2 Corporate Securities 134.5 1.2 — 135.7 Other investments 3.2 — — 3.2 Total $ 2,945.5 $ 21.7 $ (7.2) $ 2,960.0 December 31, 2019 Amortized Gross Gross Estimated Held-to Maturity State, county, and municipal securities $ 57.3 $ 2.1 $ — $ 59.4 Obligations of U.S. government agencies 19.8 — — 19.8 U.S. agency residential mortgage-backed securities & 1.2 — — 1.2 Corporate securities 13.9 0.1 — 14.0 Other investments 0.1 — — 0.1 Total $ 92.3 $ 2.2 $ — $ 94.5
Gross Unrealized Losses and Fair Values of Investment Securities	The following tables show the gross unrealized losses and fair values of investment securities, aggregated by investment category, and the length of time individual investment securities have been in a continuous unrealized loss position, as of December 31, 2020 and 2019. There were no held-to-maturity securities in a continuous unrealized loss position as of December 31, 2020. Less than 12 Months 12 Months or More Total December 31, 2020 Fair Gross Fair Gross Fair Gross Available-for-Sale State, county, and municipal securities $ 148.1 $ (1.0) $ — $ — $ 148.1 $ (1.0) Obligations of U.S. government agencies 235.6 (2.0) — — 235.6 (2.0) U.S. agency residential mortgage-backed 434.0 (2.4) 12.3 (0.1) 446.3 (2.5) Private mortgage-backed securities — — 4.3 (0.1) 4.3 (0.1) Corporate securities 20.9 (0.1) — — 20.9 (0.1) Total $ 838.6 $ (5.5) $ 16.6 $ (0.2) $ 855.2 $ (5.7) Less than 12 Months 12 Months or More Total December 31, 2019 Fair Gross Fair Gross Fair Gross Available-for-Sale Obligations of U.S. government agencies $ 185.3 $ (0.8) $ — $ — $ 185.3 $ (0.8) U.S. agency residential mortgage-backed 740.1 (4.6) 155.9 (1.4) 896.0 (6.0) Private mortgage-backed securities — — 46.6 (0.4) 46.6 (0.4) Total $ 925.4 $ (5.4) $ 202.5 $ (1.8) $ 1,127.9 $ (7.2)
Maturities of Investment Securities	All other investment securities maturities are shown at contractual maturity dates. Available-for-Sale Held-to-Maturity December 31, 2020 Amortized Estimated Amortized Estimated Within one year $ 687.6 $ 991.4 $ 8.3 $ 8.4 After one year but within five years 1,587.7 1,362.6 27.8 29.0 After five years but within ten years 736.2 996.3 14.1 16.2 After ten years 921.2 658.4 1.4 1.4 Total $ 3,932.7 $ 4,008.7 $ 51.6 $ 55.0

Premises and Equipment (Tables)

Premises and Equipment (Tables)	12 Months Ended
	Dec. 31, 2020
Property, Plant and Equipment [Abstract]
Premises and Equipment and Related Accumulated Depreciation	Premises and equipment and related accumulated depreciation are as follows: December 31, 2020 2019 Land $ 52.9 $ 50.8 Buildings and improvements 349.5 330.1 Furniture and equipment 88.9 94.2 Total premises and equipment 491.3 475.1 Less accumulated depreciation (179.0) (169.1) Premises and equipment, net $ 312.3 $ 306.0

Company-Owned Life Insurance (T

Company-Owned Life Insurance (Tables)	12 Months Ended
	Dec. 31, 2020
Investments, All Other Investments [Abstract]
Schedule of Life Insurance Corporate or Bank Owned	Company-owned life insurance consists of the following: December 31, 2020 2019 Key executive, principal shareholder $ 3.1 $ 4.4 Key executive split dollar 7.0 7.0 Group life 286.3 282.4 Total $ 296.4 $ 293.8

Other Real Estate Owned (Tables

Other Real Estate Owned (Tables)	12 Months Ended
	Dec. 31, 2020
Repossessed Assets [Abstract]
Other Real Estate Owned	Information with respect to the Company’s other real estate owned follows: Year Ended December 31, 2020 2019 2018 Balance at beginning of year $ 8.5 $ 14.4 $ 10.1 OREO acquired through acquisitions — 2.4 0.6 Additions 3.3 14.1 12.1 Capitalized improvements — 0.3 — Valuation adjustments (0.1) (0.9) (0.1) Dispositions (9.2) (21.8) (8.3) Balance at end of year $ 2.5 $ 8.5 $ 14.4

Derivatives and Hedging Activ_2

Derivatives and Hedging Activities (Tables)	12 Months Ended
	Dec. 31, 2020
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Offsetting Assets	The following table illustrates the potential effect of the Company’s master netting arrangements, by type of financial instrument, on the Company’s consolidated balance sheets as of the periods indicated: December 31, 2020 Gross Amounts Recognized Gross Amounts Offset in the Balance Sheet Net Amounts in the Balance Sheet Financial Instruments Fair Value of Financial Collateral in the Balance Sheet Net Amount Financial Assets Interest rate swap contracts $ 52.0 $ — $ 52.0 $ — $ 17.2 $ 34.8 Mortgage related derivatives 3.3 — 3.3 — — 3.3 Total derivatives 55.3 — 55.3 — 17.2 38.1 Total assets $ 55.3 $ — $ 55.3 $ — $ 17.2 $ 38.1 Financial Liabilities Interest rate swap contracts $ 52.2 $ — $ 52.2 $ — $ — $ 52.2 Mortgage related derivatives 1.1 — 1.1 — — 1.1 Total derivatives 53.3 — 53.3 — — 53.3 Repurchase agreements 1,091.4 1,091.4 — 1,091.4 — Total liabilities $ 1,144.7 $ — $ 1,144.7 $ — $ 1,091.4 $ 53.3 December 31, 2019 Gross Amounts Recognized Gross Amounts Offset in the Balance Sheet Net Amounts in the Balance Sheet Financial Instruments Fair Value of Financial Collateral in the Balance Sheet Net Amount Financial Assets Interest rate swap contracts $ 21.9 $ — $ 21.9 $ 0.1 $ 18.0 $ 3.8 Mortgage related derivatives 1.3 — 1.3 — — 1.3 Total derivatives 23.2 — 23.2 0.1 18.0 5.1 Total assets $ 23.2 $ — $ 23.2 $ 0.1 $ 18.0 $ 5.1 Financial Liabilities Interest rate swap contracts $ 21.9 $ — $ 21.9 $ 0.1 $ — $ 21.8 Mortgage related derivatives 0.3 — 0.3 — — 0.3 Total derivatives 22.2 — 22.2 0.1 — 22.1 Repurchase agreements 697.6 — 697.6 — 697.6 — Total liabilities $ 719.8 $ — $ 719.8 $ 0.1 $ 697.6 $ 22.1
Offsetting Liabilities	The following table illustrates the potential effect of the Company’s master netting arrangements, by type of financial instrument, on the Company’s consolidated balance sheets as of the periods indicated: December 31, 2020 Gross Amounts Recognized Gross Amounts Offset in the Balance Sheet Net Amounts in the Balance Sheet Financial Instruments Fair Value of Financial Collateral in the Balance Sheet Net Amount Financial Assets Interest rate swap contracts $ 52.0 $ — $ 52.0 $ — $ 17.2 $ 34.8 Mortgage related derivatives 3.3 — 3.3 — — 3.3 Total derivatives 55.3 — 55.3 — 17.2 38.1 Total assets $ 55.3 $ — $ 55.3 $ — $ 17.2 $ 38.1 Financial Liabilities Interest rate swap contracts $ 52.2 $ — $ 52.2 $ — $ — $ 52.2 Mortgage related derivatives 1.1 — 1.1 — — 1.1 Total derivatives 53.3 — 53.3 — — 53.3 Repurchase agreements 1,091.4 1,091.4 — 1,091.4 — Total liabilities $ 1,144.7 $ — $ 1,144.7 $ — $ 1,091.4 $ 53.3 December 31, 2019 Gross Amounts Recognized Gross Amounts Offset in the Balance Sheet Net Amounts in the Balance Sheet Financial Instruments Fair Value of Financial Collateral in the Balance Sheet Net Amount Financial Assets Interest rate swap contracts $ 21.9 $ — $ 21.9 $ 0.1 $ 18.0 $ 3.8 Mortgage related derivatives 1.3 — 1.3 — — 1.3 Total derivatives 23.2 — 23.2 0.1 18.0 5.1 Total assets $ 23.2 $ — $ 23.2 $ 0.1 $ 18.0 $ 5.1 Financial Liabilities Interest rate swap contracts $ 21.9 $ — $ 21.9 $ 0.1 $ — $ 21.8 Mortgage related derivatives 0.3 — 0.3 — — 0.3 Total derivatives 22.2 — 22.2 0.1 — 22.1 Repurchase agreements 697.6 — 697.6 — 697.6 — Total liabilities $ 719.8 $ — $ 719.8 $ 0.1 $ 697.6 $ 22.1
Schedule of Derivative Instruments in Statement of Financial Position, Fair Value	In the normal course of business, the Company enters into interest rate lock commitments to finance residential mortgage loans that are not designated as accounting hedges. These commitments, which contain fixed expiration dates, offer the borrower an interest rate guarantee, provided the loan meets underwriting guidelines and closes within the timeframe established by the Company. Interest rate risk arises on these commitments and subsequently closed loans if interest rates change between the time of the interest rate lock and the delivery of the loan to the investor. Loan commitments related to residential mortgage loans intended to be sold are considered derivatives and are marked to market through earnings. In addition to the effects of the change in market interest rate, the fair value measurement of the derivative also contemplates the expected cash flows to be received from the counterparty from the future sale of the loan. The Company sells residential mortgage loans on either a best efforts or mandatory delivery basis. The Company mitigates the effect of the interest rate risk inherent in providing interest rate lock commitments by entering into forward loan sales contracts. During the interest rate lock commitment period, these forward loan sales contracts are marked to market through earnings and are not designated as accounting hedges. Exclusive of the fair value component associated with the projected cash flows from the loan delivery to the investor, the changes in fair value related to movements in market rates of the interest rate lock commitments and the forward loan sales contracts generally move in opposite directions, and the net impact of changes in these valuations on net income during the loan commitment period is generally inconsequential. When the loan is funded to the borrower, the interest rate lock commitment derivative expires, and the Company records a loan held for sale. The forward loan sales contract acts as a hedge against the variability in cash to be received from the loan sale. The changes in measurement of the estimated fair values of the interest rate lock commitments and forward loan sales contracts are included in mortgage banking revenues in the accompanying consolidated statements of income. The notional amounts and estimated fair values of the Company’s derivatives are presented in the following table. Fair value estimates are obtained from third parties and are based on pricing models. December 31, 2020 December 31, 2019 Notional Amount Estimated Notional Amount Estimated Derivative Assets (included in other assets on the consolidated balance sheets) Non-hedging interest rate derivatives: Interest rate swap contracts $ 799.7 $ 52.0 $ 503.2 $ 21.9 Interest rate lock commitments 101.9 3.3 67.8 1.3 Total derivative assets $ 901.6 $ 55.3 $ 571.0 $ 23.2 Derivative Liabilities (included in accounts payable and accrued expenses on the consolidated balance sheets) Derivatives designated as hedges: Interest rate swap contracts $ 87.6 $ 0.2 $ — $ — Non-hedging interest rate derivatives: Interest rate swap contracts 799.7 $ 52.0 503.2 $ 21.9 Forward loan sales contracts 126.8 1.1 128.0 0.3 Total derivative liabilities $ 1,014.1 $ 53.3 $ 631.2 $ 22.2
Derivative Instruments, Gain (Loss)	The following table presents the pre-tax gains or losses related to derivative contracts that were recorded in accumulated other comprehensive income and other non-interest income in the Company’s statements of income: As of or For The Year Ended December 31, 2020 2019 2018 Derivatives designated as hedges: Amount of loss recognized in other comprehensive income (effective portion) $ (0.1) $ — $ — Reclassification adjustment for derivative net gain included in income (0.1) — — Non-hedging interest rate derivatives: Amount of gain recognized in other non-interest income — — 0.3 Amount of net fee income recognized in other non-interest income 7.7 2.5 1.3 Amount of net gains (losses) recognized in mortgage banking revenues $ 1.2 $ 0.3 $ (0.5)

Mortgage Servicing Rights (Tabl

Mortgage Servicing Rights (Tables)	12 Months Ended
	Dec. 31, 2020
Transfers and Servicing [Abstract]
Schedule of Servicing Assets at Amortized Value	Information with respect to the Company’s mortgage servicing rights follows: Year Ended December 31, 2020 2019 2018 Balance at beginning of year $ 30.6 $ 27.7 $ 24.8 Originations of mortgage servicing rights 11.7 7.3 6.0 Amortization expense (8.0) (4.4) (3.1) Balance at end of year 34.3 30.6 27.7 Less valuation reserve (10.3) (0.4) — Balance at end of year, net of valuation reserve $ 24.0 $ 30.2 $ 27.7 Principal balance of serviced loans underlying mortgage servicing rights $ 3,585.5 $ 3,710.1 $ 3,698.2 Mortgage servicing rights as a percentage of serviced loans 0.67 % 0.81 % 0.75 %

Deposits (Tables)

Deposits (Tables)	12 Months Ended
	Dec. 31, 2020
Deposits [Abstract]
Schedule of Deposits, By Type	Deposits are summarized as follows: December 31, 2020 2019 Non-interest bearing demand $ 4,633.5 $ 3,426.5 Interest bearing: Demand 4,118.9 3,195.4 Savings 4,405.9 3,591.6 Time, $100 and over 419.3 651.1 Time, other 639.4 798.9 Total interest bearing 9,583.5 8,237.0 Total deposits $ 14,217.0 $ 11,663.5
Schedule of Maturities of Time Deposits	Maturities of time deposits at December 31, 2020 are as follows: Time, $100 Total Time Due within 3 months or less $ 65.8 $ 271.1 Due after 3 months and within 6 months 77.2 190.8 Due after 6 months and within 12 months 173.0 355.5 Due within 2022 81.7 171.1 Due within 2023 7.3 30.0 Due within 2024 4.0 15.2 Due within 2025 and thereafter 10.3 25.0 Total $ 419.3 $ 1,058.7

Long-Term Debt and Other Borr_2

Long-Term Debt and Other Borrowed Funds (Tables)	12 Months Ended
	Dec. 31, 2020
Debt Disclosure [Abstract]
Summary of Long-Term Debt	A summary of long-term debt follows: December 31, 2020 2019 Parent Company: Fixed to floating subordinated notes, 5.25% fixed rate effective May 2020 through May 2025 $ 98.6 $ — Subsidiaries: 8.00% finance lease obligation with term ending October 25, 2029 1.1 1.2 2.28% note payable maturing July 29, 2022, principal due at maturity, interest payable monthly 5.0 5.0 1.00% note payable maturing December 31, 2041, interest only payable quarterly until December 31, 2025 and then principal and interest until maturity 5.1 5.1 Note payable maturing March 31, 2038, interest only payable at 1.30% monthly until March 31, 2025 and then principal and interest at 3.25% until maturity 2.0 2.0 1.30% note payable maturing June 1, 2034, interest only payable monthly until March 31, 2025 and then principal and interest until maturity 0.6 0.6 Total long-term debt $ 112.4 $ 13.9
Schedule of Maturities of Long-term Debt	Maturities of long-term debt at December 31, 2020 were as follows: 2021 $ 0.1 2022 5.2 2023 0.1 2024 0.1 2025 0.1 Thereafter 106.8 Total $ 112.4

Subordinated Debentures Held _2

Subordinated Debentures Held by Subsidiary Trusts (Tables)	12 Months Ended
	Dec. 31, 2020
Subordinated Borrowings [Abstract]
Schedule of Subordinated Borrowings	A summary of Subordinated Debenture issuances follows: Principal Amount Outstanding Issuance Maturity Date 2020 2019 October 2007 January 1, 2038 $ 10.3 $ 10.3 November 2007 December 15, 2037 15.5 15.5 December 2007 December 15, 2037 20.6 20.6 December 2007 April 1, 2038 15.5 15.5 January 2008 April 1, 2038 10.3 10.3 January 2008 April 1, 2038 10.3 10.3 June 2005 June 30, 2035 4.5 4.4 Total subordinated debentures held by subsidiary trusts $ 87.0 $ 86.9

Regulatory Capital (Tables)

Regulatory Capital (Tables)	12 Months Ended
	Dec. 31, 2020
Banking and Thrift [Abstract]
Schedule of Compliance with Regulatory Capital Requirements under Banking Regulations	The Company’s actual capital amounts and ratios and selected minimum regulatory thresholds and prompt corrective action provisions as of December 31, 2020 and 2019 are presented in the following tables: Actual Minimum Required for Capital Adequacy Purposes For Capital Adequacy Purposes Plus Capital Conservation Buffer Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements (1) December 31, 2020 Amount Ratio Amount Ratio Amount Ratio Amount Ratio Total risk-based capital: Consolidated $ 1,575.7 14.19 $ 888.3 8.00 % $ 1,165.8 10.50 % $ 1,110.3 10.00 % FIB 1,426.8 12.89 885.6 8.00 1,162.3 10.50 1,107.0 10.00 Tier 1 risk-based capital: Consolidated 1,369.0 12.33 666.2 6.00 943.8 8.50 888.3 8.00 FIB 1,320.1 11.93 664.2 6.00 940.9 8.50 885.6 8.00 Common equity tier 1 risk-based capital: Consolidated 1,284.9 11.57 499.6 4.50 777.2 7.00 721.7 6.50 FIB 1,320.1 11.93 498.1 4.50 774.9 7.00 719.5 6.50 Leverage capital ratio: Consolidated 1,369.0 8.16 671.0 4.00 671.0 4.00 838.7 5.00 FIB 1,320.1 7.88 669.7 4.00 669.7 4.00 837.2 5.00 (1) The ratios for the well capitalized requirement are only applicable to FIB. However, the Company manages its capital position as if the requirement applies to the consolidated entity and has presented the ratios as if they also applied on a consolidated basis. In connection with the adoption of CECL, or ASC 326, the Company recognized an after-tax cumulative effect reduction to retained earnings totaling $24.1 million. In March 2020, the Office of the Comptroller of Currency, the Board of Governors of the Federal Reserve System, and the FDIC issued an interim final rule that allows banking organizations to mitigate the effects of ASC 326 on their regulatory capital computations. This interim rule is in addition to the three-year transition period already in place under the capital transition rule previously issued in February 2019. Banking organizations can elect to mitigate the estimated cumulative regulatory capital effects for an additional two years. This rule allows an institution to defer transitioning the impact of ASC 326 into its regulatory capital calculation, including ratios, over an extended period. Additionally, the interim rule extends the transition period whereby an institution can defer the impact from ASC 326 on the current period, determined based on the difference between the new ASC 326 allowance for credit losses and the allowance for loan losses under the incurred loss method from previous GAAP, for up to two years. The total impact related to ASC 326 would then be transitioned into regulatory capital and the associated ratios over a three-year transition period, beginning after the initial two-year deferral period, for a total transition period of five years. The Company has elected to opt into the transition election and is adopting transition relief over the permissible five-year period. Actual Minimum Required for Capital Adequacy Purposes For Capital Adequacy Purposes Plus Capital Conservation Buffer Minimum to Be Well Capitalized Under Prompt Corrective Action Requirements (1) December 31, 2019 Amount Ratio Amount Ratio Amount Ratio Amount Ratio Total risk-based capital: Consolidated $ 1,495.3 14.10 % $ 848.5 8.00 % $ 1,113.6 10.50 % $ 1,060.6 10.00 % FIB 1,321.4 12.50 845.8 8.00 1,110.1 10.50 1,057.2 10.00 Tier 1 risk-based capital: Consolidated 1,422.3 13.41 636.3 6.00 901.5 8.50 848.5 8.00 FIB 1,248.4 11.81 634.3 6.00 898.6 8.50 845.8 8.00 Common equity tier 1 risk-based capital: Consolidated 1,338.2 12.62 477.3 4.50 742.4 7.00 689.4 6.50 FIB 1,248.4 11.81 475.7 4.50 740.0 7.00 687.2 6.50 Leverage capital ratio: Consolidated 1,422.3 10.13 561.6 4.00 561.6 4.00 702.0 5.00 FIB 1,248.4 8.91 560.4 4.00 560.4 4.00 700.4 5.00 (1) The ratios for the well capitalized requirement are only applicable to FIB. However, the Company manages its capital position as if the requirement applies to the consolidated entity and has presented the ratios as if they also applied on a consolidated basis.

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract]
Schedule of Future Minimum Rental Payments for Operating Leases	The total future minimum rental commitments, exclusive of maintenance and operating costs, required under operating leases that have initial or remaining noncancelable lease terms in excess of one year at December 31, 2020, are as follows: Third Related Total For the year ending December 31: 2021 $ 5.6 $ 1.4 $ 7.0 2022 5.2 1.4 6.6 2023 4.0 1.1 5.1 2024 3.5 1.0 4.5 2025 3.1 1.0 4.1 Thereafter 13.4 2.0 15.4 Total $ 34.8 $ 7.9 $ 42.7

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2020
Income Tax Disclosure [Abstract]
Schedule of Income Tax Expense	Income tax expense consists of the following: Year ended December 31, 2020 2019 2018 Current: Federal $ 42.4 $ 40.5 $ 23.1 State 12.3 8.2 7.2 Total current 54.7 48.7 30.3 Deferred: Federal (5.7) 3.7 12.3 State (0.9) 1.7 3.5 Total deferred (6.6) 5.4 15.8 Total income tax expense $ 48.1 $ 54.1 $ 46.1
Schedule of Effective Income Tax Rate Reconciliation	Total income tax provision differs from the amount of income tax determined by applying the statutory federal income tax rate of 21% for the periods presented to income before income taxes due to the following: Year ended December 31, 2020 2019 2018 Tax expense at the statutory tax rate $ 44.0 $ 49.4 $ 43.3 Increase (decrease) in tax resulting from: Tax-exempt income (2.1) (2.8) (2.8) State income tax, net of federal income tax benefit 9.0 9.9 8.5 Benefit of stock-based compensation plans (0.4) (1.2) (1.1) Federal tax credits (2.3) (2.0) (2.6) Other, net (0.1) 0.8 0.8 Tax expense at effective tax rate $ 48.1 $ 54.1 $ 46.1
Schedule of Deferred Tax Assets and Liabilities	The tax effects of temporary differences between the financial statement carrying amounts and tax bases of assets and liabilities that give rise to significant portions of the net deferred tax asset (liability) relate to the following: December 31, 2020 2019 Deferred tax assets: Loans, principally due to allowance for credit losses $ 36.5 $ 18.1 Loan discount 4.0 7.2 Deferred compensation 19.5 18.1 Non-performing loan interest 1.2 1.0 Other real estate owned write-downs and carrying costs — 0.1 Tax credit carryforwards — 0.2 Net operating loss carryforwards (1) 2.0 2.6 Lease liabilities 9.5 9.9 Other 3.2 0.9 Deferred tax assets 75.9 58.1 Deferred tax liabilities: Fixed assets, principally differences in bases and depreciation (10.9) (8.8) Deferred loan costs (4.4) (2.8) Investment securities, unrealized gains (19.2) (3.9) Investment in joint venture partnership, principally due to differences in depreciation of partnership assets (0.9) (0.7) Right of use assets (9.2) (9.7) Prepaid amounts (0.5) (0.5) Government agency stock dividends (1.2) (1.2) Goodwill and core deposit intangibles (50.3) (49.2) Mortgage servicing rights (5.6) (7.2) Other (0.9) (0.8) Deferred tax liabilities (103.1) (84.8) Net deferred tax assets (liabilities) $ (27.2) $ (26.7) (1) As of December 31, 2020, we had remaining federal net operating loss carryforwards of $3.4 million from acquired companies, which is available to offset federal taxable income and state net operating loss carryforwards in amounts which vary by state. The federal net operating losses will expire beginning in 2030 and ending in 2036 and the state net operating losses will expire beginning in 2023 and ending in 2034. The use of these carryforwards is subject to annual limitations.

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2020
Share-based Payment Arrangement [Abstract]
Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity	The following table presents information regarding the Company’s restricted stock: As of December 31, 2020 Number of Weighted-Average Restricted stock, beginning of year 363,022 $ 41.47 Granted 332,085 28.56 Vested (135,366) 40.11 Forfeited (34,912) 35.50 Canceled — — Restricted stock, end of year 524,829 $ 33.65

Other Comprehensive Income (Tab

Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2020
Equity [Abstract]
Schedule of Comprehensive Income (Loss)	The gross amounts of each component of other comprehensive income and the related tax effects for the periods indicated are as follows: Year Ended December 31, 2020 Before Tax Amount Tax Expense (Benefit) Net of Tax Amount Investment securities available-for sale: Change in net unrealized gains during period $ 61.8 $ 15.8 $ 46.0 Reclassification adjustment for net gains included in net income (0.3) (0.1) (0.2) Change in net unrealized loss on derivatives 0.2 — 0.2 Defined benefits post-retirement benefit plan: Change in net actuarial gains (0.5) (0.1) (0.4) Total other comprehensive income $ 61.2 $ 15.6 $ 45.6 Year Ended December 31, 2019 Before Tax Amount Tax Expense (Benefit) Net of Tax Amount Investment securities available-for sale: Change in net unrealized gains during period $ 54.9 $ 14.1 $ 40.8 Reclassification adjustment for net gains included in net income (0.1) — (0.1) Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale (6.0) (1.6) (4.4) Defined benefits post-retirement benefit plan: Change in net actuarial gains (0.8) (0.1) (0.7) Total other comprehensive income $ 48.0 $ 12.4 $ 35.6 Year Ended December 31, 2018 Before Tax Amount Tax Expense (Benefit) Net of Tax Amount Investment securities available-for sale: Change in net unrealized loss during period $ (13.9) $ (3.6) $ (10.3) Reclassification adjustment for net loss included in net income 0.1 — 0.1 Change in unamortized loss on available-for-sale securities transferred into held-to-maturity 1.6 0.4 1.2 Defined benefits post-retirement benefit plan: Change in net actuarial loss (0.6) (0.1) (0.5) Total other comprehensive loss $ (12.8) $ (3.3) $ (9.5)
Schedule of Accumulated Other Comprehensive Income (Loss)	The components of accumulated other comprehensive income, net of income taxes, are as follows: Year ended December 31, 2020 2019 Net unrealized gain on investment securities available-for-sale $ 56.8 $ 10.6 Net unrealized loss on derivatives (0.2) — Net actuarial gain on defined benefit post-retirement benefit plans — 0.4 Net accumulated other comprehensive income (loss) $ 56.6 $ 11.0

Condensed Financial Informati_2

Condensed Financial Information (Parent Company Only) (Tables)	12 Months Ended
	Dec. 31, 2020
Condensed Financial Information Disclosure [Abstract]
Schedule of Condensed Financial Statements	Following is condensed financial information of First Interstate BancSystem, Inc. December 31, 2020 2019 Condensed balance sheets: Cash and cash equivalents $ 123.2 $ 147.1 Investment in bank subsidiary 1,986.6 1,899.3 Advances to subsidiaries, net 41.0 48.9 Other assets 61.3 61.7 Total assets $ 2,212.1 $ 2,157.0 Other liabilities $ 66.7 $ 56.2 Long-term debt 98.6 — Subordinated debentures held by subsidiary trusts 87.0 86.9 Total liabilities 252.3 143.1 Stockholders’ equity 1,959.8 2,013.9 Total liabilities and stockholders’ equity $ 2,212.1 $ 2,157.0 Years Ended December 31, 2020 2019 2018 Condensed statements of income: Dividends from subsidiaries $ 130.0 $ 178.0 $ 148.5 Other interest income 0.1 0.3 0.1 Other income, primarily management fees from subsidiaries 28.7 25.9 17.0 Total income 158.8 204.2 165.6 Salaries and benefits 31.5 34.2 25.3 Interest expense 6.6 4.7 4.5 Acquisition expenses — 17.0 8.1 Other operating expenses, net 15.6 14.8 14.5 Total expenses 53.7 70.7 52.4 Earnings before income tax benefit 105.1 133.5 113.2 Income tax benefit (6.1) (11.9) (9.3) Income before undistributed earnings of subsidiaries 111.2 145.4 122.5 Undistributed earnings of subsidiaries 50.0 35.6 37.7 Net income $ 161.2 $ 181.0 $ 160.2 Years Ended December 31, 2020 2019 2018 Condensed statements of cash flows: Cash flows from operating activities: Net income $ 161.2 $ 181.0 $ 160.2 Adjustments to reconcile net income to cash provided by operating activities: Undistributed earnings of subsidiaries (50.0) (35.6) (37.7) Stock-based compensation expense 7.5 8.0 5.6 Other, net (13.6) 8.1 16.4 Net cash provided by operating activities 105.1 161.5 144.5 Cash flows from investing activities: Acquisition of bank holding company, net of cash and cash equivalents received — — (14.7) Net cash used in investing activities — — (14.7) Years Ended December 31, 2020 2019 2018 Cash flows from financing activities: Net (decrease) increase in advances from subsidiaries 16.7 (6.6) (9.9) Proceeds from issuance of long-term debt 98.6 — — Repayment of long-term debt — — (26.0) Proceeds from issuance of common stock, net of stock issuance costs 1.1 1.0 1.8 Purchase and retirement of common stock (116.8) (2.5) (1.0) Dividends paid to common stockholders (128.6) (79.2) (64.1) Net cash used in financing activities (129.0) (87.3) (99.2) Net change in cash and cash equivalents (23.9) 74.2 30.6 Cash and cash equivalents, beginning of year 147.1 72.9 42.3 Cash and cash equivalents, end of year $ 123.2 $ 147.1 $ 72.9

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value Disclosures [Abstract]
Schedule of Financial Assets and Financial Liabilities Measured at Fair Value on a Recurring Basis	Financial assets and financial liabilities measured at fair value on a recurring basis are as follows: Fair Value Measurements at Reporting Date Using As of December 31, 2020 Balance Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Investment debt securities available-for-sale: State, county, and municipal securities $ 465.9 $ — $ 465.9 $ — Obligations of U.S. government agencies 331.9 — 331.9 — U.S. agency mortgage-backed securities & collateralized mortgage obligations 2,897.6 — 2,897.6 — Private mortgage-backed securities 10.9 — 10.9 — Corporate securities 302.2 — 302.2 — Other investments 0.2 — 0.2 — Loans held for sale 74.0 — 74.0 — Derivative assets: Interest rate swap contracts 52.0 — 52.0 — Interest rate lock commitments 3.3 — 3.3 — Derivative liabilities: Interest rate swap contracts 52.2 — 52.2 — Forward loan sales contracts 1.1 — 1.1 — Deferred compensation plan assets 19.1 — 19.1 — Deferred compensation plan liabilities 19.1 — 19.1 — Fair Value Measurements at Reporting Date Using As of December 31, 2019 Balance Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Investment debt securities available-for-sale: U.S. Treasury Notes $ 9.0 $ — $ 9.0 $ — State, county, and municipal securities 80.9 — 80.9 — Obligations of U.S. government agencies 366.8 — 366.8 — U.S. agency mortgage-backed securities & collateralized mortgage obligations 2,317.2 — 2,317.2 — Private mortgage-backed securities 47.2 — 47.2 — Corporate securities 135.7 — 135.7 — Other investments 3.2 — 3.2 — Loans held for sale 100.9 — 100.9 — Derivative assets: Interest rate swap contracts 21.9 — 21.9 — Interest rate lock commitments 1.3 — 1.3 — Derivative liabilities: Interest rate swap contracts 21.9 — 21.9 — Forward loan sales contracts 0.3 — 0.3 — Deferred compensation plan assets 18.2 — 18.2 — Deferred compensation plan liabilities 18.2 — 18.2 —
Schedule of Financial Assets and Financial Liabilities Measured at Fair Value on a Non-Recurring Basis	The following table presents information about the Company’s assets and liabilities measured at fair value on a non-recurring basis: Fair Value Measurements at Reporting Date Using As of December 31, 2020 Total Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Total Gains (Losses) Collateral-dependent loans* $ 14.7 $ — $ — $ 14.7 $ (2.8) Long-lived assets to be disposed of by sale 5.3 — — 5.3 (0.2) Fair Value Measurements at Reporting Date Using As of December 31, 2019 Total Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Total Impaired loans* $ 27.6 $ — $ — $ 27.6 $ (13.7) Other real estate owned 2.2 — — 2.2 (1.2) Long-lived assets to be disposed of by sale 6.2 — — 6.2 (0.2) *The Company adopted ASC 326 as of January 1, 2020 which changes the methodology of impaired loans. The comparable period presents impaired loans under previously applicable GAAP.
Fair Value Inputs, Assets, Quantitative Information	The following table presents additional quantitative information about assets measured at fair value on a non-recurring basis and for which the Company has utilized Level 3 inputs to determine fair values: As of December 31, 2020 Fair Value Valuation Technique Unobservable Inputs Range (Weighted Average) Collateral-dependent loans* $ 14.7 Appraisal Appraisal adjustment 0% - 18% (9%) Long-lived assets to be disposed of by sale 5.3 Appraisal Appraisal adjustment 0% - 0% 0% As of December 31, 2019 Fair Value Valuation Technique Unobservable Inputs Range (Weighted Average) Impaired loans* $ 27.6 Appraisal Appraisal adjustment 0% - 56% (22%) Other real estate owned 2.2 Appraisal Appraisal adjustment 8% - 65% (27%) Long-lived assets to be disposed of by sale 6.2 Appraisal Appraisal adjustment 0% - 37% (3%) *The Company adopted ASC 326 as of January 1, 2020 which changes the methodology of impaired loans. The comparable period presents impaired loans under previously applicable GAAP.
Fair Value, by Balance Sheet Grouping	The estimated fair values of financial instruments that are reported in the Company’s consolidated balance sheets, segregated by the level of the valuation inputs within the fair value hierarchy utilized to measure fair value, are as follows: ` Fair Value Measurements at Reporting Date Using As of December 31, 2020 Carrying Amount Estimated Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Financial assets: Cash and cash equivalents $ 2,276.8 $ 2,276.8 $ 2,276.8 $ — $ — Investment debt securities available-for-sale 4,008.7 4,008.7 — 4,008.7 — Investment debt securities held-to-maturity 51.6 55.0 — 55.0 — Accrued interest receivable 51.1 51.1 — 51.1 — Mortgage servicing rights, net 24.0 24.0 — 24.0 — Loans held for sale 74.0 74.0 — 74.0 — Net loans held for investment 9,663.2 9,785.6 — 9,770.9 14.7 Derivative assets 55.3 55.3 — 55.3 — Deferred compensation plan assets 19.1 19.1 — 19.1 — Total financial assets $ 16,223.8 $ 16,349.6 $ 2,276.8 $ 14,058.1 $ 14.7 Financial liabilities: Total deposits, excluding time deposits $ 13,158.3 $ 13,158.3 $ 13,158.3 $ — $ — Time deposits 1,058.7 1,061.1 — 1,061.1 — Securities sold under repurchase agreements 1,091.4 1,091.4 — 1,091.4 — Accrued interest payable 5.8 5.8 — 5.8 — Long-term debt 112.4 116.5 — 116.5 — Subordinated debentures held by subsidiary trusts 87.0 81.3 — 81.3 — Derivative liabilities 53.3 53.3 — 53.3 — Deferred compensation plan liabilities 19.1 19.1 — 19.1 — Total financial liabilities $ 15,586.0 $ 15,586.8 $ 13,158.3 $ 2,428.5 $ — Fair Value Measurements at Reporting Date Using As of December 31, 2019 Carrying Amount Estimated Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Financial assets: Cash and cash equivalents $ 1,076.8 $ 1,076.8 $ 1,076.8 $ — $ — Investment debt securities available-for-sale 2,960.0 2,960.0 — 2,960.0 — Investment debt securities held-to-maturity 92.3 94.5 — 94.5 — Accrued interest receivable 46.7 46.7 — 46.7 — Mortgage servicing rights, net 30.2 34.8 — 34.8 — Loans held for sale 100.9 100.9 — 100.9 — Net loans held for investment 8,857.7 8,930.7 — 8,906.7 24.0 Derivative assets 23.2 23.2 — 23.2 — Deferred compensation plan assets 18.2 18.2 — 18.2 — Total financial assets $ 13,206.0 $ 13,285.8 $ 1,076.8 $ 12,185.0 $ 24.0 Financial liabilities: Total deposits, excluding time deposits $ 10,213.5 $ 10,213.5 $ 10,213.5 $ — $ — Time deposits 1,450.0 1,446.6 — 1,446.6 — Securities sold under repurchase agreements 697.6 697.6 — 697.6 — Accrued interest payable 12.1 12.1 — 12.1 — Long-term debt 13.9 10.4 — 10.4 — Subordinated debentures held by subsidiary trusts 86.9 81.3 — 81.3 — Derivative liabilities 22.2 22.2 — 22.2 — Deferred compensation plan liabilities 18.2 18.2 — 18.2 — Total financial liabilities $ 12,514.4 $ 12,501.9 $ 10,213.5 $ 2,288.4 $ —

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2020USD ($)subsidiary	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Property, Plant and Equipment
Benefit of stock-based compensation plans			$ (1,100,000)
Impairment losses	$ 0	$ 0	0
Number of significant subsidiaries | subsidiary	1
Income tax examination, penalties	$ 0	0	0
Cash on deposit with Federal Reserve Bank	1,989,300,000	769,300,000
Compensating balances with Federal Reserve Bank	0	46,300,000
Federal Reserve Bank stock	42,800,000	42,800,000
Federal Home Loan Bank stock	10,600,000	10,700,000
Advertising expense	2,800,000	4,200,000	$ 3,200,000
Loans and Leases Receivable Disclosure [Line Items]
Accrued interest receivable	51,100,000	46,700,000
Accrued Investment Income Receivable	12,600,000	9,800,000
Hosting Arrangement, Service Contract, Implementation Cost, Expense, Amortization	6,000,000	9,200,000
Impairment losses	$ 0	0
Buildings and improvements | Minimum
Property, Plant and Equipment
Estimated useful lives	5 years
Buildings and improvements | Maximum [Member]
Property, Plant and Equipment
Estimated useful lives	45 years
Furniture and equipment | Minimum
Property, Plant and Equipment
Estimated useful lives	3 years
Furniture and equipment | Maximum [Member]
Property, Plant and Equipment
Estimated useful lives	15 years
Loans
Loans and Leases Receivable Disclosure [Line Items]
Accrued interest receivable	$ 37,900,000	36,900,000
Common Stock Including Additional Paid in Capital [Member]
Property, Plant and Equipment
Benefit of stock-based compensation plans	$ (400,000)	$ (1,200,000)

Acquisitions - Narrative (Detai

Acquisitions - Narrative (Details) $ / shares in Units, $ in Millions	Apr. 08, 2019USD ($)office$ / sharesshares	Dec. 31, 2020USD ($)segment	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Business Acquisition [Line Items]
Goodwill		$ 621.6	$ 621.6	$ 546.7
Number of operating segments | segment		1
Acquisition related expenses		$ 0	20.3	$ 12.4
Core Deposits Intangibles
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Net	$ 3	51.2	62.1
Finite-lived intangible assets acquired		$ 0	$ 16.6
Estimated useful lives of related deposits	10 years
Idaho Independent Bank [Member]
Business Acquisition [Line Items]
Core deposit intangible assets	$ 13.6
Number of banking offices | office	11
Total consideration paid	$ 157.3
Goodwill	$ 73
Idaho Independent Bank [Member] | Class A Common Stock
Business Acquisition [Line Items]
Business Acquisition Conversion Share Rate | shares	0.50
Common stock issued (in shares) | shares	3,871,422
Business Acquisition, Share Price | $ / shares	$ 40.64
Idaho Independent Bank [Member] | Core Deposits Intangibles
Business Acquisition [Line Items]
Estimated useful lives of related deposits	10 years
Community 1st Bank [Member]
Business Acquisition [Line Items]
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Financial Liabilities	$ 110.1
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Assets	$ 129.1
Number of banking offices | office	3
Total consideration paid	$ 18.8
Goodwill	2.3
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Net Assets	16.5
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Financial Assets	78.8
Business Combination, Acquired Receivables, Estimated Uncollectible	0.7
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Liabilities	$ 112.6
Community 1st Bank [Member] | Class A Common Stock
Business Acquisition [Line Items]
Business Acquisition Conversion Share Rate | shares	0.3784
Common stock issued (in shares) | shares	463,134
Business Acquisition, Share Price | $ / shares	$ 40.64

Acquisitions - Schedule of Reco

Acquisitions - Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed (Details) $ / shares in Units, $ in Millions	Apr. 08, 2019USD ($)office$ / sharesshares	Dec. 31, 2020USD ($)segment	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Business Acquisition [Line Items]
Goodwill		$ 621.6	$ 621.6	$ 546.7
Number of operating segments | segment		1
Idaho Independent Bank, Fair Value Adjustments [Member]
Business Acquisition [Line Items]
Cash and cash equivalents	$ 0
Investment securities	0.5
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held for Investment	(9.8)
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held For Sale	0
Allowance for loan losses	6.3
Premises and equipment	4.8
Other real estate owned (“OREO”)	2
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Life Insurance, Corporate Or Bank Owned	0
Core deposit intangible assets	13.6
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Deferred Tax Asset	(2.6)
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Accounts Payable And Accrued Expense	2.6
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Assets	(0.7)
Total assets acquired	14.1
Deposits	0.1
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities	0.1
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Securities Sold Under Repurchase Agreements	0
Total liabilities assumed	2.8
Net assets acquired	$ 11.3
Idaho Independent Bank [Member]
Business Acquisition [Line Items]
Number of banking offices | office	11
Cash and cash equivalents	$ 270.7
Investment securities	63.2
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held for Investment	337.8
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held For Sale	0.5
Allowance for loan losses	0
Premises and equipment	21.3
Other real estate owned (“OREO”)	2.4
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Life Insurance, Corporate Or Bank Owned	15.2
Core deposit intangible assets	13.6
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Deferred Tax Asset	0.6
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Accounts Payable And Accrued Expense	17.8
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Assets	7.9
Total assets acquired	733.2
Deposits	596.6
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities	4.1
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Securities Sold Under Repurchase Agreements	30.4
Total liabilities assumed	648.9
Net assets acquired	84.3
Class A common stock	157.3
Total consideration paid	157.3
Goodwill	$ 73
Idaho Independent Bank [Member] | Class A Common Stock
Business Acquisition [Line Items]
Business Acquisition Conversion Share Rate | shares	0.50
Common stock issued (in shares) | shares	3,871,422
Business Acquisition, Share Price | $ / shares	$ 40.64
Idaho Independent Bank, As Recorded By Idaho Independent Bank [Member]
Business Acquisition [Line Items]
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held For Sale	$ 0.5
Allowance for loan losses	(6.3)
Premises and equipment	16.5
Other real estate owned (“OREO”)	0.4
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Life Insurance, Corporate Or Bank Owned	15.2
Core deposit intangible assets	0
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Deferred Tax Asset	3.2
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Accounts Payable And Accrued Expense	15.2
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Assets	8.6
Total assets acquired	719.1
Deposits	596.5
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities	4
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Securities Sold Under Repurchase Agreements	30.4
Total liabilities assumed	646.1
Net assets acquired	$ 73
Community 1st Bank [Member]
Business Acquisition [Line Items]
Number of banking offices | office	3
Total consideration paid	$ 18.8
Goodwill	$ 2.3
Community 1st Bank [Member] | Class A Common Stock
Business Acquisition [Line Items]
Business Acquisition Conversion Share Rate | shares	0.3784
Common stock issued (in shares) | shares	463,134
Business Acquisition, Share Price | $ / shares	$ 40.64
Northwest Bank Acquisition, As Recorded By Inland Northwest Bank [Member]
Business Acquisition [Line Items]
Cash and cash equivalents	$ 270.7
Investment securities	62.7
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held for Investment	$ 347.6
Core Deposits Intangibles
Business Acquisition [Line Items]
Estimated useful lives of related deposits	10 years
Core Deposits Intangibles | Idaho Independent Bank [Member]
Business Acquisition [Line Items]
Estimated useful lives of related deposits	10 years

Acquisitions - Schedule of Acqu

Acquisitions - Schedule of Acquisition Related Expenses (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Business Combinations [Abstract]
Acquisition related expenses	$ 0	$ 20.3	$ 12.4

Acquisitions - Schedules of Loa

Acquisitions - Schedules of Loans Acquired (Details) - Idaho Independent Bank [Member] $ in Millions	Apr. 08, 2019USD ($)
Business Acquisition [Line Items]
Contractually required principal and interest payments	$ 24.1
Contractual cash flows not expected to be collected (“non-accretable discount”)	3.9
Cash flows expected to be collected	20.2
Interest component of cash flows expected to be collected (“accretable discount”)	3.4
Fair value of acquired credit-impaired loans	16.8
Contractually required principal and interest payments	398.7
Contractual cash flows not expected to be collected	15.2
Fair value at acquisition	$ 321.5

Acquisitions - Schedule of Unau

Acquisitions - Schedule of Unaudited Pro Forma Revenue and Income (Details)	Apr. 08, 2019
Core Deposits Intangibles
Business Acquisition [Line Items]
Estimated useful lives of related deposits	10 years

Goodwill and Core Deposit Int_3

Goodwill and Core Deposit Intangibles - Schedule of Goodwill (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Line Items]
Goodwill, Acquired During Period	$ 0	$ 74.9
Property, Plant and Equipment, Net	312.3	306
Goodwill [Roll Forward]
Net carrying value at beginning of period	621.6	546.7
Net carrying value at end of period	$ 621.6	$ 621.6

Goodwill and Core Deposit Int_4

Goodwill and Core Deposit Intangibles - Goodwill Narrative (Details) - USD ($) $ in Millions	Jul. 01, 2020	Jul. 01, 2019	Jul. 01, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Line Items]
Finite-Lived Intangible Asset, Useful Life				10 years
Goodwill				$ 621.6	$ 621.6	$ 546.7
Impairment of goodwill	$ 0	$ 0	$ 0

Goodwill and Core Deposit Int_5

Goodwill and Core Deposit Intangibles - Schedule of Core Deposit Intangibles Activity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Finite-lived Intangible Assets [Roll Forward]
Accumulated amortization	$ 10.9	$ 11.2	$ 7.9
Finite-Lived Intangible Asset, Useful Life	10 years
Core Deposits Intangibles
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross		106	$ 89.7
Finite-lived Intangible Assets [Roll Forward]
Gross CDI, beginning of period	$ 62.1
Established through acquisitions	0	16.6
Reductions due to sale of accounts	0	(0.3)
Finite-Lived Intangible Assets, Accumulated Amortization	54.8	43.9
Net CDI, end of period	$ 51.2	$ 62.1

Goodwill and Core Deposit Int_6

Goodwill and Core Deposit Intangibles - Intangible Assets Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	10 years
Core Deposits Intangibles
Finite-Lived Intangible Assets [Line Items]
Reductions due to sale of accounts	$ 0	$ (0.3)

Goodwill and Core Deposit Int_7

Goodwill and Core Deposit Intangibles - Schedule of Future Estimated CDI Amortization Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Apr. 08, 2019
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	10 years
Core Deposits Intangibles
Finite-Lived Intangible Assets [Line Items]
2020	$ 9.9
2021	9
2022	8.2
2023	7.3
2024	6.5
Thereafter	10.3
Total	$ 51.2	$ 62.1	$ 3

Investment Securities - Amortiz

Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Schedule of Available for Sale and Held-to-Maturity Securities
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Amortized Cost	$ 687.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	5.7	$ 7.2
Available-for-sale Securities, Gross Realized Gains	0
Available-for-sale Securities, Gross Realized Losses		0
Available-for-Sale:
Amortized Cost	3,932.7	2,945.5
Gross Unrealized Gains	81.7	21.7
Gross Unrealized Losses	(5.7)	(7.2)
Estimated Fair Value	4,008.7	2,960
Held-to-Maturity:
Amortized Cost	51.6	92.3
Gross Unrealized Gains	3.4	2.2
Gross Unrealized Losses	0	0
Estimated Fair Value	55	94.5
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	991.4
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	8.3
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	8.4
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost	1,587.7
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	1,362.6
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	27.8
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	29
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost	736.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	996.3
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	14.1
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	16.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost	921.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	658.4
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost	1.4
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	1.4
US Treasury notes
Available-for-Sale:
Amortized Cost		9
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value		9
State, county and municipal securities
Schedule of Available for Sale and Held-to-Maturity Securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1
Available-for-Sale:
Amortized Cost	462.1
Gross Unrealized Gains	4.8	0.8
Gross Unrealized Losses	(1)	0
Estimated Fair Value	465.9	80.9
Held-to-Maturity:
Amortized Cost	46.6	57.3
Gross Unrealized Gains	3.2	2.1
Gross Unrealized Losses	0	0
Estimated Fair Value	49.8	59.4
Available-for-sale securities, amortized cost basis	38.6	80.1
US Government Agencies Debt Securities [Member]
Schedule of Available for Sale and Held-to-Maturity Securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	2	0.8
Available-for-Sale:
Amortized Cost	332.9	367.5
Gross Unrealized Gains	1	0.1
Gross Unrealized Losses	(2)	(0.8)
Estimated Fair Value	331.9	366.8
Held-to-Maturity:
Amortized Cost		19.8
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value		19.8
Corporate Debt Securities [Member]
Schedule of Available for Sale and Held-to-Maturity Securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0.1
Available-for-Sale:
Amortized Cost	295.8	134.5
Gross Unrealized Gains	6.5	1.2
Gross Unrealized Losses	(0.1)	0
Estimated Fair Value	302.2	135.7
Held-to-Maturity:
Amortized Cost	3.9	13.9
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	0	0
Estimated Fair Value	4	14
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available for Sale and Held-to-Maturity Securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	2.5	6
Available-for-Sale:
Amortized Cost	2,830.8	2,303.6
Gross Unrealized Gains	69.3	19.6
Gross Unrealized Losses	(2.5)	(6)
Estimated Fair Value	2,897.6	2,317.2
Held-to-Maturity:
Amortized Cost	1	1.2
Gross Unrealized Gains	0.1	0
Gross Unrealized Losses	0	0
Estimated Fair Value	1.1	1.2
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available for Sale and Held-to-Maturity Securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0.1	0.4
Available-for-Sale:
Amortized Cost	10.9	47.6
Gross Unrealized Gains	0.1	0
Gross Unrealized Losses	(0.1)	(0.4)
Estimated Fair Value	10.9	47.2
Other Debt Obligations [Member]
Available-for-Sale:
Amortized Cost	0.2	3.2
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	0.2	3.2
Other Investments
Held-to-Maturity:
Amortized Cost	0.1	0.1
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	0.1	$ 0.1
Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming | State, county and municipal securities
Held-to-Maturity:
Available-for-sale securities, amortized cost basis	26
Callable Within One Year [Member]
Schedule of Available for Sale and Held-to-Maturity Securities
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Amortized Cost Basis	282.4
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Fair Value	281.7
Held-to-Maturity:
Investment Securities, Callable Structured, Amortized Cost Basis	$ 0

Investment Securities - Realize

Investment Securities - Realized Gains (Losses) on Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities
Gross realized gains	$ 0
Gross realized losses		$ 0
Less than 12 Months	838.6	925.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	5.5	5.4
12 Months or More	16.6	202.5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.2	1.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	855.2	1,127.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	5.7	7.2
US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities
Less than 12 Months	235.6	185.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2	0.8
12 Months or More	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	235.6	185.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	2	0.8
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities
Less than 12 Months	434	740.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2.4	4.6
12 Months or More	12.3	155.9
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.1	1.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	446.3	896
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	2.5	6
State, county and municipal securities
Schedule of Available-for-sale Securities
Less than 12 Months	148.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1
12 Months or More	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	148.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1
General obligation securities, amortized cost	38.6	80.1
State, county and municipal securities | Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming
Schedule of Available-for-sale Securities
General obligation securities, amortized cost	26
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities
Less than 12 Months	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
12 Months or More	4.3	46.6
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.1	0.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	4.3	46.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0.1	$ 0.4
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities
Less than 12 Months	20.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.1
12 Months or More	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	20.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 0.1

Investment Securities - Gross U

Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details)	12 Months Ended
	Dec. 31, 2020USD ($)Investments	Dec. 31, 2019USD ($)Investments
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	$ 51,600,000	$ 92,300,000
Available-for-Sale and Held-to-Maturity:
Investment securities in an unrealized loss position (number of securities) | Investments	181	338
Impairment losses	$ 0	$ 0
Less than 12 Months	838,600,000	925,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	5,500,000	5,400,000
12 Months or More	16,600,000	202,500,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	200,000	1,800,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	855,200,000	1,127,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	5,700,000	7,200,000
Gross Unrealized Losses
Less than 12 Months	(5,500,000)	(5,400,000)
12 Months or More	16,600,000	202,500,000
12 Months or More	(200,000)	(1,800,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	855,200,000	1,127,900,000
Total	(5,700,000)	(7,200,000)
Less than 12 Months	838,600,000	925,400,000
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	1,000,000	1,200,000
Available-for-Sale and Held-to-Maturity:
Less than 12 Months	434,000,000	740,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,400,000	4,600,000
12 Months or More	12,300,000	155,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	100,000	1,400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	446,300,000	896,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	2,500,000	6,000,000
Gross Unrealized Losses
Less than 12 Months	(2,400,000)	(4,600,000)
12 Months or More	12,300,000	155,900,000
12 Months or More	(100,000)	(1,400,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	446,300,000	896,000,000
Total	(2,500,000)	(6,000,000)
Less than 12 Months	434,000,000	740,100,000
US Government Agencies Debt Securities [Member]
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity		19,800,000
Available-for-Sale and Held-to-Maturity:
Less than 12 Months	235,600,000	185,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,000,000	800,000
12 Months or More	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	235,600,000	185,300,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	2,000,000	800,000
Gross Unrealized Losses
Less than 12 Months	(2,000,000)	(800,000)
12 Months or More	0	0
12 Months or More	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	235,600,000	185,300,000
Total	(2,000,000)	(800,000)
Less than 12 Months	235,600,000	185,300,000
Corporate Debt Securities [Member]
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	3,900,000	13,900,000
Available-for-Sale and Held-to-Maturity:
Less than 12 Months	20,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	100,000
12 Months or More	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	20,900,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	100,000
Gross Unrealized Losses
Less than 12 Months	(100,000)
12 Months or More	0
12 Months or More	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	20,900,000
Total	(100,000)
Less than 12 Months	20,900,000
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Available-for-Sale and Held-to-Maturity:
Less than 12 Months	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
12 Months or More	4,300,000	46,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	100,000	400,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	4,300,000	46,600,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	100,000	400,000
Gross Unrealized Losses
Less than 12 Months	0	0
12 Months or More	4,300,000	46,600,000
12 Months or More	(100,000)	(400,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	4,300,000	46,600,000
Total	(100,000)	(400,000)
Less than 12 Months	0	0
State, county and municipal securities
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	46,600,000	$ 57,300,000
Available-for-Sale and Held-to-Maturity:
Less than 12 Months	148,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,000,000
12 Months or More	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	148,100,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1,000,000
Gross Unrealized Losses
Less than 12 Months	(1,000,000)
12 Months or More	0
12 Months or More	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	148,100,000
Total	(1,000,000)
Less than 12 Months	$ 148,100,000

Investment Securities - Maturit

Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Amortized Cost	$ 687.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost	1,587.7
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost	736.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost	921.2
Amortized Cost	3,932.7	$ 2,945.5
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	991.4
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	1,362.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	996.3
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	658.4
Estimated Fair Value	4,008.7	2,960
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	8.3
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	27.8
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	14.1
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost	1.4
Amortized Cost	51.6	92.3
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	8.4
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	29
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	16.2
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	1.4
Estimated Fair Value	55	94.5
Available-for-sale Securities and Held-to-maturity Securities
Securities sold under agreements to repurchase	2,323	2,132
Pledged assets separately reported, securities pledged for repurchase agreements, at fair value	2,383.6	2,144.9
Mortgage Backed Securities, Variable Rate
Available-for-sale Securities and Held-to-maturity Securities
Available-for-sale securities, amortized cost basis	$ 220.7	$ 298.1

Loans - Schedule of Loans by Cl

Loans - Schedule of Loans by Class (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Dec. 31, 2017
Loans and Leases Receivable
Loans and Leases Receivable, Deferred Income	$ (21)	$ (5.5)
Mortgage loans held for sale, at fair value	74	100.9
Total loans	9,807.5	8,930.7
Accounts Receivable, Allowance for Credit Loss	(144.3)	(73)	$ (73)	$ (73)	$ (72.1)
Loans and Leases Receivable, Net Amount	9,663.2	8,857.7
Loans, principally due to allowance for credit losses	(36.5)	(18.1)
Less loans charged-off	(20.9)	(23.6)	$ (19.7)
Financing Receivable, Allowance for Credit Loss, Recovery	6.7
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	55.5
Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(30)
Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable
Loans held for investment	3,743.2	3,487.8
Accounts Receivable, Allowance for Credit Loss	(54.8)			(19.5)
Loans, principally due to allowance for credit losses	(20.1)
Less loans charged-off	(0.4)
Financing Receivable, Allowance for Credit Loss, Recovery	0.3
Commercial Real Estate Portfolio Segment [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(15.3)
Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable
Loans held for investment	250.9	244.1
Total loans	250.9
Accounts Receivable, Allowance for Credit Loss	(1.6)			(1.5)
Loans, principally due to allowance for credit losses	(1)
Less loans charged-off	0
Financing Receivable, Allowance for Credit Loss, Recovery	0
Residential Real Estate Construction Financing Receivable [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	0.9
Commercial Real Estate Construction [Member]
Loans and Leases Receivable
Loans held for investment	523.5	431.5
Total loans	523.5
Accounts Receivable, Allowance for Credit Loss	(7.3)			(2.7)
Loans, principally due to allowance for credit losses	(3.3)
Less loans charged-off	0
Financing Receivable, Allowance for Credit Loss, Recovery	0
Commercial Real Estate Construction [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(1.3)
Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable
Loans held for investment	1,039.4	977.7
Residential Mortgage [Member]
Loans and Leases Receivable
Loans held for investment	1,396.3	1,246.1
Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable
Loans held for investment	220.6	226.6
Total loans	220.6
Accounts Receivable, Allowance for Credit Loss	(2.7)			(0.5)
Loans, principally due to allowance for credit losses	(0.4)
Less loans charged-off	0
Financing Receivable, Allowance for Credit Loss, Recovery	0
Agricultural Real Estate Financing Receivable [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(1.8)
Real Estate Financing Receivable [Member]
Loans and Leases Receivable
Loans held for investment	6,399.5	5,938.2
Accounts Receivable, Allowance for Credit Loss	(80.5)			(28.9)
Loans, principally due to allowance for credit losses	(22.9)
Less loans charged-off	(0.9)
Financing Receivable, Allowance for Credit Loss, Recovery	1.1
Real Estate Financing Receivable [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(28.5)
Indirect consumer
Loans and Leases Receivable
Loans held for investment	805.1	784.6
Total loans	805.1
Accounts Receivable, Allowance for Credit Loss	(16.7)			(4.5)
Loans, principally due to allowance for credit losses	(5.4)
Less loans charged-off	(4.1)
Financing Receivable, Allowance for Credit Loss, Recovery	2.1
Indirect consumer | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(8.8)
Other consumer
Loans and Leases Receivable
Loans held for investment	150.6	179
Total loans	150.6
Accounts Receivable, Allowance for Credit Loss	(4.6)			(2.9)
Loans, principally due to allowance for credit losses	(1.6)
Less loans charged-off	(3.9)
Financing Receivable, Allowance for Credit Loss, Recovery	1
Other consumer | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(3)
Credit card consumer
Loans and Leases Receivable
Loans held for investment	70.2	81.6
Accounts Receivable, Allowance for Credit Loss	(2.6)			(2.5)
Loans, principally due to allowance for credit losses	(1.8)
Less loans charged-off	(2.8)
Financing Receivable, Allowance for Credit Loss, Recovery	0.8
Credit card consumer | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(0.3)
Total consumer loans
Loans and Leases Receivable
Loans held for investment	1,025.9	1,045.2
Accounts Receivable, Allowance for Credit Loss	(23.9)			(9.9)
Loans, principally due to allowance for credit losses	(8.8)
Less loans charged-off	(10.8)
Financing Receivable, Allowance for Credit Loss, Recovery	3.9
Total consumer loans | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(12.1)
Commercial Portfolio Segment [Member]
Loans and Leases Receivable
Loans held for investment	2,153.9	1,673.7
Accounts Receivable, Allowance for Credit Loss	(39.2)			(32.6)
Loans, principally due to allowance for credit losses	(24)
Less loans charged-off	(9.1)
Financing Receivable, Allowance for Credit Loss, Recovery	1.7
Commercial Portfolio Segment [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	10
Agricultural Financing Receivable [Member]
Loans and Leases Receivable
Loans held for investment	247.6	279.1
Accounts Receivable, Allowance for Credit Loss	(0.7)			(1.6)
Loans, principally due to allowance for credit losses	(0.2)
Less loans charged-off	(0.1)
Financing Receivable, Allowance for Credit Loss, Recovery	0
Agricultural Financing Receivable [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	0.6
Other, including overdrafts
Loans and Leases Receivable
Loans held for investment	1.6	0
Loans held for investment
Loans and Leases Receivable
Loans held for investment	9,828.5	8,936.2
FIBK Commercial Real Estate Non Owner Occupied Loans
Loans and Leases Receivable
Total loans	1,674.5
Accounts Receivable, Allowance for Credit Loss	(25.5)			(8.8)
Loans, principally due to allowance for credit losses	(11.7)
Less loans charged-off	0
FIBK Commercial Real Estate Non Owner Occupied Loans | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(4.9)
Financing Receivable, Allowance for Credit Loss, Recovery	0.1
Commercial Real Estate Owner Occupied
Loans and Leases Receivable
Total loans	1,705
Accounts Receivable, Allowance for Credit Loss	(18.3)			(10)
Loans, principally due to allowance for credit losses	(5)
Less loans charged-off	(0.4)
Financing Receivable, Allowance for Credit Loss, Recovery	0.2
Commercial Real Estate Owner Occupied | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(3.5)
Commercial Real Estate Multifamily
Loans and Leases Receivable
Total loans	363.7
Accounts Receivable, Allowance for Credit Loss	(11)			(0.7)
Loans, principally due to allowance for credit losses	(3.4)
Less loans charged-off	0
Financing Receivable, Allowance for Credit Loss, Recovery	0
Commercial Real Estate Multifamily | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(6.9)
Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable
Loans held for investment	265	$ 302.1
Total loans	265
Accounts Receivable, Allowance for Credit Loss	(1.3)			(1.9)
Loans, principally due to allowance for credit losses	(0.4)
Less loans charged-off	(0.5)
Financing Receivable, Allowance for Credit Loss, Recovery	0.4
Land Acquisition And Development Construction Financing Receivable [Member] | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	0.1
Construction Loans
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(10.2)			(6.1)
Loans, principally due to allowance for credit losses	(3.9)
Less loans charged-off	(0.5)
Financing Receivable, Allowance for Credit Loss, Recovery	0.4
Construction Loans | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(0.3)
Residential Real Estate 1-4 Family
Loans and Leases Receivable
Total loans	1,012.3
Accounts Receivable, Allowance for Credit Loss	(11.4)			(1.8)
Loans, principally due to allowance for credit losses	(1.1)
Less loans charged-off	0
Financing Receivable, Allowance for Credit Loss, Recovery	0.1
Residential Real Estate 1-4 Family | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(10.6)
Home Equity Loan
Loans and Leases Receivable
Total loans	384
Accounts Receivable, Allowance for Credit Loss	(1.4)			(1)
Loans, principally due to allowance for credit losses	(0.4)
Less loans charged-off	0
Financing Receivable, Allowance for Credit Loss, Recovery	0.3
Home Equity Loan | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(0.5)
Residential Real Estate
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(12.8)			(2.8)
Loans, principally due to allowance for credit losses	(1.5)
Less loans charged-off	0
Financing Receivable, Allowance for Credit Loss, Recovery	0.4
Residential Real Estate | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(11.1)
Commercial and Floor Plans
Loans and Leases Receivable
Total loans	1,814.5
Accounts Receivable, Allowance for Credit Loss	(34.2)			(25.5)
Loans, principally due to allowance for credit losses	(20.4)
Less loans charged-off	(8)
Financing Receivable, Allowance for Credit Loss, Recovery	1.4
Commercial and Floor Plans | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	5.1
Commercial Purpose Secured by 1-4 Family
Loans and Leases Receivable
Total loans	272.8
Accounts Receivable, Allowance for Credit Loss	(4.7)			(5.9)
Loans, principally due to allowance for credit losses	(2.5)
Less loans charged-off	(0.1)
Financing Receivable, Allowance for Credit Loss, Recovery	0.2
Commercial Purpose Secured by 1-4 Family | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	3.8
Commercial Borrower
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(0.3)			$ (1.2)
Loans, principally due to allowance for credit losses	(1.1)
Less loans charged-off	(1)
Financing Receivable, Allowance for Credit Loss, Recovery	0.1
Commercial Borrower | Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	$ 1.1

Loans - Schedule of Recorded In

Loans - Schedule of Recorded Investment in Past Due Loans by Class (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Loans and Leases Receivable Recorded Investment, Past Due
Total loans	$ 9,807.5	$ 8,930.7
Impaired Financing Receivable, Interest Income, Accrual Method	0	0.3	$ 0.3
Financing Receivable, Allowance for Credit Loss, Writeoff	0
Loans and Leases Receivable, Nonperforming, Accrual of Interest	0
Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	12.8	7.2
Current Loans	3,720.8	3,467.6
Non-accrual Loans	9.6	13
Loans held for investment	3,743.2	3,487.8
Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	3.7	1.8
Current Loans	260.6	298.9
Non-accrual Loans	0.7	1.4
Loans held for investment	265	302.1
Total loans	265
Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	1.9	2.3
Current Loans	247.9	241.8
Non-accrual Loans	1.1	0
Loans held for investment	250.9	244.1
Total loans	250.9
Commercial Real Estate Construction [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	12.2	0
Current Loans	511.2	431
Non-accrual Loans	0.1	0.5
Loans held for investment	523.5	431.5
Total loans	523.5
Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	17.8	4.1
Current Loans	1,019.7	971.7
Non-accrual Loans	1.9	1.9
Loans held for investment	1,039.4	977.7
Residential Mortgage [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	6.8	6.3
Current Loans	1,384.9	1,235.2
Non-accrual Loans	4.6	4.6
Loans held for investment	1,396.3	1,246.1
Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	2	1.3
Current Loans	212.4	220.1
Non-accrual Loans	6.2	5.2
Loans held for investment	220.6	226.6
Total loans	220.6
Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	39.4	18.9
Current Loans	6,337.8	5,894.6
Non-accrual Loans	22.3	24.7
Loans held for investment	6,399.5	5,938.2
Impaired Financing Receivable, Interest Income, Accrual Method		0.1	0.1
Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	8.9	10
Current Loans	794.3	773
Non-accrual Loans	1.9	1.6
Loans held for investment	805.1	784.6
Total loans	805.1
Other consumer
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	1.2	1.8
Current Loans	149	176.7
Non-accrual Loans	0.4	0.5
Loans held for investment	150.6	179
Total loans	150.6
Credit card consumer
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	1.6	2.1
Current Loans	68.6	79.5
Non-accrual Loans	0	0
Loans held for investment	70.2	81.6
Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	11.7	13.9
Current Loans	1,011.9	1,029.2
Non-accrual Loans	2.3	2.1
Loans held for investment	1,025.9	1,045.2
Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	9.2	9.7
Current Loans	2,132.9	1,650.3
Non-accrual Loans	11.8	13.7
Loans held for investment	2,153.9	1,673.7
Impaired Financing Receivable, Interest Income, Accrual Method		0.2	0.2
Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0	0
Current Loans	1.6	0
Non-accrual Loans	0	0
Loans held for investment	1.6	0
Loans held for investment
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	62.7	43.5
Current Loans	9,726.3	8,849.8
Non-accrual Loans	39.5	42.9
Loans held for investment	9,828.5	8,936.2
Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	2.4	1
Current Loans	242.1	275.7
Non-accrual Loans	3.1	2.4
Loans held for investment	247.6	279.1
Impaired Financing Receivable, Interest Income, Accrual Method		0	$ 0
Financing Receivables, 30 to 59 Days Past Due | Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	7.6	5.5
Financing Receivables, 30 to 59 Days Past Due | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	2.5	0.7
Financing Receivables, 30 to 59 Days Past Due | Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	1.5	1.5
Financing Receivables, 30 to 59 Days Past Due | Commercial Real Estate Construction [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	12.2	0
Financing Receivables, 30 to 59 Days Past Due | Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	16.2	2.2
Financing Receivables, 30 to 59 Days Past Due | Residential Mortgage [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	4.7	3.8
Financing Receivables, 30 to 59 Days Past Due | Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	2	0.8
Financing Receivables, 30 to 59 Days Past Due | Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	30.5	12.3
Financing Receivables, 30 to 59 Days Past Due | Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	6.4	7.6
Financing Receivables, 30 to 59 Days Past Due | Other consumer
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0.8	1.2
Financing Receivables, 30 to 59 Days Past Due | Credit card consumer
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0.6	0.8
Financing Receivables, 30 to 59 Days Past Due | Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	7.8	9.6
Financing Receivables, 30 to 59 Days Past Due | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	6.2	4.8
Financing Receivables, 30 to 59 Days Past Due | Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0	0
Financing Receivables, 30 to 59 Days Past Due | Loans held for investment
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	44.9	27.6
Financing Receivables, 30 to 59 Days Past Due | Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0.4	0.9
Financing Receivables, 60 to 89 Days Past Due | Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	1.2	1.1
Financing Receivables, 60 to 89 Days Past Due | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	1.1	0.8
Financing Receivables, 60 to 89 Days Past Due | Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0.4	0.8
Financing Receivables, 60 to 89 Days Past Due | Commercial Real Estate Construction [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0	0
Financing Receivables, 60 to 89 Days Past Due | Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	1.5	1.6
Financing Receivables, 60 to 89 Days Past Due | Residential Mortgage [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	1.6	1.4
Financing Receivables, 60 to 89 Days Past Due | Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0	0.5
Financing Receivables, 60 to 89 Days Past Due | Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	4.3	4.6
Financing Receivables, 60 to 89 Days Past Due | Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	2	1.9
Financing Receivables, 60 to 89 Days Past Due | Other consumer
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0.2	0.5
Financing Receivables, 60 to 89 Days Past Due | Credit card consumer
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0.4	0.5
Financing Receivables, 60 to 89 Days Past Due | Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	2.6	2.9
Financing Receivables, 60 to 89 Days Past Due | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	1.8	2.6
Financing Receivables, 60 to 89 Days Past Due | Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0	0
Financing Receivables, 60 to 89 Days Past Due | Loans held for investment
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	9.3	10.2
Financing Receivables, 60 to 89 Days Past Due | Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0.6	0.1
Financing Receivables, Equal to Greater than 90 Days Past Due | Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	4	0.6
Financing Receivables, Equal to Greater than 90 Days Past Due | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0.1	0.3
Financing Receivables, Equal to Greater than 90 Days Past Due | Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0	0
Financing Receivables, Equal to Greater than 90 Days Past Due | Commercial Real Estate Construction [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0	0
Financing Receivables, Equal to Greater than 90 Days Past Due | Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0.1	0.3
Financing Receivables, Equal to Greater than 90 Days Past Due | Residential Mortgage [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0.5	1.1
Financing Receivables, Equal to Greater than 90 Days Past Due | Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0	0
Financing Receivables, Equal to Greater than 90 Days Past Due | Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	4.6	2
Financing Receivables, Equal to Greater than 90 Days Past Due | Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0.5	0.5
Financing Receivables, Equal to Greater than 90 Days Past Due | Other consumer
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0.2	0.1
Financing Receivables, Equal to Greater than 90 Days Past Due | Credit card consumer
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0.6	0.8
Financing Receivables, Equal to Greater than 90 Days Past Due | Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	1.3	1.4
Financing Receivables, Equal to Greater than 90 Days Past Due | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	1.2	2.3
Financing Receivables, Equal to Greater than 90 Days Past Due | Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0	0
Financing Receivables, Equal to Greater than 90 Days Past Due | Loans held for investment
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	8.5	5.7
Financing Receivables, Equal to Greater than 90 Days Past Due | Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	$ 1.4	$ 0

Loans - Schedule of Recorded _2

Loans - Schedule of Recorded Investment in Impaired Loans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2020	Dec. 31, 2017
Impaired Financing Receivable, Unpaid Principal Balance
Total Recorded Investment	$ 63.5
Average Recorded Investment		$ 64.7	$ 74
Accounts Receivable, Allowance for Credit Loss	144.3	73	73	$ 73	$ 72.1
Provision charged (credited) to operating expense	(56.9)	(13.9)	(8.6)
Less loans charged-off	(20.9)	(23.6)	(19.7)
Add back recoveries of loans previously charged-off		9.7	12
Loans and Leases Receivable, Collateral for Secured Borrowings	17.5
Individually evaluated for impairment		3.6	6.8
Collectively evaluated for impairment		69.4	66.2
Loans and Leases Receivable, Allowance	144.3	73	73
Individually evaluated for impairment		64.7	64.8
Collectively evaluated for impairment		8,871.5	8,410
Loans And Leases Receivable Held For Investment, Evaluated for Impairment		8,936.2	8,474.8
Impaired Financing Receivable, Interest Income, Accrual Method	0	0.3	0.3
Business Assets Pledged as Collateral
Impaired Financing Receivable, Unpaid Principal Balance
Loans and Leases Receivable, Collateral for Secured Borrowings	7.4
Real Property Pledged as Collateral
Impaired Financing Receivable, Unpaid Principal Balance
Loans and Leases Receivable, Collateral for Secured Borrowings	8.6
Other Property
Impaired Financing Receivable, Unpaid Principal Balance
Loans and Leases Receivable, Collateral for Secured Borrowings	1.5
Real Estate
Impaired Financing Receivable, Unpaid Principal Balance
Accounts Receivable, Allowance for Credit Loss		28.9	31		31.7
Provision charged (credited) to operating expense		(1.3)	(0.7)
Less loans charged-off		(3.5)	(3.7)
Add back recoveries of loans previously charged-off		2.7	3.7
Individually evaluated for impairment		1.7	1.3
Collectively evaluated for impairment		27.2	29.7
Loans and Leases Receivable, Allowance		28.9	31
Individually evaluated for impairment		41.1	41.8
Collectively evaluated for impairment		5,897.1	5,546.1
Loans And Leases Receivable Held For Investment, Evaluated for Impairment		5,938.2	5,587.9
Total consumer loans
Impaired Financing Receivable, Unpaid Principal Balance
Accounts Receivable, Allowance for Credit Loss		9.9	8.7		8.7
Provision charged (credited) to operating expense		(10.6)	(6.8)
Less loans charged-off		(13)	(11.3)
Add back recoveries of loans previously charged-off		3.6	4.5
Individually evaluated for impairment		0	0
Collectively evaluated for impairment		9.9	8.7
Loans and Leases Receivable, Allowance		9.9	8.7
Individually evaluated for impairment		0	0
Collectively evaluated for impairment		1,045.2	1,070.2
Loans And Leases Receivable Held For Investment, Evaluated for Impairment		1,045.2	1,070.2
Commercial Portfolio Segment [Member]
Impaired Financing Receivable, Unpaid Principal Balance
Accounts Receivable, Allowance for Credit Loss		32.6	31.3		30.5
Provision charged (credited) to operating expense		(4.5)	(1.9)
Less loans charged-off		(6.6)	(4.7)
Add back recoveries of loans previously charged-off		3.4	3.6
Individually evaluated for impairment		1.7	5.2
Collectively evaluated for impairment		30.9	26.1
Loans and Leases Receivable, Allowance		32.6	31.3
Individually evaluated for impairment		17.3	19.9
Collectively evaluated for impairment		1,656.4	1,540.4
Loans And Leases Receivable Held For Investment, Evaluated for Impairment		1,673.7	1,560.3
Agriculture
Impaired Financing Receivable, Unpaid Principal Balance
Accounts Receivable, Allowance for Credit Loss		1.6	2		1.2
Provision charged (credited) to operating expense		(0.1)	(0.6)
Less loans charged-off		(0.5)	0
Add back recoveries of loans previously charged-off		0	0.2
Individually evaluated for impairment		0.2	0.3
Collectively evaluated for impairment		1.4	1.7
Loans and Leases Receivable, Allowance		1.6	2
Individually evaluated for impairment		6.3	3.1
Collectively evaluated for impairment		272.8	251.7
Loans And Leases Receivable Held For Investment, Evaluated for Impairment		279.1	254.8
Other Segments
Impaired Financing Receivable, Unpaid Principal Balance
Accounts Receivable, Allowance for Credit Loss		0	0		$ 0
Provision charged (credited) to operating expense		0	0
Less loans charged-off		0	0
Add back recoveries of loans previously charged-off		0	0
Other
Impaired Financing Receivable, Unpaid Principal Balance
Individually evaluated for impairment		0	0
Collectively evaluated for impairment		0	0
Loans and Leases Receivable, Allowance		0	0
Individually evaluated for impairment		0	0
Collectively evaluated for impairment		0	1.6
Loans And Leases Receivable Held For Investment, Evaluated for Impairment		0	1.6
Commercial Real Estate Portfolio Segment [Member]
Impaired Financing Receivable, Unpaid Principal Balance
Recorded Investment With No Allowance		12.9	8.6
Recorded Investment With Allowance		10.8	7.7
Total Recorded Investment		23.7	16.3
Related Allowance		0.7	0.7
Accounts Receivable, Allowance for Credit Loss	54.8			19.5
Less loans charged-off	(0.4)
Unpaid Total Principal Balance		29.2	22.2
Land Acquisition And Development Construction Financing Receivable [Member]
Impaired Financing Receivable, Unpaid Principal Balance
Recorded Investment With No Allowance		0.4	0.4
Recorded Investment With Allowance		2.6	3.5
Total Recorded Investment		3	3.9
Related Allowance		0.5	0.2
Accounts Receivable, Allowance for Credit Loss	1.3			1.9
Less loans charged-off	(0.5)
Unpaid Total Principal Balance		9.2	10
Residential Real Estate Construction Financing Receivable [Member]
Impaired Financing Receivable, Unpaid Principal Balance
Recorded Investment With No Allowance		0	0.6
Recorded Investment With Allowance		0	0.4
Total Recorded Investment		0	1
Related Allowance		0	0.1
Accounts Receivable, Allowance for Credit Loss	1.6			1.5
Less loans charged-off	0
Unpaid Total Principal Balance		0.1	1.1
Commercial Real Estate Construction [Member]
Impaired Financing Receivable, Unpaid Principal Balance
Recorded Investment With No Allowance		0.5	0.2
Recorded Investment With Allowance		0	0
Total Recorded Investment		0.5	0.2
Related Allowance		0.1	0
Accounts Receivable, Allowance for Credit Loss	7.3			2.7
Less loans charged-off	0
Unpaid Total Principal Balance		1	0.7
Real Estate Construction Financing Receivable [Member]
Impaired Financing Receivable, Unpaid Principal Balance
Recorded Investment With No Allowance		0.9	1.2
Recorded Investment With Allowance		2.6	3.9
Total Recorded Investment		3.5	5.1
Related Allowance		0.6	0.3
Unpaid Total Principal Balance		10.3	11.8
Agricultural Real Estate Financing Receivable [Member]
Impaired Financing Receivable, Unpaid Principal Balance
Recorded Investment With No Allowance		5.2	12.5
Recorded Investment With Allowance		3	0.2
Total Recorded Investment		8.2	12.7
Related Allowance		0.2	0
Accounts Receivable, Allowance for Credit Loss	2.7			0.5
Less loans charged-off	0
Unpaid Total Principal Balance		8.6	12.9
Real Estate Financing Receivable [Member]
Impaired Financing Receivable, Unpaid Principal Balance
Recorded Investment With No Allowance		22.9	28
Recorded Investment With Allowance		18.2	13.8
Total Recorded Investment		41.1	41.8
Related Allowance		1.7	1.3
Average Recorded Investment		41.4	50
Accounts Receivable, Allowance for Credit Loss	80.5			28.9
Less loans charged-off	(0.9)
Loans and Leases Receivable, Collateral for Secured Borrowings	8.9
Unpaid Total Principal Balance		55	55.7
Impaired Financing Receivable, Interest Income, Accrual Method		0.1	0.1
Real Estate Financing Receivable [Member] | Business Assets Pledged as Collateral
Impaired Financing Receivable, Unpaid Principal Balance
Loans and Leases Receivable, Collateral for Secured Borrowings	1.3
Real Estate Financing Receivable [Member] | Real Property Pledged as Collateral
Impaired Financing Receivable, Unpaid Principal Balance
Loans and Leases Receivable, Collateral for Secured Borrowings	6.5
Real Estate Financing Receivable [Member] | Other Property
Impaired Financing Receivable, Unpaid Principal Balance
Loans and Leases Receivable, Collateral for Secured Borrowings	1.1
Agricultural Financing Receivable [Member]
Impaired Financing Receivable, Unpaid Principal Balance
Recorded Investment With No Allowance		2.3	2.5
Recorded Investment With Allowance		4	0.6
Total Recorded Investment		6.3	3.1
Related Allowance		0.2	0.3
Average Recorded Investment		4.6	2.1
Accounts Receivable, Allowance for Credit Loss	0.7			1.6
Less loans charged-off	(0.1)
Loans and Leases Receivable, Collateral for Secured Borrowings	0.8
Unpaid Total Principal Balance		6.9	3.2
Impaired Financing Receivable, Interest Income, Accrual Method		0	0
Agricultural Financing Receivable [Member] | Business Assets Pledged as Collateral
Impaired Financing Receivable, Unpaid Principal Balance
Loans and Leases Receivable, Collateral for Secured Borrowings	0
Agricultural Financing Receivable [Member] | Real Property Pledged as Collateral
Impaired Financing Receivable, Unpaid Principal Balance
Loans and Leases Receivable, Collateral for Secured Borrowings	0.8
Agricultural Financing Receivable [Member] | Other Property
Impaired Financing Receivable, Unpaid Principal Balance
Loans and Leases Receivable, Collateral for Secured Borrowings	0
Residential Real Estate
Impaired Financing Receivable, Unpaid Principal Balance
Recorded Investment With No Allowance		3.9	5.7
Recorded Investment With Allowance		1.8	2
Total Recorded Investment		5.7	7.7
Related Allowance		0.2	0.3
Accounts Receivable, Allowance for Credit Loss	12.8			2.8
Less loans charged-off	0
Unpaid Total Principal Balance		6.9	8.8
Commercial Portfolio Segment [Member]
Impaired Financing Receivable, Unpaid Principal Balance
Recorded Investment With No Allowance		12	5.5
Recorded Investment With Allowance		5.3	14.4
Total Recorded Investment		17.3	19.9
Related Allowance		1.7	5.2
Average Recorded Investment		18.7	21.9
Accounts Receivable, Allowance for Credit Loss	39.2			$ 32.6
Less loans charged-off	(9.1)
Loans and Leases Receivable, Collateral for Secured Borrowings	7.8
Unpaid Total Principal Balance		25.5	24.1
Impaired Financing Receivable, Interest Income, Accrual Method		0.2	0.2
Commercial Portfolio Segment [Member] | Business Assets Pledged as Collateral
Impaired Financing Receivable, Unpaid Principal Balance
Loans and Leases Receivable, Collateral for Secured Borrowings	6.1
Commercial Portfolio Segment [Member] | Real Property Pledged as Collateral
Impaired Financing Receivable, Unpaid Principal Balance
Loans and Leases Receivable, Collateral for Secured Borrowings	1.3
Commercial Portfolio Segment [Member] | Other Property
Impaired Financing Receivable, Unpaid Principal Balance
Loans and Leases Receivable, Collateral for Secured Borrowings	$ 0.4
Loans held for investment
Impaired Financing Receivable, Unpaid Principal Balance
Recorded Investment With No Allowance		37.2	36
Recorded Investment With Allowance		27.5	28.8
Total Recorded Investment		64.7	64.8
Related Allowance		3.6	6.8
Unpaid Total Principal Balance		$ 87.4	$ 83

Loans - Schedule of Loans Reneg

Loans - Schedule of Loans Renegotiated in Troubled Debt Restructurings (Details)	12 Months Ended
	Dec. 31, 2020USD ($)note	Dec. 31, 2019USD ($)note	Dec. 31, 2018USD ($)note
Loans and Leases Receivable, Troubled Debt Restructuring
Loans renegotiated in troubled debt restructurings	$ 14,500,000	$ 24,900,000
Loans renegotiated in troubled debt restructurings, non-accrual loans	11,300,000	19,400,000
Loans renegotiated in troubled debt restructurings, accrual loans	$ 3,200,000	$ 5,500,000
Number of notes under troubled debt restructurings | note	3	11	5
Charge-offs directly related to modified loans	$ 0	$ 0	$ 0
Financing Receivable, Troubled Debt Restructuring, Postmodification	300,000	10,400,000	$ 4,100,000
Impaired Financing Receivable, with Related Allowance, Unpaid Principal Balance		$ 300,000
Financing Receivable, Troubled Debt Restructuring, Commitment to Lend	0
Balance of notes for which there was a payment default	0
Impaired Financing Receivable, Recorded Investment	63,500,000
Financing Receivable, Allowance for Credit Loss, Noncurrent	$ 2,900,000
Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note	3	4	3
Financing Receivable, Troubled Debt Restructuring, Postmodification	$ 300,000	$ 3,300,000	$ 3,600,000
Impaired Financing Receivable, Recorded Investment		23,700,000	16,300,000
Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Financing Receivable, Troubled Debt Restructuring, Postmodification			200,000
Impaired Financing Receivable, Recorded Investment		8,200,000	12,700,000
Real Estate Financing Receivable [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Impaired Financing Receivable, Recorded Investment		$ 41,100,000	$ 41,800,000
Commercial Portfolio Segment [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note		1	1
Concessions in troubled debt restructurings, amount		$ 0
Financing Receivable, Troubled Debt Restructuring, Postmodification		5,000,000
Impaired Financing Receivable, Recorded Investment		17,300,000	$ 19,900,000
Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Impaired Financing Receivable, Recorded Investment		$ 3,500,000	5,100,000
Agricultural Financing Receivable [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note		6
Concessions in troubled debt restructurings, amount		$ 0
Financing Receivable, Troubled Debt Restructuring, Postmodification		2,100,000
Impaired Financing Receivable, Recorded Investment		6,300,000	$ 3,100,000
522291 Consumer Lending
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note			1
Concessions in troubled debt restructurings, amount			$ 0
Financing Receivable, Troubled Debt Restructuring, Postmodification			300,000
Interest Only Period Concession
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		200,000	3,600,000
Financing Receivable, Troubled Debt Restructuring, Postmodification	200,000
Interest Only Period Concession | Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		200,000	3,600,000
Financing Receivable, Troubled Debt Restructuring, Postmodification	200,000
Extension of terms or maturity
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Troubled Debt Restructuring, Postmodification	0	200,000	0
Extension of terms or maturity | Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Troubled Debt Restructuring, Postmodification	0	200,000	0
Extension of terms or maturity | Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Troubled Debt Restructuring, Postmodification			0
Extension of terms or maturity | Commercial Portfolio Segment [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Troubled Debt Restructuring, Postmodification		0
Extension of terms or maturity | Agricultural Financing Receivable [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Troubled Debt Restructuring, Postmodification		0
Extension of terms or maturity | 522291 Consumer Lending
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Troubled Debt Restructuring, Postmodification			0
Interest rate adjustment
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Financing Receivable, Troubled Debt Restructuring, Postmodification	0	0
Interest rate adjustment | Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Financing Receivable, Troubled Debt Restructuring, Postmodification	0	0
Interest rate adjustment | Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Interest rate adjustment | Commercial Portfolio Segment [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Troubled Debt Restructuring, Postmodification		0
Interest rate adjustment | Agricultural Financing Receivable [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Troubled Debt Restructuring, Postmodification		0
Interest rate adjustment | 522291 Consumer Lending
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Other
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Troubled Debt Restructuring, Postmodification	100,000	10,000,000	500,000
Other | Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Troubled Debt Restructuring, Postmodification	$ 100,000	2,900,000	0
Other | Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Troubled Debt Restructuring, Postmodification			200,000
Other | Commercial Portfolio Segment [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Troubled Debt Restructuring, Postmodification		5,000,000
Other | Agricultural Financing Receivable [Member]
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Troubled Debt Restructuring, Postmodification		$ 2,100,000
Other | 522291 Consumer Lending
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Troubled Debt Restructuring, Postmodification			$ 300,000

Loans - Schedule of Recorded _3

Loans - Schedule of Recorded Investment in Criticized Loans by Class and Credit Quality Indicator (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans and Leases Receivable, Allowance	$ 9,807,500,000	$ 8,930,700,000
Loans and Leases Receivable, Net of Deferred Income		8,936,200,000
Loans and Leases Receivable, Commitments to Purchase or Sell	0
Proceeds from Sale of Loans and Leases Held-for-investment	0
FIBK Commercial Real Estate Non Owner Occupied Loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	511,900,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	309,900,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	218,100,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	109,500,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	147,200,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(364,100,000)
Financing Receivable, Revolving	13,800,000
Loans and Leases Receivable, Allowance	1,674,500,000
Commercial Real Estate Owner Occupied
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	432,500,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	328,600,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	227,500,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	130,700,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	187,400,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(389,100,000)
Financing Receivable, Revolving	9,200,000
Loans and Leases Receivable, Allowance	1,705,000,000
Commercial Real Estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans and Leases Receivable, Net of Deferred Income		3,487,800,000
Commercial Real Estate Multifamily
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	132,500,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	58,900,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	23,500,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	41,600,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	25,800,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(80,600,000)
Financing Receivable, Revolving	800,000
Loans and Leases Receivable, Allowance	363,700,000
Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	265,000,000	302,100,000
Financing Receivable, Originated in Current Fiscal Year	105,100,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	58,900,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	27,600,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	31,700,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	7,600,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(27,700,000)
Financing Receivable, Revolving	6,400,000
Loans and Leases Receivable, Allowance	265,000,000
Loans and Leases Receivable, Net of Deferred Income		302,100,000
Residential Real Estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans and Leases Receivable, Net of Deferred Income		1,246,100,000
Commercial Real Estate Construction [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	523,500,000	431,500,000
Financing Receivable, Originated in Current Fiscal Year	236,100,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	195,400,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	62,700,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	11,900,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	6,000,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(200,000)
Financing Receivable, Revolving	11,600,000
Loans and Leases Receivable, Allowance	523,500,000
Loans and Leases Receivable, Net of Deferred Income		431,500,000
Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	250,900,000	244,100,000
Financing Receivable, Originated in Current Fiscal Year	80,600,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	64,700,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	16,700,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	5,600,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(100,000)
Financing Receivable, Revolving	83,200,000
Loans and Leases Receivable, Allowance	250,900,000
Loans and Leases Receivable, Net of Deferred Income		244,100,000
Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	220,600,000	226,600,000
Financing Receivable, Originated in Current Fiscal Year	54,200,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	58,200,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	32,600,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	19,600,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	17,000,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(32,600,000)
Financing Receivable, Revolving	6,400,000
Loans and Leases Receivable, Allowance	220,600,000
Loans and Leases Receivable, Net of Deferred Income		226,600,000
Commercial and Floor Plans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	1,044,100,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	156,200,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	142,200,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	77,200,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	51,100,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(96,500,000)
Financing Receivable, Revolving	247,200,000
Loans and Leases Receivable, Allowance	1,814,500,000
Commercial Purpose Secured by 1-4 Family
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	85,200,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	55,200,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	38,800,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	21,100,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	17,400,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(36,700,000)
Financing Receivable, Revolving	18,400,000
Loans and Leases Receivable, Allowance	272,800,000
Agriculture
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	52,600,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	20,300,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	15,300,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	4,800,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	3,200,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(2,000,000)
Financing Receivable, Revolving	147,900,000
Loans and Leases Receivable, Allowance	246,100,000
Residential Real Estate 1-4 Family
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	491,200,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	114,600,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	57,200,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	47,800,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	65,700,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(235,800,000)
Financing Receivable, Revolving	0
Loans and Leases Receivable, Allowance	1,012,300,000
Residential Real Estate 1-4 Family | Performing Financial Instruments
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	491,100,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	113,900,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	57,200,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	47,800,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	65,700,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(234,600,000)
Financing Receivable, Revolving	0
Loans and Leases Receivable, Allowance	1,010,300,000
Residential Real Estate 1-4 Family | Nonperforming Financial Instruments
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	100,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	700,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	0
Financing Receivable, Originated Three Years before Latest Fiscal Year	0
Financing Receivable, Originated Four Years before Latest Fiscal Year	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(1,200,000)
Financing Receivable, Revolving	0
Loans and Leases Receivable, Allowance	2,000,000
Home Equity Loan
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	12,100,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	7,300,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	7,100,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	9,700,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	4,800,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(14,800,000)
Financing Receivable, Revolving	328,200,000
Loans and Leases Receivable, Allowance	384,000,000
Home Equity Loan | Performing Financial Instruments
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	12,000,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	7,100,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	7,100,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	9,700,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	4,700,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(14,400,000)
Financing Receivable, Revolving	328,100,000
Loans and Leases Receivable, Allowance	383,100,000
Home Equity Loan | Nonperforming Financial Instruments
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	100,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	200,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	0
Financing Receivable, Originated Three Years before Latest Fiscal Year	0
Financing Receivable, Originated Four Years before Latest Fiscal Year	100,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(400,000)
Financing Receivable, Revolving	100,000
Loans and Leases Receivable, Allowance	900,000
Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	805,100,000	784,600,000
Financing Receivable, Originated in Current Fiscal Year	334,600,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	188,100,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	118,000,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	74,000,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	47,700,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(42,700,000)
Financing Receivable, Revolving	0
Loans and Leases Receivable, Allowance	805,100,000
Loans and Leases Receivable, Net of Deferred Income		784,600,000
Indirect consumer | Performing Financial Instruments
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	334,500,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	187,900,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	117,900,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	73,800,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	47,600,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(42,600,000)
Financing Receivable, Revolving	0
Loans and Leases Receivable, Allowance	804,300,000
Indirect consumer | Nonperforming Financial Instruments
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	100,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	200,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	100,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	200,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	100,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(100,000)
Financing Receivable, Revolving	0
Loans and Leases Receivable, Allowance	800,000
Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	150,600,000	179,000,000
Financing Receivable, Originated in Current Fiscal Year	47,200,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	29,400,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	28,200,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	11,900,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	5,300,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(8,600,000)
Financing Receivable, Revolving	20,000,000
Loans and Leases Receivable, Allowance	150,600,000
Loans and Leases Receivable, Net of Deferred Income		1,045,200,000
Other consumer | Performing Financial Instruments
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	47,100,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	29,400,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	28,100,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	11,900,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	5,300,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(8,600,000)
Financing Receivable, Revolving	19,900,000
Loans and Leases Receivable, Allowance	150,300,000
Other consumer | Nonperforming Financial Instruments
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	100,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	0
Financing Receivable, Originated Two Years before Latest Fiscal Year	100,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	0
Financing Receivable, Originated Four Years before Latest Fiscal Year	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	0
Financing Receivable, Revolving	100,000
Loans and Leases Receivable, Allowance	300,000
Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,025,900,000	1,045,200,000
Loans and Leases Receivable, Net of Deferred Income		179,000,000
Credit card consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	70,200,000	81,600,000
Financing Receivable, Originated in Current Fiscal Year	138.3
Loans and Leases Receivable, Net of Deferred Income		81,600,000
Credit card consumer | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	70.2
Credit card consumer | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	66.6
Credit card consumer | Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	1.5
Credit card consumer | Performing Financial Instruments
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	137.4
Credit card consumer | Performing Financial Instruments | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	69.6
Credit card consumer | Performing Financial Instruments | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	66.3
Credit card consumer | Performing Financial Instruments | Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	1.5
Credit card consumer | Nonperforming Financial Instruments
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	0.9
Credit card consumer | Nonperforming Financial Instruments | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	0.6
Credit card consumer | Nonperforming Financial Instruments | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	0.3
Credit card consumer | Nonperforming Financial Instruments | Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	0
Construction Loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans and Leases Receivable, Net of Deferred Income		977,700,000
Real Estate Loan
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans and Leases Receivable, Net of Deferred Income		5,938,200,000
Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2,153,900,000	1,673,700,000
Loans and Leases Receivable, Net of Deferred Income		1,673,700,000
Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	247,600,000	279,100,000
Loans and Leases Receivable, Net of Deferred Income		279,100,000
Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3,743,200,000	3,487,800,000
Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,039,400,000	977,700,000
Residential Mortgage [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,396,300,000	1,246,100,000
Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	6,399,500,000	5,938,200,000
Other Assets Especial Mentioned
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		157,500,000
Other Assets Especial Mentioned | FIBK Commercial Real Estate Non Owner Occupied Loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	300,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	2,300,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	900,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	100,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	6,400,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(13,700,000)
Financing Receivable, Revolving	0
Loans and Leases Receivable, Allowance	23,700,000
Other Assets Especial Mentioned | Commercial Real Estate Owner Occupied
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	7,100,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	9,600,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	4,800,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	3,100,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	18,600,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(20,000,000)
Financing Receivable, Revolving	0
Loans and Leases Receivable, Allowance	63,200,000
Other Assets Especial Mentioned | Commercial Real Estate Multifamily
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	0
Financing Receivable, Originated Two Years before Latest Fiscal Year	0
Financing Receivable, Originated Three Years before Latest Fiscal Year	0
Financing Receivable, Originated Four Years before Latest Fiscal Year	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(100,000)
Financing Receivable, Revolving	0
Loans and Leases Receivable, Allowance	100,000
Other Assets Especial Mentioned | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		3,800,000
Financing Receivable, Originated in Current Fiscal Year	200,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	100,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	0
Financing Receivable, Originated Three Years before Latest Fiscal Year	900,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(1,200,000)
Financing Receivable, Revolving	500,000
Loans and Leases Receivable, Allowance	2,900,000
Other Assets Especial Mentioned | Commercial Real Estate Construction [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		1,700,000
Financing Receivable, Originated in Current Fiscal Year	0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	0
Financing Receivable, Originated Two Years before Latest Fiscal Year	1,500,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	0
Financing Receivable, Originated Four Years before Latest Fiscal Year	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	0
Financing Receivable, Revolving	0
Loans and Leases Receivable, Allowance	1,500,000
Other Assets Especial Mentioned | Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		900,000
Other Assets Especial Mentioned | Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		14,300,000
Financing Receivable, Originated in Current Fiscal Year	2,800,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	6,700,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	1,000,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	1,500,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	900,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(1,000,000)
Financing Receivable, Revolving	500,000
Loans and Leases Receivable, Allowance	14,400,000
Other Assets Especial Mentioned | Commercial and Floor Plans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	5,600,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	1,000,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	1,900,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	7,000,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	3,900,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(500,000)
Financing Receivable, Revolving	2,400,000
Loans and Leases Receivable, Allowance	22,300,000
Other Assets Especial Mentioned | Commercial Purpose Secured by 1-4 Family
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	500,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	500,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	300,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	100,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	500,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(800,000)
Financing Receivable, Revolving	800,000
Loans and Leases Receivable, Allowance	3,500,000
Other Assets Especial Mentioned | Agriculture
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	1,500,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	700,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	400,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	0
Financing Receivable, Originated Four Years before Latest Fiscal Year	100,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(300,000)
Financing Receivable, Revolving	13,400,000
Loans and Leases Receivable, Allowance	16,400,000
Other Assets Especial Mentioned | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		200,000
Other Assets Especial Mentioned | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		400,000
Other Assets Especial Mentioned | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		600,000
Other Assets Especial Mentioned | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		40,400,000
Other Assets Especial Mentioned | Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		8,500,000
Other Assets Especial Mentioned | Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		84,700,000
Other Assets Especial Mentioned | Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		6,400,000
Other Assets Especial Mentioned | Residential Mortgage [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		2,600,000
Other Assets Especial Mentioned | Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		108,000,000
Substandard
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		224,000,000
Substandard | FIBK Commercial Real Estate Non Owner Occupied Loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	15,700,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	2,700,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	1,000,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	4,100,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	1,100,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(13,900,000)
Financing Receivable, Revolving	0
Loans and Leases Receivable, Allowance	38,500,000
Substandard | Commercial Real Estate Owner Occupied
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	8,900,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	6,500,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	11,500,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	5,000,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	15,100,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(11,100,000)
Financing Receivable, Revolving	300,000
Loans and Leases Receivable, Allowance	58,400,000
Substandard | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		1,900,000
Financing Receivable, Originated in Current Fiscal Year	300,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	0
Financing Receivable, Originated Two Years before Latest Fiscal Year	1,200,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	100,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(100,000)
Financing Receivable, Revolving	100,000
Loans and Leases Receivable, Allowance	1,800,000
Substandard | Commercial Real Estate Construction [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		1,500,000
Financing Receivable, Originated in Current Fiscal Year	0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	0
Financing Receivable, Originated Two Years before Latest Fiscal Year	0
Financing Receivable, Originated Three Years before Latest Fiscal Year	0
Financing Receivable, Originated Four Years before Latest Fiscal Year	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(100,000)
Financing Receivable, Revolving	0
Loans and Leases Receivable, Allowance	100,000
Substandard | Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		2,200,000
Financing Receivable, Originated in Current Fiscal Year	200,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	0
Financing Receivable, Originated Two Years before Latest Fiscal Year	0
Financing Receivable, Originated Three Years before Latest Fiscal Year	0
Financing Receivable, Originated Four Years before Latest Fiscal Year	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	0
Financing Receivable, Revolving	1,100,000
Loans and Leases Receivable, Allowance	1,300,000
Substandard | Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	1,400,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	5,900,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	3,400,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	900,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	3,400,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(4,400,000)
Financing Receivable, Revolving	600,000
Loans and Leases Receivable, Allowance	20,000,000
Substandard | Commercial and Floor Plans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	8,800,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	1,800,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	4,300,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	400,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	4,100,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(1,100,000)
Financing Receivable, Revolving	11,500,000
Loans and Leases Receivable, Allowance	32,000,000
Substandard | Commercial Purpose Secured by 1-4 Family
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	2,400,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	1,000,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	4,800,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	300,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	1,400,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(1,000,000)
Financing Receivable, Revolving	100,000
Loans and Leases Receivable, Allowance	11,000,000
Substandard | Agriculture
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	3,700,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	1,500,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	4,200,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	600,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	100,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(400,000)
Financing Receivable, Revolving	3,600,000
Loans and Leases Receivable, Allowance	14,100,000
Substandard | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		2,900,000
Substandard | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		800,000
Substandard | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		3,700,000
Substandard | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		60,300,000
Substandard | Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		22,700,000
Substandard | Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		97,300,000
Substandard | Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		5,600,000
Substandard | Residential Mortgage [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		7,800,000
Substandard | Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		137,300,000
Doubtful
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		5,900,000
Doubtful | FIBK Commercial Real Estate Non Owner Occupied Loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	0
Financing Receivable, Originated Two Years before Latest Fiscal Year	200,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	0
Financing Receivable, Originated Four Years before Latest Fiscal Year	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	0
Financing Receivable, Revolving	0
Loans and Leases Receivable, Allowance	200,000
Doubtful | Commercial Real Estate Owner Occupied
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	200,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	0
Financing Receivable, Originated Two Years before Latest Fiscal Year	0
Financing Receivable, Originated Three Years before Latest Fiscal Year	200,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(100,000)
Financing Receivable, Revolving	0
Loans and Leases Receivable, Allowance	500,000
Doubtful | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		1,000,000
Financing Receivable, Originated in Current Fiscal Year	0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	0
Financing Receivable, Originated Two Years before Latest Fiscal Year	0
Financing Receivable, Originated Three Years before Latest Fiscal Year	0
Financing Receivable, Originated Four Years before Latest Fiscal Year	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(100,000)
Financing Receivable, Revolving	0
Loans and Leases Receivable, Allowance	100,000
Doubtful | Commercial Real Estate Construction [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		0
Doubtful | Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		0
Doubtful | Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		0
Financing Receivable, Originated in Current Fiscal Year	0
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	300,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	0
Financing Receivable, Originated Three Years before Latest Fiscal Year	0
Financing Receivable, Originated Four Years before Latest Fiscal Year	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	0
Financing Receivable, Revolving	0
Loans and Leases Receivable, Allowance	300,000
Doubtful | Commercial and Floor Plans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	300,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	400,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	0
Financing Receivable, Originated Three Years before Latest Fiscal Year	0
Financing Receivable, Originated Four Years before Latest Fiscal Year	100,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(2,600,000)
Financing Receivable, Revolving	200,000
Loans and Leases Receivable, Allowance	3,600,000
Doubtful | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		0
Doubtful | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		100,000
Doubtful | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		100,000
Doubtful | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		3,600,000
Doubtful | Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		100,000
Doubtful | Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		800,000
Doubtful | Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		1,000,000
Doubtful | Residential Mortgage [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		300,000
Doubtful | Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		2,100,000
Total Criticized Loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		387,400,000
Total Criticized Loans | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		6,700,000
Total Criticized Loans | Commercial Real Estate Construction [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		3,200,000
Total Criticized Loans | Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		3,100,000
Total Criticized Loans | Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		40,900,000
Total Criticized Loans | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		3,100,000
Total Criticized Loans | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		1,300,000
Total Criticized Loans | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		4,400,000
Total Criticized Loans | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		104,300,000
Total Criticized Loans | Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		31,300,000
Total Criticized Loans | Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		182,800,000
Total Criticized Loans | Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		13,000,000
Total Criticized Loans | Residential Mortgage [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		10,700,000
Total Criticized Loans | Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment		247,400,000
Pass
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans and Leases Receivable, Net of Deferred Income		8,548,800,000
Pass | FIBK Commercial Real Estate Non Owner Occupied Loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	495,900,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	304,900,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	216,000,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	105,300,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	139,700,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(336,500,000)
Financing Receivable, Revolving	13,800,000
Loans and Leases Receivable, Allowance	1,612,100,000
Pass | Commercial Real Estate Owner Occupied
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	416,300,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	312,500,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	211,200,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	122,400,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	153,700,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(357,900,000)
Financing Receivable, Revolving	8,900,000
Loans and Leases Receivable, Allowance	1,582,900,000
Pass | Commercial Real Estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans and Leases Receivable, Net of Deferred Income		3,305,000,000
Pass | Commercial Real Estate Multifamily
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	132,500,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	58,900,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	23,500,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	41,600,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	25,800,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(80,500,000)
Financing Receivable, Revolving	800,000
Loans and Leases Receivable, Allowance	363,600,000
Pass | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	104,600,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	58,800,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	26,400,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	30,700,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	7,600,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(26,300,000)
Financing Receivable, Revolving	5,800,000
Loans and Leases Receivable, Allowance	260,200,000
Loans and Leases Receivable, Net of Deferred Income		295,400,000
Pass | Residential Real Estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans and Leases Receivable, Net of Deferred Income		1,235,400,000
Pass | Commercial Real Estate Construction [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	236,100,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	195,400,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	61,200,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	11,900,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	6,000,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(300,000)
Financing Receivable, Revolving	11,600,000
Loans and Leases Receivable, Allowance	521,900,000
Loans and Leases Receivable, Net of Deferred Income		428,300,000
Pass | Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	80,400,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	64,700,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	16,700,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	5,600,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(100,000)
Financing Receivable, Revolving	82,100,000
Loans and Leases Receivable, Allowance	249,600,000
Loans and Leases Receivable, Net of Deferred Income		241,000,000
Pass | Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	50,000,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	45,300,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	28,200,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	17,200,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	12,700,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(27,200,000)
Financing Receivable, Revolving	5,300,000
Loans and Leases Receivable, Allowance	185,900,000
Loans and Leases Receivable, Net of Deferred Income		185,700,000
Pass | Commercial and Floor Plans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	1,029,400,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	153,000,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	136,000,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	69,800,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	43,000,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(92,300,000)
Financing Receivable, Revolving	233,100,000
Loans and Leases Receivable, Allowance	1,756,600,000
Pass | Commercial Purpose Secured by 1-4 Family
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	82,300,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	53,700,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	33,700,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	20,700,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	15,500,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(34,900,000)
Financing Receivable, Revolving	17,500,000
Loans and Leases Receivable, Allowance	258,300,000
Pass | Agriculture
Loans and Leases Receivable Recorded Investment, Criticized Loans
Financing Receivable, Originated in Current Fiscal Year	47,400,000
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	18,100,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	10,700,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	4,200,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	3,000,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	(1,300,000)
Financing Receivable, Revolving	130,900,000
Loans and Leases Receivable, Allowance	$ 215,600,000
Pass | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans and Leases Receivable, Net of Deferred Income		781,500,000
Pass | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans and Leases Receivable, Net of Deferred Income		1,040,800,000
Pass | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans and Leases Receivable, Net of Deferred Income		177,700,000
Pass | Credit card consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans and Leases Receivable, Net of Deferred Income		81,600,000
Pass | Construction Loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans and Leases Receivable, Net of Deferred Income		964,700,000
Pass | Real Estate Loan
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans and Leases Receivable, Net of Deferred Income		5,690,800,000
Pass | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans and Leases Receivable, Net of Deferred Income		1,569,400,000
Pass | Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans and Leases Receivable, Net of Deferred Income		$ 247,800,000

Premises and Equipment (Details

Premises and Equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment
Depreciation	$ 25.8	$ 23.3	$ 16.2
Premises and equipment, gross	491.3	475.1
Less accumulated depreciation	(179)	(169.1)
Premises and equipment, net	312.3	306
Land
Property, Plant and Equipment
Premises and equipment, gross	52.9	50.8
Buildings and improvements
Property, Plant and Equipment
Premises and equipment, gross	349.5	330.1
Furniture and equipment
Property, Plant and Equipment
Premises and equipment, gross	$ 88.9	$ 94.2

Company-Owned Life Insurance (D

Company-Owned Life Insurance (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Schedule of Company - Owned Life Insurance, By Type
Total	$ 296.4	$ 293.8
Key executive, principal shareholder
Schedule of Company - Owned Life Insurance, By Type
Cash surrender value of life insurance	3.1	4.4
Key executive split dollar
Schedule of Company - Owned Life Insurance, By Type
Cash surrender value of life insurance	7	7
FIB | Group life
Schedule of Company - Owned Life Insurance, By Type
Cash surrender value of life insurance	$ 286.3	$ 282.4

Other Real Estate Owned (Detail

Other Real Estate Owned (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Repossessed Assets [Abstract]
Real Estate Acquired Through Foreclosure, Valuation Adjustments	$ 0.1	$ 0.9
Real Estate Acquired Through Foreclosure [Roll Forward]
Balance at beginning of year	8.5	14.4	$ 10.1
OREO acquired through acquisitions	0	2.4	0.6
Additions	3.3	14.1	12.1
Valuation adjustments	(0.1)	(0.9)	(0.1)
Dispositions	(9.2)	(21.8)	(8.3)
Balance at end of year	2.5	8.5	14.4
Real Estate Acquired Through Foreclosure, Improvements	0	0.3	0
Write-downs of OREO	(0.1)	(0.9)	$ (0.1)
Mortgage Loans in Process of Foreclosure, Amount	0.2	0.4
Carrying value of foreclosed OREO	$ 0	$ 2.3

Derivatives and Hedging Activ_3

Derivatives and Hedging Activities - Notional Amounts and Estimated Fair Values of Derivatives (Details) $ in Millions	Dec. 31, 2020USD ($)	May 01, 2020USD ($)	Dec. 31, 2019USD ($)
Derivative [Line Items]
Derivative Asset, Notional Amount	$ 901.6		$ 571
Derivative Assets, Estimated Fair Value	55.3		23.2
Derivative Liability, Notional Amount	1,014.1		631.2
Derivative Liabilities, Estimated Fair Value	53.3		22.2
Interest Rate Cash Flow Hedge Derivative at Fair Value, Net	0
Interest Rate Swap
Derivative [Line Items]
Derivative Liability, Notional Amount		$ 46.4
Derivative, Notional Amount, Contract 2		36.1
Derivative, Notional Amount, Contract 3		$ 5.1
Derivative, Fixed Interest Rate, Contract 1		0.0040
Derivative, Fixed Interest Rate, Contract 2		0.0034
Derivative, Fixed Interest Rate, Contract 3		0.0040
Not Designated as Hedging Instrument | Interest Rate Swap
Derivative [Line Items]
Derivative Asset, Notional Amount	799.7		503.2
Derivative Assets, Estimated Fair Value	52		21.9
Derivative Liability, Notional Amount	799.7		503.2
Derivative Liabilities, Estimated Fair Value	52		21.9
Not Designated as Hedging Instrument | Interest rate lock commitments
Derivative [Line Items]
Derivative Asset, Notional Amount	101.9		67.8
Derivative Assets, Estimated Fair Value	3.3		1.3
Not Designated as Hedging Instrument | Forward loan sales contracts
Derivative [Line Items]
Derivative Liability, Notional Amount	126.8		128
Derivative Liabilities, Estimated Fair Value	1.1		0.3
Designated as Hedging Instrument | Interest Rate Swap
Derivative [Line Items]
Derivative Asset, Notional Amount	87.6		0
Derivative Assets, Estimated Fair Value	$ 0.2		$ 0

Derivatives and Hedging Activ_4

Derivatives and Hedging Activities - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative liability, notional amount	$ 1,014.1	$ 631.2

Derivatives and Hedging Activ_5

Derivatives and Hedging Activities - Schedule of Master Netting Arrangements (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Financial Assets
Gross Amounts Recognized	$ 55.3	$ 23.2
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	55.3	23.2
Financial Instruments	0	0.1
Fair Value of Financial Collateral in the Balance Sheet	17.2	18
Net Amount	38.1	5.1
Offsetting Derivative Liabilities [Abstract]
Gross Amounts Recognized	53.3	22.2
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	53.3	22.2
Financial Instruments	0	0.1
Fair Value of Financial Collateral in the Balance Sheet	0	0
Net Amount	53.3	22.1
Repurchase agreements
Gross Amounts Recognized	1,091.4	697.6
Gross Amounts Offset in the Balance Sheet		0
Net Amounts in the Balance Sheet	1,091.4	697.6
Financial Instruments	0	0
Fair Value of Financial Collateral in the Balance Sheet	1,091.4	697.6
Net Amount	0	0
Offsetting Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned [Abstract]
Gross Amounts Recognized	1,144.7	719.8
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	1,144.7	719.8
Financial Instruments	0	0.1
Fair Value of Financial Collateral in the Balance Sheet	1,091.4	697.6
Net Amount	53.3	22.1
Interest rate swap contracts
Financial Assets
Gross Amounts Recognized	52	21.9
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	52	21.9
Financial Instruments	0	0.1
Fair Value of Financial Collateral in the Balance Sheet	17.2	18
Net Amount	34.8	3.8
Offsetting Derivative Liabilities [Abstract]
Gross Amounts Recognized	52.2	21.9
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	52.2	21.9
Financial Instruments	0	0.1
Fair Value of Financial Collateral in the Balance Sheet	0	0
Net Amount	52.2	21.8
Mortgage related derivatives
Financial Assets
Gross Amounts Recognized	3.3	1.3
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	3.3	1.3
Financial Instruments	0	0
Fair Value of Financial Collateral in the Balance Sheet	0	0
Net Amount	3.3	1.3
Offsetting Derivative Liabilities [Abstract]
Gross Amounts Recognized	1.1	0.3
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	1.1	0.3
Financial Instruments	0	0
Fair Value of Financial Collateral in the Balance Sheet	0	0
Net Amount	$ 1.1	$ 0.3

Derivatives and Hedging Activ_6

Derivatives and Hedging Activities - Pre-tax Gains and Losses on Derivative Contracts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Amount of loss recognized in other comprehensive income (effective portion)	$ (0.2)
Designated as Hedging Instrument
Derivative [Line Items]
Amount of loss recognized in other comprehensive income (effective portion)	0.1	$ 0	$ 0
Reclassification adjustment for derivative net gain included in income	(0.1)	0	0
Designated as Hedging Instrument | Interest Rate Swap | Cash Flow Hedging
Derivative [Line Items]
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, before Tax	0
Not Designated as Hedging Instrument
Derivative [Line Items]
Amount of gain recognized in other non-interest income	0	0
Amount of net fee income recognized in other non-interest income	7.7	2.5
Gain (Loss) on Fair Value Hedges Recognized in Earnings	$ 1.2	$ 0.3
Not Designated as Hedging Instrument | Interest Rate Swap
Derivative [Line Items]
Amount of gain recognized in other non-interest income			0.3
Amount of net fee income recognized in other non-interest income			1.3
Amount of net gains (losses) recognized in mortgage banking revenues			$ (0.5)

Mortgage Servicing Rights - Sch

Mortgage Servicing Rights - Schedule of Mortgage Servicing Rights (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
Servicing Asset at Amortized Value, Balance [Roll Forward]
Balance at end of year, net of valuation reserve	$ 24	$ 30.2
Mortgage loans
Servicing Asset at Amortized Value, Balance [Roll Forward]
Servicing asset at amortized cost, before valuation reserve, beginning	30.6	27.7	$ 24.8
Originations of mortgage servicing rights	11.7	7.3	6
Amortization expense	(8)	(4.4)	(3.1)
Servicing asset at amortized cost, before valuation reserve, ending	34.3	30.6	27.7
Less valuation reserve	(10.3)	(0.4)	0
Balance at end of year, net of valuation reserve	24	30.2	27.7
Principal balance of serviced loans underlying mortgage servicing rights	$ 3,585.5	$ 3,710.1	$ 3,698.2
Mortgage servicing rights as a percentage of serviced loans	0.0067	0.0081	0.0075

Mortgage Servicing Rights - Nar

Mortgage Servicing Rights - Narrative (Details) - Mortgage loans - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]
Mortgage servicing rights, fair value	$ 24	$ 34.8
Servicing asset at fair value, assumptions used to estimate fair value, weighted average life	3 years 10 months 24 days	6 years
Impairment reversals	$ 0
Permanent impairment charged against carrying value	$ 0
Minimum
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]
Servicing assets at fair value, assumptions used to estimate fair value, discount rate	8.30%	9.30%
Servicing assets at fair value, assumptions used to estimate fair value, prepayment speed	1.20%	0.60%
Maximum [Member]
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]
Servicing assets at fair value, assumptions used to estimate fair value, discount rate	10.00%	11.00%
Servicing assets at fair value, assumptions used to estimate fair value, prepayment speed	2.50%	1.50%

Deposits (Details)

Deposits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deposits, by Type [Abstract]
Non-interest bearing demand	$ 4,633,500	$ 3,426,500
Interest bearing:
Demand	4,118,900	3,195,400
Savings	4,405,900	3,591,600
Total	419,300	651,100
Time, other	639,400	798,900
Total interest bearing	9,583,500	8,237,000
Total deposits	14,217,000	11,663,500
Brokered time deposits	0	2,900
Time deposits obtained through Certificate of Deposit Account Registry Service (CDARS)	97,300	117,700
Time deposits of $250,000 or more	193,000	278,400
FDIC deposit insurance limit	250,000
Time, $100 and Over
Due within 3 months or less	65,800
Due after 3 months and within 6 months	77,200
Due after 6 months and within 12 months	173,000
Due within 2022	81,700
Due within 2023	7,300
Due within 2024	4,000
Due within 2025 and thereafter	10,300
Total	419,300	651,100
Total Time
Due within 3 months or less	271,100
Due after 3 months and within 6 months	190,800
Due after 6 months and within 12 months	355,500
Due within 2022	171,100
Due within 2023	30,000
Due within 2024	15,200
Due within 2025 and thereafter	25,000
Total	1,058,700
Interest expense, time deposits, $100,000 or more	$ 6,500	$ 11,500	$ 5,000

Long-Term Debt and Other Borr_3

Long-Term Debt and Other Borrowed Funds (Details) - USD ($) $ in Millions	Dec. 31, 2020	May 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument
Stated interest rate		5.25%
Long-term debt	$ 98.6		$ 0
Long-term debt and capital lease obligations	112.4		13.9
Fiscal Year Maturities of Long-term Debt
2018	0.1
2019	5.2
2020	0.1
2021	0.1
2022	0.1
Thereafter	106.8
Long-term debt and capital lease obligations	112.4		$ 13.9
Advances from FHLB	0
Other borrowings	0
Debt Instrument, Unamortized Discount (Premium) and Debt Issuance Costs, Net	$ 1.4
FIB
Fiscal Year Maturities of Long-term Debt
Ownership percentage in subsidiary	99.90%
Parent
Debt Instrument
Long-term debt	$ 98.6
Capital lease obligations | Capital Lease Obligation, 8.00% | Subsidiaries
Debt Instrument
Stated interest rate			8.00%
Capital Lease Obligations	1.1		$ 1.2
Notes payable | Subsidiaries
Fiscal Year Maturities of Long-term Debt
Long Term Debt, Acquired or Assumed	12.6
Notes payable | Note Payable, 6.24% | Subsidiaries
Debt Instrument
Stated interest rate			6.24%
Notes payable | Note Payable, 1.30%, maturing March 31, 2038 [Member] | Subsidiaries
Debt Instrument
Long-term debt	2		$ 2
Notes payable | Note Payable, 1.30%, Maturing June 1, 2034 [Member] | Subsidiaries
Debt Instrument
Long-term debt	$ 0.6		0.6
Notes payable | Note Payable, Two Point Two Eight Percent | Subsidiaries
Debt Instrument
Stated interest rate	2.28%
Notes payable | Note Payable, One Percent | Subsidiaries
Debt Instrument
Stated interest rate	1.00%
Notes payable | Note Payable, 2.28% | Subsidiaries
Debt Instrument
Long-term debt	$ 5		5
Notes payable | Note Payable, 1.00% | Subsidiaries
Debt Instrument
Long-term debt	5.1		$ 5.1
Federal Home Loan Bank | Notes Payable to FHLB | Subsidiaries
Fiscal Year Maturities of Long-term Debt
FHLB, advances, maximum amount available	1,697.5
Secured Debt
Fiscal Year Maturities of Long-term Debt
Line of credit facility, maximum borrowing capacity	478.6
Federal Funds Purchased
Fiscal Year Maturities of Long-term Debt
Line of credit facility, maximum borrowing capacity	205
Unsecured Debt [Member]
Fiscal Year Maturities of Long-term Debt
Line of credit facility, maximum borrowing capacity	$ 50
Subordinated term loan | Subordinated Notes Due May 2030
Fiscal Year Maturities of Long-term Debt
Debt Instrument, Face Amount		$ 100
Subordinated term loan | Subordinated Term Loan, 6.81% | Parent Company
Debt Instrument
Stated interest rate				6.81%
Minimum | Note Payable, 6.24% | Subsidiaries
Debt Instrument
Stated interest rate	1.00%
Maximum [Member] | Note Payable, 6.24% | Subsidiaries
Debt Instrument
Stated interest rate	3.25%

Subordinated Debentures Held _3

Subordinated Debentures Held by Subsidiary Trusts (Details) $ in Millions	Aug. 16, 2018USD ($)	Dec. 31, 2007USD ($)	Dec. 31, 2020USD ($)trusts	Dec. 31, 2009	May 31, 2020	Dec. 31, 2019USD ($)	Jan. 31, 2008USD ($)	Nov. 30, 2007	Oct. 31, 2007USD ($)
Subordinated Borrowing
Number of company sponsored wholly-owned business trusts | trusts			7
Subordinated debentures held by subsidiary trusts			$ 87			$ 86.9
Debt instrument, interest rate at period end			2.98%
Stated interest rate					5.25%
Debt Instrument, Redemption, At Issuance through Year Five [Member]
Subordinated Borrowing
Stated interest rate							6.78%
LIBOR | Debt Instrument, Redemption, After Period Five
Subordinated Borrowing
Debt instrument, basis spread on variable rate				2.75%
First Interstate Statutory Trust II | Junior Subordinated Deferrable Interest Debentures Issued by FIST II
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts			$ 10.3			10.3			$ 10.3
Debt instrument, interest rate at period end			2.48%
First Interstate Statutory Trust II | Junior Subordinated Deferrable Interest Debentures Issued by FIST II | LIBOR | Debt Instrument, Redemption, After Period Five
Subordinated Borrowing
Debt instrument, basis spread on variable rate		2.25%
First Interstate Statutory Trust I | Junior Subordinated Deferrable Interest Debentures Issued by FIST I
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts			$ 15.5			15.5			$ 15.5
Debt instrument, interest rate at period end			2.97%
First Interstate Statutory Trust I | Junior Subordinated Deferrable Interest Debentures Issued by FIST I | Debt Instrument, Redemption, At Issuance through Year Five [Member]
Subordinated Borrowing
Stated interest rate								7.50%
First Interstate Statutory Trust I | Junior Subordinated Deferrable Interest Debentures Issued by FIST I | LIBOR | Debt Instrument, Redemption, After Period Five
Subordinated Borrowing
Debt instrument, basis spread on variable rate		2.75%
First Interstate Statutory Trust III | Junior Subordinated Deferrable Interest Debentures Issued by FIST III
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts		$ 20.6	$ 20.6			20.6
Debt instrument, interest rate at period end			2.62%
First Interstate Statutory Trust III | Junior Subordinated Deferrable Interest Debentures Issued by FIST III | Debt Instrument, Redemption, At Issuance through Year Five [Member]
Subordinated Borrowing
Stated interest rate		6.88%
First Interstate Statutory Trust III | Junior Subordinated Deferrable Interest Debentures Issued by FIST III | LIBOR | Debt Instrument, Redemption, After Period Five
Subordinated Borrowing
Debt instrument, basis spread on variable rate		2.40%
First Interstate Statutory Trust IV | Junior Subordinated Deferrable Interest Debentures Issued by FIST IV
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts			$ 15.5			15.5
First Interstate Statutory Trust V | Junior Subordinated Deferrable Interest Debentures Issued by FIST V
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts			$ 10.3			10.3	$ 10.3
First Interstate Statutory Trust VI | Junior Subordinated Deferrable Interest Debentures Issued by FIST IV
Subordinated Borrowing
Debt instrument, basis spread on variable rate			2.93%
Debt instrument, interest rate at period end		2.70%
First Interstate Statutory Trust VI | Junior Subordinated Deferrable Interest Debentures Issued by FIST VI
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts		$ 15.5	$ 10.3			10.3	$ 10.3
Debt instrument, basis spread on variable rate			2.98%	2.75%
Northwest Bancorporation Capital Trust 1 [Member] | Junior Subordinated Deferrable Interest Debentures Issued by FIST VI
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts			$ 4.5			$ 4.4
Northwest Bancorporation Capital Trust I [Member] | Junior Subordinated Deferrable Interest Debentures Issued by NWBC I [Member]
Subordinated Borrowing
Debt instrument, basis spread on variable rate	1.70%
Debt instrument, interest rate at period end			1.94%
Northwest Bancorporation Capital Trust I [Member] | Junior Subordinated Deferrable Interest Debentures Issued by NWBC I [Member] | Debt Instrument, Redemption, At Issuance through Year Five [Member]
Subordinated Borrowing
Stated interest rate	5.95%
Trust Preferred Securities Subject to Mandatory Redemption | Trusts
Subordinated Borrowing
Amount of financial instruments subject to mandatory redemption			$ 84.4
Business trust term (in years)			30 years
Trust Preferred Securities Subject to Mandatory Redemption | Northwest Bancorporation Capital Trust 1 [Member]
Subordinated Borrowing
Amount of financial instruments subject to mandatory redemption	$ 5
Common Stock Subject to Mandatory Redemption | Trusts
Subordinated Borrowing
Amount of financial instruments subject to mandatory redemption			$ 2.6
Common Stock Subject to Mandatory Redemption | Northwest Bancorporation Capital Trust I [Member]
Subordinated Borrowing
Amount of financial instruments subject to mandatory redemption	$ 0.2
Subordinated Debentures Subject to Mandatory Redemption | Trusts
Subordinated Borrowing
Business trust term (in years)			30 years

Capital Stock and Dividend Re_2

Capital Stock and Dividend Restrictions (Details) - USD ($) $ / shares in Units, $ in Millions	Apr. 08, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 12, 2020	Jun. 11, 2019
Class of Stock
Common stock, shares authorized		200,000,000
Stock compensation expense		$ 7.5	$ 8	$ 5.6
Aggregate purchase price of shares repurchased and retired		$ 116.8	$ 2.5	$ 1
Stock Repurchase Program, Number of Shares Authorized to be Repurchased					5,500,000
Incremental Common Shares Attributable to Dilutive Effect of Accelerated Share Repurchase Agreements		3,000,000
Stock Repurchased and Retired During Period, Shares		3,578,743	43,560	24,271
Class A Common Stock
Class of Stock
Stock Repurchased and Retired During Period, Shares		3,538,142
Class A Common Stock
Class of Stock
Common stock, shares authorized		100,000,000
Common Stock, Voting Rights, Votes Per Share		$ 1
Common stock, shares, outstanding		40,335,113	43,129,085
Stock Repurchase Program, Number of Shares Authorized to be Repurchased						2,500,000
Class B Common Stock
Class of Stock
Common stock, shares authorized		100,000,000
Common Stock, Voting Rights, Votes Per Share		$ 5
Common stock, shares, outstanding		21,760,686	22,117,254
Community 1st Bank [Member]
Class of Stock
Total consideration paid	$ 18.8
Community 1st Bank [Member] | Class A Common Stock
Class of Stock
Common stock issued (in shares)	463,134
Idaho Independent Bank [Member]
Class of Stock
Total consideration paid	$ 157.3
Class A common stock	$ 157.3
Idaho Independent Bank [Member] | Class A Common Stock
Class of Stock
Common stock issued (in shares)	3,871,422
Director [Member] | Class A Common Stock
Class of Stock
Shares issued to directors during period		19,491
Stock compensation expense		$ 0.6
Board of Directors
Class of Stock
Aggregate value of shares issued to directors			$ 0.8
Board of Directors | Class A Common Stock
Class of Stock
Shares issued to directors during period			22,417

Earnings per Common Share Compu

Earnings per Common Share Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2020	Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Income (Loss) Available to Common Stockholders, Basic [Abstract]
Net income, basic and diluted			$ 161.2	$ 181	$ 160.2
Weighted Average Number of Shares Outstanding, Basic and Diluted [Abstract]
Weighted average common shares outstanding for basic earnings per share computation			63,611,891	63,645,029	57,778,857
Dilutive effect of stock-based compensation (in shares)			117,579	239,839	438,266
Weighted average common shares outstanding for diluted earnings per common share computation			63,729,470	63,884,868	58,217,123
Earnings Per Share, Basic and Diluted [Abstract]
Basic earnings per common share (in dollars per share)			$ 2.53	$ 2.84	$ 2.77
Diluted earnings per common share (in dollars per share)			$ 2.53	$ 2.83	$ 2.75
Restricted Stock [Member]
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)		150	3,094		83,475
Performance Shares
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)	138,298		291,540		448

Regulatory Capital - Schedule o

Regulatory Capital - Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Total risk-based capital:
Actual amount	$ 1,575.7	$ 1,495.3
Capital to Risk Weighted Assets	14.19%	14.10%
Adequately capitalized Basel III phase-in schedule, amount	$ 888.3	$ 848.5
Adequately capitalized Basel III phase-in schedule, ratio (percent)	8.00%	8.00%
Adequately capitalized Basel III fully phased-in, amount	$ 1,165.8	$ 1,113.6
Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	10.50%	10.50%
Well capitalized, amount	$ 1,110.3	$ 1,060.6
Capital Required to be Well Capitalized to Risk Weighted Assets	10.00%	10.00%
Tier 1 risk-based capital:
Actual amount	$ 1,369	$ 1,422.3
Tier One Risk Based Capital to Risk Weighted Assets	12.33%	13.41%
Adequately capitalized Basel III phase-in schedule, amount	$ 666.2	$ 636.3
Adequately capitalized Basel III phase-in schedule, ratio (percent)	6.00%	6.00%
Adequately capitalized Basel III fully phased-in, amount	$ 943.8	$ 901.5
Tier One Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	8.50%	8.50%
Well capitalized, amount	$ 888.3	$ 848.5
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	8.00%	8.00%
Common Equity Tier One Capital [Abstract]
Actual amount	$ 1,284.9	$ 1,338.2
Common Equity Tier One Risk Based Capital To Risk Weighted Assets	11.57%	12.62%
Adequately capitalized Basel III phase-in schedule, amount	$ 499.6	$ 477.3
Adequately capitalized Basel III phase-in schedule, ratio (percent)	4.50%	4.50%
Adequately capitalized Basel III fully phased-in, amount	$ 777.2	$ 742.4
Tier One Common Equity Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	7.00%	7.00%
Well capitalized, amount	$ 721.7	$ 689.4
Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets	6.50%	6.50%
Leverage capital ratio:
Actual amount	$ 1,369	$ 1,422.3
Tier One Leverage Capital to Average Assets	8.16%	10.13%
Adequately capitalized, amount	$ 671	$ 561.6
Adequately capitalized, ratio (percent)	4.00%	4.00%
Well capitalized, amount	$ 838.7	$ 702
Well capitalized, ratio (percent)	5.00%	5.00%
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	$ (24.1)
Retained Earnings [Member]
Leverage capital ratio:
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification	(24.1)
FIB
Total risk-based capital:
Actual amount	$ 1,426.8	$ 1,321.4
Capital to Risk Weighted Assets	12.89%	12.50%
Adequately capitalized Basel III phase-in schedule, amount	$ 885.6	$ 845.8
Adequately capitalized Basel III phase-in schedule, ratio (percent)	8.00%	8.00%
Adequately capitalized Basel III fully phased-in, amount	$ 1,162.3	$ 1,110.1
Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	10.50%	10.50%
Well capitalized, amount	$ 1,107	$ 1,057.2
Capital Required to be Well Capitalized to Risk Weighted Assets	10.00%	10.00%
Tier 1 risk-based capital:
Actual amount	$ 1,320.1	$ 1,248.4
Tier One Risk Based Capital to Risk Weighted Assets	11.93%	11.81%
Adequately capitalized Basel III phase-in schedule, amount	$ 664.2	$ 634.3
Adequately capitalized Basel III phase-in schedule, ratio (percent)	6.00%	6.00%
Adequately capitalized Basel III fully phased-in, amount	$ 940.9	$ 898.6
Tier One Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	8.50%	8.50%
Well capitalized, amount	$ 885.6	$ 845.8
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	8.00%	8.00%
Common Equity Tier One Capital [Abstract]
Actual amount	$ 1,320.1	$ 1,248.4
Common Equity Tier One Risk Based Capital To Risk Weighted Assets	11.93%	11.81%
Adequately capitalized Basel III phase-in schedule, amount	$ 498.1	$ 475.7
Adequately capitalized Basel III phase-in schedule, ratio (percent)	4.50%	4.50%
Adequately capitalized Basel III fully phased-in, amount	$ 774.9	$ 740
Tier One Common Equity Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	7.00%	7.00%
Well capitalized, amount	$ 719.5	$ 687.2
Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets	6.50%	6.50%
Leverage capital ratio:
Actual amount	$ 1,320.1	$ 1,248.4
Tier One Leverage Capital to Average Assets	7.88%	8.91%
Adequately capitalized, amount	$ 669.7	$ 560.4
Adequately capitalized, ratio (percent)	4.00%	4.00%
Well capitalized, amount	$ 837.2	$ 700.4
Well capitalized, ratio (percent)	5.00%	5.00%

Commitments and Contingencies -

Commitments and Contingencies - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Apr. 08, 2019office
Unrecorded Unconditional Purchase Obligation
Operating leases, rent expense	$ 5.1	$ 4.8	$ 3.2
Construction Contracts
Unrecorded Unconditional Purchase Obligation
Commitments under construction contracts	$ 4.6
Related Entity
Unrecorded Unconditional Purchase Obligation
Ownership percentage in partnership	50.00%
Idaho Independent Bank [Member]
Unrecorded Unconditional Purchase Obligation
Number of banking offices | office				11
Community 1st Bank [Member]
Unrecorded Unconditional Purchase Obligation
Number of banking offices | office				3
Mortgage loans originated for sale
Unrecorded Unconditional Purchase Obligation
Guarantor Obligations, Current Carrying Value	$ 0.6	$ 0.9

Commitments and Contingencies_2

Commitments and Contingencies - Operating Lease Commitments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018	$ 7
2019	6.6
2020	5.1
2021	4.5
2022	4.1
Thereafter	15.4
Operating leases, future minimum payments due	42.7
Operating leases, rent expense	5.1	$ 4.8	$ 3.2
Commitments
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018	5.6
2019	5.2
2020	4
2021	3.5
2022	3.1
Thereafter	13.4
Operating leases, future minimum payments due	34.8
Related Entity
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018	1.4
2019	1.4
2020	1.1
2021	1
2022	1
Thereafter	2
Operating leases, future minimum payments due	$ 7.9
Ownership percentage in partnership	50.00%

Financial Instruments with Of_2

Financial Instruments with Off-Balance Sheet Risk (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Entity Information
Unused credit card lines		$ 2,067
Off-Balance Sheet, Credit Loss, Liability	$ 3.7	0
Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal)	1.4	0
Cumulative-Effect Adjustment, Consolidation of Variable Interest Entity
Entity Information
Off-Balance Sheet, Credit Loss, Liability	2.3	0
Unused Credit Card Lines
Entity Information
Credit extension commitments	682.8	671.8
Unused credit card lines	2,280
Standby Letters of Credit
Entity Information
Credit extension commitments	$ 59	$ 42.7

Income Taxes - Schedule of Inco

Income Taxes - Schedule of Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Deferred Tax Liabilities, Unrealized Gains on Trading Securities	$ (19.2)
Current:
Federal	42.4	$ 40.5	$ 23.1
State	12.3	8.2	7.2
Total current	54.7	48.7	30.3
Deferred:
Federal	(5.7)	3.7	12.3
State	(0.9)	1.7	3.5
Total deferred	(6.6)	5.4	15.8
Total income tax expense	48.1	54.1	46.1
Operating Loss Carryforwards [Line Items]
Benefit of stock-based compensation plans			$ (1.1)
Common Stock Including Additional Paid in Capital [Member]
Operating Loss Carryforwards [Line Items]
Benefit of stock-based compensation plans	$ (0.4)	$ (1.2)

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Tax Expense at Effective Tax Rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Expense, Continuing Operations Income Tax Reconciliation [Abstract]
Tax expense at the statutory tax rate	$ 44	$ 49.4	$ 43.3
Increase (decrease) in tax resulting from:
Tax-exempt income	(2.1)	(2.8)	(2.8)
State income tax, net of federal income tax benefit	9	9.9	8.5
Benefit of stock-based compensation plans			(1.1)
Federal tax credits	(2.3)	(2)	(2.6)
Other, net	(0.1)	0.8	0.8
Total income tax expense	$ 48.1	$ 54.1	$ 46.1

Income Taxes - Deferred Tax Ass

Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Loans, principally due to allowance for credit losses	$ 36.5	$ 18.1
Loan discount	4	7.2
Deferred compensation	19.5	18.1
Non-performing loan interest	1.2	1
Other real estate owned write-downs and carrying costs	0	0.1
Tax credit carryforwards	0	0.2
Net operating loss carryforwards	2	2.6
Lease liabilities	9.5	9.9
Other	3.2	0.9
Deferred tax assets	75.9	58.1
Deferred tax liabilities:
Fixed assets, principally differences in bases and depreciation	(10.9)	(8.8)
Deferred loan costs	(4.4)	(2.8)
Deferred Tax Liabilities, Unrealized Gains on Trading Securities	19.2
Deferred Tax Liabilities, Unrealized Gains on Available For Sale Securities		(3.9)
Investment in joint venture partnership, principally due to differences in depreciation of partnership assets	(0.9)	(0.7)
Right of use assets	(9.2)	(9.7)
Prepaid amounts	(0.5)	(0.5)
Government agency stock dividends	(1.2)	(1.2)
Goodwill and core deposit intangibles	(50.3)	(49.2)
Mortgage servicing rights	(5.6)	(7.2)
Other	(0.9)	(0.8)
Deferred tax liabilities	(103.1)	(84.8)
Deferred Tax Liabilities, Net	27.2	26.7
Operating Loss Carryforwards [Line Items]
Deferred Income Tax Expense (Benefit)	(6.6)	$ 5.4	$ 15.8
Tax Credit Carryforward, Amount	$ 3.4

Income Taxes - Narrative (Detai

Income Taxes - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Current net income tax receivable	$ 8.1	$ 6.1

Stock-Based Compensation - Narr

Stock-Based Compensation - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020USD ($)planshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock-based compensation plans | plan	2
Options expiration period	10 years
Intrinsic value of fully-vested stock options outstanding | $	$ 2.1
2006 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for grant | shares	1,141,186

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Stock Options Activity (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term	9 months 29 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term	9 months 29 days
Stock option awards granted	0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Expirations in Period	0
Share-based Compensation Arrangements by Share-based Payment Award, Options, Expirations in Period, Weighted Average Exercise Price	$ 0
Intrinsic value of fully-vested stock options outstanding	$ 2.1
Share-based Payment Arrangement, Exercise of Option, Tax Benefit	$ 0.5	$ 0.9	$ 0.9
Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price	$ 0
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding options, beginning of year, number of shares	221,197
Forfeited, number of shares	(2,084)
Outstanding options, end of year, number of shares	79,318	221,197
Outstanding options exercisable, end of year, number of shares	79,318
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]
Outstanding options, beginning of year, weighted average exercise price (in dollars per share)	$ 15.33
Exercised, weighted average exercise price (in dollars per share)	15.80
Forfeited, weighted average exercise price (in dollars per share)	16.35
Outstanding options, end of year, weighted average exercise price (in dollars per share)	14.49	$ 15.33
Outstanding options exercisable, end of year, weighted average exercise price (in dollars per share)	$ 14.49
Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total intrinsic value of options exercised	$ 3.1	$ 4.9	4.9
Proceeds from Stock Options Exercised	1.1	1	1.8
Common Stock Redeemed For Payment of Stock Option Exercises, Aggregate Value	$ 1	$ 2	$ 1.6

Stock-Based Compensation - Sc_2

Stock-Based Compensation - Schedule of Restricted Stock Activity (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	$ 7.5	$ 8	$ 5.6
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Vested, number of shares	(139,795)
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Ending Balance	$ 1,959.8	2,013.9
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares, restricted stock award	332,085			1,352
Stock compensation expense	$ 7.5	$ 8	5.6
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Restricted stock, beginning of year, number of shares	363,022
Granted, number of shares	332,085
Vested, number of shares	(135,366)
Forfeited, number of shares	(34,912)
Canceled, number of shares	0
Restricted stock, end of year, number of shares	524,829	363,022
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Restricted stock, beginning of year, weighted-average measurement date fair value (in dollars per share)	$ 41.47
Granted, weighted average grant date fair value (in dollars per share)		$ 28.56
Vested, weighted average grant date fair value (in dollars per share)		40.11
Forfeited, weighted average grant date fair value (in dollars per share)		35.50
Canceled, weighted average grant date fair value (in dollars per share)		0
Restricted stock, end of year, weighted-average measurement date fair value (in dollars per share)	$ 33.65	$ 41.47
Tax benefits from stock-based compensation	$ 0.1	$ 0.4	0.2
Unrecognized compensation cost related to nonvested restricted stock awards	$ 9.5
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition	1 year 3 months 29 days
Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	$ 7.5	$ 8	$ 5.6
Upon Achievement of Defined Total Return to Shareholder Goals | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares, restricted stock award	98,139
Upon One-Third Annual Anniversary | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares, restricted stock award	134,455
Upon Achievement of Performance Goals | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares, restricted stock award	196,278
Upon Achievement of Defined Return on Equity Performance Goals | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares, restricted stock award	98,139

Employee Benefit Plans - Profit

Employee Benefit Plans - Profit Sharing and Savings Plans (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits
Other Comprehensive Income (Loss), Tax	$ 15,600,000	$ 15,600,000	$ 12,400,000	$ (3,300,000)
Eligibility requirement, service hours to be completed		20 hours
Deferred Profit Sharing
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits
Eligibility requirement, requisite service period		2 years
Compensation expense			2,100,000	2,400,000
Savings Plan, Employee Elected
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits
Compensation expense		$ 8,900,000	$ 7,000,000	$ 6,300,000
Maximum annual contribution per employee, amount per dollar of employee contribution		$ 1
Maximum annual contribution per employee, percent		5.00%

Employee Benefit Plans - Postre

Employee Benefit Plans - Postretirement Healthcare Plan (Details) - Postretirement Healthcare Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ 0.3	$ 0.4
Minimum age requirement	55 years
Requisite service period	15 years
Net periodic benefit cost	$ 0.5	$ 0.7	$ 0.8

Other Comprehensive Income - Sc

Other Comprehensive Income - Schedule of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Equity [Abstract]
Other Comprehensive Income (Loss), Tax	$ 15.6	$ 15.6	$ 12.4	$ (3.3)
Other Comprehensive Income (Loss), Transfers From Available-For-Sale To Held-To-Maturity Securities, Net Of Tax				1.2
Other Comprehensive Income (Loss), Net of Tax	45.6		35.6
Before Tax Amount
Change in net unrealized gains during period		61.8	54.9	(13.9)
Reclassification adjustment for net gains included in net income		(0.3)	(0.1)	0.1
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Investment Transferred from Available-for-sale to Equity Method, Tax			(1.6)
Change in net unrealized loss on derivatives		0.2
Change in net actuarial gains		(0.5)	(0.8)	(0.6)
Total other comprehensive income				(12.8)
Tax Expense (Benefit)
Change in net unrealized gains during period		15.8	14.1	(3.6)
Reclassification adjustment for net gains included in net income		(0.1)	0	0
Change in net unrealized loss on derivatives		0
Change in net actuarial gains		(0.1)	(0.1)	(0.1)
Total other comprehensive income (loss)				(3.3)
Net of Tax Amount
Change in net unrealized loss during period		46	40.8	(10.3)
Reclassification adjustment for net loss included in net income		(0.2)	(0.1)	0.1
Change in net unrealized loss on derivatives		0.2
Change in net actuarial loss		(0.4)	(0.7)	(0.5)
Other comprehensive income (loss), net of tax		45.6	35.6	(9.5)
Other comprehensive income (loss), before tax	$ 61.2	61.2	48	(12.8)
Change in unamortized loss on available-for-sale investment securities transferred into held-to-maturity		0	0	1.6
Other Comprehensive Income (Loss), Transfers From Available-For-Sale To Held-To-Maturity Securities, Tax				0.4
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale		0	(6)	0
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, Net of Tax			(4.4)
Change in net unrealized loss on derivatives		$ (0.2)	$ 0	$ 0

Other Comprehensive Income - Ac

Other Comprehensive Income - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in net unrealized gains during period	$ 61.8	$ 54.9	$ (13.9)
Reclassification adjustment for net gains included in net income	(0.3)	(0.1)	$ 0.1
Accumulated other comprehensive income (loss)	1,959.8	2,013.9
Net unrealized gain on investment securities available-for-sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	56.8	10.6
Total other comprehensive income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	56.6	11
Postretirement Healthcare Plan | Net actuarial gain on defined benefit post-retirement benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), before tax, related to the Plan	$ 0	$ 0.4

Condensed Financial Informati_3

Condensed Financial Information (Parent Company Only) - Condensed Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Condensed balance sheets:
Cash and cash equivalents	$ 2,276.8	$ 1,076.8
Investment in subsidiaries, at equity:
Other assets	215.3	187.6
Total assets	17,648.7	14,644.2
Long-term debt	98.6	0
Subordinated debentures held by subsidiary trusts	87	86.9
Total liabilities	15,688.9	12,630.3
Stockholders’ equity	1,959.8	2,013.9
Total liabilities and stockholders’ equity	17,648.7	14,644.2
Parent Company
Condensed balance sheets:
Cash and cash equivalents	123.2	147.1
Investment in subsidiaries, at equity:
Equity method investments	1,986.6	1,899.3
Advances from subsidiaries, net	41	48.9
Other assets	61.3	61.7
Total assets	2,212.1	2,157
Other liabilities	66.7	56.2
Subordinated debentures held by subsidiary trusts	87	86.9
Total liabilities	252.3	143.1
Stockholders’ equity	1,959.8	2,013.9
Total liabilities and stockholders’ equity	$ 2,212.1	$ 2,157

Condensed Financial Informati_4

Condensed Financial Information (Parent Company Only) - Condensed Statements of Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed Financial Statements, Captions
Salaries and benefits	$ 173.7	$ 155.3	$ 146.4
Interest expense	26.6	59	40.9
Acquisition related expenses	0	20.3	12.4
Earnings before income tax benefit	209.3	235.1	206.3
Income tax benefit	48.1	54.1	46.1
Net income	161.2	181	160.2
Parent Company
Condensed Financial Statements, Captions
Dividends from subsidiaries	130	178	148.5
Other interest income	0.1	0.3	0.1
Other income, primarily management fees from subsidiaries	28.7	25.9	17
Total income	158.8	204.2	165.6
Salaries and benefits	31.5	34.2	25.3
Interest expense	6.6	4.7	4.5
Acquisition related expenses	0	17	8.1
Other operating expenses, net	15.6	14.8	14.5
Total expenses	53.7	70.7	52.4
Earnings before income tax benefit	105.1	133.5	113.2
Income tax benefit	(6.1)	(11.9)	(9.3)
Income before undistributed earnings of subsidiaries	111.2	145.4	122.5
Undistributed earnings of subsidiaries	50	35.6	37.7
Net income	$ 161.2	$ 181	$ 160.2

Condensed Financial Informati_5

Condensed Financial Information (Parent Company Only) - Condensed Statements of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net income	$ 161.2	$ 181	$ 160.2
Adjustments to reconcile net income to cash provided by operating activities:
Stock-based compensation expense	7.5	8	5.6
Net cash provided by operating activities	268.3	127.3	219
Cash flows from investing activities:
Acquisition of banks and bank holding companies, net of cash and cash equivalents received	0	(298.4)	(28.1)
Net cash used in investing activities	(1,869.8)	(16.8)	(181.1)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of stock issuance costs	1.1	1	1.8
Purchase and retirement of common stock	(116.8)	(2.5)	(1)
Dividends paid to common stockholders	(128.6)	(79.2)	(64.1)
Net cash used in financing activities	2,801.5	144.3	25.2
Net increase (decrease) in cash and cash equivalents	1,200	254.8	63.1
Cash and cash equivalents at beginning of period	1,076.8	822	758.9
Cash and cash equivalents at end of period	2,276.8	1,076.8	822
Parent Company
Cash flows from operating activities:
Net income	161.2	181	160.2
Adjustments to reconcile net income to cash provided by operating activities:
Undistributed earnings of subsidiaries	(50)	(35.6)	(37.7)
Stock-based compensation expense	7.5	8	5.6
Other, net	(13.6)	8.1	16.4
Net cash provided by operating activities	105.1	161.5	144.5
Cash flows from investing activities:
Acquisition of banks and bank holding companies, net of cash and cash equivalents received	0	0	(14.7)
Net cash used in investing activities	0	0	(14.7)
Cash flows from financing activities:
Net (decrease) increase in advances from subsidiaries	16.7	(6.6)	(9.9)
Proceeds from issuance of long-term debt	98.6	0	0
Repayment of long-term debt	0	0	(26)
Proceeds from issuance of common stock, net of stock issuance costs	1.1	1	1.8
Purchase and retirement of common stock	(116.8)	(2.5)	(1)
Dividends paid to common stockholders	(128.6)	(79.2)	(64.1)
Net cash used in financing activities	(129)	(87.3)	(99.2)
Net increase (decrease) in cash and cash equivalents	(23.9)	74.2	30.6
Cash and cash equivalents at beginning of period	147.1	72.9	42.3
Cash and cash equivalents at end of period	123.2	147.1	72.9
Community 1st Bank [Member]
Cash flows from financing activities:
Stock Issued	$ 0
Inland Northwest Bank [Member]
Cash flows from financing activities:
Stock Issued		$ 176.1
BOTC Acquisition
Cash flows from financing activities:
Stock Issued			$ 173.3

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Financial Assets and Financial Liabilities Measured at Fair Value on a Recurring and Non-Recurring Basis (Details) $ in Millions	Dec. 31, 2020USD ($)	Dec. 31, 2019USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	$ 4,008.7	$ 2,960
Derivative assets	55.3	23.2
Derivative liabilities	53.3	22.2
Deferred compensation plan assets	19.1	18.2
Deferred compensation plan liabilities	19.1	18.2
Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other real estate owned		1.2
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for sale	0	0
Deferred compensation plan assets	0	0
Deferred compensation plan liabilities	0	0
Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for sale	74	100.9
Deferred compensation plan assets	19.1	18.2
Deferred compensation plan liabilities	19.1	18.2
Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for sale	0	0
Deferred compensation plan assets	0	0
Deferred compensation plan liabilities	0	0
Loans And Leases Receivable, Other Real Estate Owned [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other real estate owned		0
Loans And Leases Receivable, Other Real Estate Owned [Member] | Significant Other Observable Inputs (Level 2) | Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other real estate owned		0
Loans And Leases Receivable, Other Real Estate Owned [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other real estate owned		2.2
US Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value		9
US Treasury Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value		0
US Treasury Securities [Member] | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value		9
US Treasury Securities [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value		0
State, county and municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	465.9	80.9
State, county and municipal securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0
State, county and municipal securities | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	465.9
State, county and municipal securities | Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0
US Government Agencies Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	331.9	366.8
US Government Agencies Debt Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0	0
US Government Agencies Debt Securities [Member] | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	331.9	366.8
US Government Agencies Debt Securities [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0	0
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	2,897.6	2,317.2
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0	0
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	2,897.6	2,317.2
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0	0
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	10.9	47.2
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0	0
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	10.9	47.2
Mortgage-backed Securities, Issued by Private Enterprises [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0	0
Corporate securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	302.2	135.7
Corporate securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0	0
Corporate securities | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	302.2	135.7
Corporate securities | Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0	0
Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	4,008.7	2,960
Derivative assets	55.3	23.2
Derivative liabilities	53.3	22.2
Deferred compensation plan assets	19.1	18.2
Deferred compensation plan liabilities	19.1	18.2
Estimated Fair Value | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for sale	74	100.9
Deferred compensation plan assets	19.1	18.2
Deferred compensation plan liabilities	19.1	18.2
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0	0
Derivative assets	0	0
Derivative liabilities	0	0
Deferred compensation plan assets	0	0
Deferred compensation plan liabilities	0	0
Estimated Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	4,008.7	2,960
Derivative assets	55.3	23.2
Derivative liabilities	53.3	22.2
Deferred compensation plan assets	19.1	18.2
Deferred compensation plan liabilities	19.1	18.2
Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	0	0
Derivative assets	0	0
Derivative liabilities	0	0
Deferred compensation plan assets	0	0
Deferred compensation plan liabilities	0	0
Estimated Fair Value | Loans And Leases Receivable, Other Real Estate Owned [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other real estate owned		2.2
Estimated Fair Value | US Treasury Securities [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value		9
Estimated Fair Value | State, county and municipal securities | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	465.9
Estimated Fair Value | US Government Agencies Debt Securities [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	331.9	366.8
Estimated Fair Value | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	2,897.6	2,317.2
Estimated Fair Value | Mortgage-backed Securities, Issued by Private Enterprises [Member] | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	10.9	47.2
Estimated Fair Value | Corporate securities | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	302.2	135.7
Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	52	21.9
Derivative liabilities	52.2	21.9
Interest Rate Swap | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Interest Rate Swap | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	52	21.9
Derivative liabilities	52.2	21.9
Interest Rate Swap | Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Interest Rate Swap | Estimated Fair Value | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	52	21.9
Derivative liabilities	52.2	21.9
Interest rate lock commitments | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	0
Interest rate lock commitments | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	3.3	1.3
Interest rate lock commitments | Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	0	0
Interest rate lock commitments | Estimated Fair Value | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	3.3	1.3
Forward loan sales contracts | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	0	0
Forward loan sales contracts | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	1.1	0.3
Forward loan sales contracts | Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	0	0
Forward loan sales contracts | Estimated Fair Value | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities	$ 1.1	$ 0.3
Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member] | Loans And Leases Receivable, Impaired Loans [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing Asset, Measurement Input	0.18
Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Weighted Average [Member] | Loans And Leases Receivable, Impaired Loans [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Servicing Asset, Measurement Input	0.09

Fair Value Measurements - Fair

Fair Value Measurements - Fair Value Inputs, Quantitative Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)	Dec. 31, 2020USD ($)
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	$ 2,960	$ 4,008.7
Minimum | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Discount Rate for Level 3 Fair Value Disclosure	0.00%
Maximum [Member] | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Discount Rate for OREO Level 3 Fair Value	(65.00%)
Discount Rate for Assets HFS Level 3 Fair Value	(37.00%)
Weighted Average [Member] | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Discount Rate for Level 3 Fair Value Disclosure	(22.00%)
Discount Rate for OREO Level 3 Fair Value	(27.00%)
Discount Rate for Assets HFS Level 3 Fair Value	(3.00%)
Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Collateral-dependent loans*	$ (13.7)	(2.8)
Other real estate owned	(1.2)
Long-lived assets to be disposed of by sale	(0.2)	(0.2)
Estimated Fair Value
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	2,960	4,008.7
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	0	0
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	2,960	4,008.7
Significant Unobservable Inputs (Level 3) | Estimated Fair Value
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	0	0
Significant Unobservable Inputs (Level 3) | Estimated Fair Value | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Collateral-dependent loans*	$ (27.6)	(14.7)
Measurement Input, Discount Rate [Member] | Minimum | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Discount Rate for OREO Level 3 Fair Value	(8.00%)
Discount Rate for Assets HFS Level 3 Fair Value	0.00%
Measurement Input, Discount Rate [Member] | Maximum [Member] | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Discount Rate for Level 3 Fair Value Disclosure	(56.00%)
Other Debt Obligations [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	$ 3.2	0.2
Other Debt Obligations [Member] | Estimated Fair Value | Fair Value, Recurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	3.2	0.2
Other Debt Obligations [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Recurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	0	0
Other Debt Obligations [Member] | Significant Other Observable Inputs (Level 2) | Fair Value, Recurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	3.2	0.2
Other Debt Obligations [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Recurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	0	0
Loans And Leases Receivable, Impaired Loans [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Collateral-dependent loans*	0	0
Loans And Leases Receivable, Impaired Loans [Member] | Significant Other Observable Inputs (Level 2) | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Collateral-dependent loans*	0	0
Loans And Leases Receivable, Impaired Loans [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Collateral-dependent loans*	(27.6)
Loans And Leases Receivable, Impaired Loans [Member] | Significant Unobservable Inputs (Level 3) | Estimated Fair Value | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Collateral-dependent loans*	(27.6)	$ (14.7)
Loans And Leases Receivable, Impaired Loans [Member] | Measurement Input, Discount Rate [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member] | Minimum | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Servicing Asset, Measurement Input		0
Loans And Leases Receivable, Impaired Loans [Member] | Measurement Input, Discount Rate [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member] | Maximum [Member] | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Servicing Asset, Measurement Input		0.18
Loans And Leases Receivable, Impaired Loans [Member] | Measurement Input, Discount Rate [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member] | Weighted Average [Member] | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Servicing Asset, Measurement Input		0.09
Loans And Leases Receivable, Other Real Estate Owned [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other real estate owned	0
Loans And Leases Receivable, Other Real Estate Owned [Member] | Significant Other Observable Inputs (Level 2) | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other real estate owned	0
Loans And Leases Receivable, Other Real Estate Owned [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other real estate owned	(2.2)
Loans And Leases Receivable, Other Real Estate Owned [Member] | Significant Unobservable Inputs (Level 3) | Estimated Fair Value | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other real estate owned	(2.2)
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Long-lived assets to be disposed of by sale	0	$ 0
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Significant Other Observable Inputs (Level 2) | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Long-lived assets to be disposed of by sale	0	0
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Long-lived assets to be disposed of by sale	(6.2)	(5.3)
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Significant Unobservable Inputs (Level 3) | Estimated Fair Value | Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Long-lived assets to be disposed of by sale	$ (6.2)	$ (5.3)
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Measurement Input, Discount Rate [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member] | Minimum | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Servicing Asset, Measurement Input		0
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Measurement Input, Discount Rate [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member] | Maximum [Member] | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Servicing Asset, Measurement Input		0
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Measurement Input, Discount Rate [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Nonrecurring [Member] | Weighted Average [Member] | Valuation, Market Approach [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Servicing Asset, Measurement Input		0

Fair Value Measurements - Narra

Fair Value Measurements - Narrative (Details) - Fair Value, Nonrecurring [Member] - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	$ 2.8	$ 13.7
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Related allowance	2.8	3.6
Partial loan charge-off		10.1
Carrying Amount | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	17.5	41.3
Assets Held-for-sale, Long Lived, Fair Value Disclosure	5.5	6.4
Changes Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Held-for-sale, Long Lived, Fair Value Disclosure	0.2
Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Held-for-sale, Long Lived, Fair Value Disclosure	5.3	6.2
Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	$ 14.7	$ 27.6

Fair Value Measurements - Sch_2

Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Financial assets:
Estimated Fair Value	$ 4,008.7	$ 2,960
Held-to-maturity investment securities, estimated fair value	55	94.5
Derivative assets	55.3	23.2
Deferred compensation plan assets	19.1	18.2
Financial liabilities:
Derivative liabilities	53.3	22.2
Deferred compensation plan liabilities	19.1	18.2
Carrying Amount
Financial assets:
Cash and cash equivalents	2,276.8	1,076.8
Estimated Fair Value	4,008.7	2,960
Held-to-maturity investment securities, estimated fair value	51.6	92.3
Accrued interest receivable	51.1	46.7
Mortgage servicing rights, net	24	30.2
Loans Held-for-sale, Fair Value Disclosure	74	100.9
Net loans held for investment	9,663.2	8,857.7
Derivative assets	55.3	23.2
Deferred compensation plan assets	19.1	18.2
Total financial assets	16,223.8	13,206
Financial liabilities:
Total deposits, excluding time deposits	13,158.3	10,213.5
Time deposits	1,058.7	1,450
Securities sold under repurchase agreements	1,091.4	697.6
Accrued interest payable	5.8	12.1
Long-term debt	112.4	13.9
Subordinated debentures held by subsidiary trusts	87	86.9
Derivative liabilities	53.3	22.2
Deferred compensation plan liabilities	19.1	18.2
Total financial liabilities	15,586	12,514.4
Estimated Fair Value
Financial assets:
Cash and cash equivalents	2,276.8	1,076.8
Estimated Fair Value	4,008.7	2,960
Held-to-maturity investment securities, estimated fair value	55	94.5
Accrued interest receivable	51.1	46.7
Mortgage servicing rights, net	24	34.8
Loans Held-for-sale, Fair Value Disclosure	74	100.9
Net loans held for investment	9,785.6	8,930.7
Derivative assets	55.3	23.2
Deferred compensation plan assets	19.1	18.2
Total financial assets	16,349.6	13,285.8
Financial liabilities:
Total deposits, excluding time deposits	13,158.3	10,213.5
Time deposits	1,061.1	1,446.6
Securities sold under repurchase agreements	1,091.4	697.6
Accrued interest payable	5.8	12.1
Long-term debt	116.5	10.4
Subordinated debentures held by subsidiary trusts	81.3	81.3
Derivative liabilities	53.3	22.2
Deferred compensation plan liabilities	19.1	18.2
Total financial liabilities	15,586.8	12,501.9
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	2,276.8	1,076.8
Estimated Fair Value	0	0
Held-to-maturity investment securities, estimated fair value	0	0
Accrued interest receivable	0	0
Mortgage servicing rights, net	0	0
Loans Held-for-sale, Fair Value Disclosure	0	0
Net loans held for investment	0	0
Derivative assets	0	0
Deferred compensation plan assets	0	0
Total financial assets	2,276.8	1,076.8
Financial liabilities:
Total deposits, excluding time deposits	13,158.3	10,213.5
Time deposits	0	0
Securities sold under repurchase agreements	0	0
Accrued interest payable	0	0
Long-term debt	0	0
Subordinated debentures held by subsidiary trusts	0	0
Derivative liabilities	0	0
Deferred compensation plan liabilities	0	0
Total financial liabilities	13,158.3	10,213.5
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0	0
Estimated Fair Value	4,008.7	2,960
Held-to-maturity investment securities, estimated fair value	55	94.5
Accrued interest receivable	51.1	46.7
Mortgage servicing rights, net	24	34.8
Loans Held-for-sale, Fair Value Disclosure	74	100.9
Net loans held for investment	9,770.9	8,906.7
Derivative assets	55.3	23.2
Deferred compensation plan assets	19.1	18.2
Total financial assets	14,058.1	12,185
Financial liabilities:
Total deposits, excluding time deposits	0	0
Time deposits	1,061.1	1,446.6
Securities sold under repurchase agreements	1,091.4	697.6
Accrued interest payable	5.8	12.1
Long-term debt	116.5	10.4
Subordinated debentures held by subsidiary trusts	81.3	81.3
Derivative liabilities	53.3	22.2
Deferred compensation plan liabilities	19.1	18.2
Total financial liabilities	2,428.5	2,288.4
Significant Unobservable Inputs (Level 3) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0	0
Estimated Fair Value	0	0
Held-to-maturity investment securities, estimated fair value	0	0
Accrued interest receivable	0	0
Mortgage servicing rights, net	0	0
Loans Held-for-sale, Fair Value Disclosure	0	0
Net loans held for investment	14.7	24
Derivative assets	0	0
Deferred compensation plan assets	0	0
Total financial assets	14.7	24
Financial liabilities:
Total deposits, excluding time deposits	0	0
Time deposits	0	0
Securities sold under repurchase agreements	0	0
Accrued interest payable	0	0
Long-term debt	0	0
Subordinated debentures held by subsidiary trusts	0	0
Derivative liabilities	0	0
Deferred compensation plan liabilities	0	0
Total financial liabilities	$ 0	$ 0

Related Party Transactions (Det

Related Party Transactions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Loan repayments	$ 700	$ 500	$ 400
Amount of loans removed due to changes in related parties	18,100
Airplane Usage by Company
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Related party transaction, expenses from transactions with related party		22	53
Executive Officers, Directors, Shareholders Greater than Five Percent, and Related Entities and Individuals to Such Persons [Member]
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Loans and leases receivable, related parties	22,400	40,300
New loans and advances on existing loans	17,200
Loan repayments	17,000
Board of Directors Chairman | Airplane Usage by Company
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Related party transactions, other revenues	54	30	25
Entity Majority Owned by Shareholders and Directors [Member] | Education, Communication, Strategic Enterprise Planning and Corporate Governance Consultation [Member]
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Related party transaction, expenses from transactions with related party	$ 85	$ 85	$ 80

Recent Authoritative Accounting

Recent Authoritative Accounting Guidance (Details) - USD ($) $ in Millions	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity Attributable to Parent	$ 1,959.8		$ 2,013.9
Off-Balance Sheet, Credit Loss, Liability	$ 3.7		$ 0
Accounting Standards Update 2016-13 | Adjustments for New Accounting Principle, Early Adoption [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Financing Receivable, Allowance for Credit Loss		$ 30
Off-Balance Sheet, Credit Loss, Liability		2.3
Deferred Income Tax Assets, Net		8.2
Accounting Standards Update 2016-13 | Adjustments for New Accounting Principle, Early Adoption [Member] | Retained Earnings [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders' Equity Attributable to Parent		$ 24.1

Subsequent Events Subsequent Ev

Subsequent Events Subsequent Events (Details) - USD ($)	Jan. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Subsequent Event [Line Items]
Financing Receivable, CARES Act		$ 296,300,000
Financing Receivable, CARES Act Applications		3,150
Commercial Portfolio Segment [Member]
Subsequent Event [Line Items]
Loans held for investment		$ 2,153,900,000	$ 1,673,700,000
Class A Common Stock | Subsequent Event
Subsequent Event [Line Items]
Dividends (in dollars per share)	$ 0.41

Uncategorized Items - fibk-2020

Label	Element	Value
Stock Issued, Value, Stock Options Exercised, Net of Tax Benefit (Expense)	us-gaap_StockIssuedDuringPeriodValueStockOptionsExercisedNetOfTaxBenefitExpense	$ 1,800,000
APIC, Share-based Payment Arrangement, Option, Increase for Cost Recognition	us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition	(64,100,000)
Stock Issued During Period, Value, Restricted Stock Award, Forfeitures	us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures	0
Dividends, Common Stock, Cash	us-gaap_DividendsCommonStockCash	1,427,600,000
Dividends, Common Stock, Cash	us-gaap_DividendsCommonStockCash	1,693,900,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	1,989,800,000
Stock Repurchased and Retired During Period, Value	us-gaap_StockRepurchasedAndRetiredDuringPeriodValue	1,000,000
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect	us-gaap_TaxCutsAndJobsActOf2017ReclassificationFromAociToRetainedEarningsTaxEffect	0
Stock Issued During Period, Value, New Issues	us-gaap_StockIssuedDuringPeriodValueNewIssues	173,300,000
Common Stock Including Additional Paid in Capital [Member]
Stock Issued, Value, Stock Options Exercised, Net of Tax Benefit (Expense)	us-gaap_StockIssuedDuringPeriodValueStockOptionsExercisedNetOfTaxBenefitExpense	1,800,000
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	0
APIC, Share-based Payment Arrangement, Option, Increase for Cost Recognition	us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition	0
Stock Issued During Period, Value, Restricted Stock Award, Forfeitures	us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures	0
Dividends, Common Stock, Cash	us-gaap_DividendsCommonStockCash	866,700,000
Dividends, Common Stock, Cash	us-gaap_DividendsCommonStockCash	687,000,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	1,049,300,000
Stock Repurchased and Retired During Period, Value	us-gaap_StockRepurchasedAndRetiredDuringPeriodValue	1,000,000
Stock Issued During Period, Value, New Issues	us-gaap_StockIssuedDuringPeriodValueNewIssues	173,300,000
Net Income (Loss) Attributable to Parent	us-gaap_NetIncomeLoss	0
Retained Earnings [Member]
Stock Issued, Value, Stock Options Exercised, Net of Tax Benefit (Expense)	us-gaap_StockIssuedDuringPeriodValueStockOptionsExercisedNetOfTaxBenefitExpense	0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	0
APIC, Share-based Payment Arrangement, Option, Increase for Cost Recognition	us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition	(64,100,000)
Stock Issued During Period, Value, Restricted Stock Award, Forfeitures	us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures	0
Share-based Payment Arrangement, Expense	us-gaap_AllocatedShareBasedCompensationExpense	0
Dividends, Common Stock, Cash	us-gaap_DividendsCommonStockCash	851,800,000
Dividends, Common Stock, Cash	us-gaap_DividendsCommonStockCash	752,600,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	929,500,000
Stock Repurchased and Retired During Period, Value	us-gaap_StockRepurchasedAndRetiredDuringPeriodValue	0
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect	us-gaap_TaxCutsAndJobsActOf2017ReclassificationFromAociToRetainedEarningsTaxEffect	3,100,000
Stock Issued During Period, Value, New Issues	us-gaap_StockIssuedDuringPeriodValueNewIssues	0
Net Income (Loss) Attributable to Parent	us-gaap_NetIncomeLoss	160,200,000
AOCI Attributable to Parent [Member]
Stock Issued, Value, Stock Options Exercised, Net of Tax Benefit (Expense)	us-gaap_StockIssuedDuringPeriodValueStockOptionsExercisedNetOfTaxBenefitExpense	0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent	(9,500,000)
APIC, Share-based Payment Arrangement, Option, Increase for Cost Recognition	us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition	0
Stock Issued During Period, Value, Restricted Stock Award, Forfeitures	us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardForfeitures	0
Share-based Payment Arrangement, Expense	us-gaap_AllocatedShareBasedCompensationExpense	0
Dividends, Common Stock, Cash	us-gaap_DividendsCommonStockCash	24,600,000
Dividends, Common Stock, Cash	us-gaap_DividendsCommonStockCash	12,000,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance	us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterestAdjustedBalance1	11,000,000
Stock Repurchased and Retired During Period, Value	us-gaap_StockRepurchasedAndRetiredDuringPeriodValue	0
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect	us-gaap_TaxCutsAndJobsActOf2017ReclassificationFromAociToRetainedEarningsTaxEffect	(3,100,000)
Stock Issued During Period, Value, New Issues	us-gaap_StockIssuedDuringPeriodValueNewIssues	0
Net Income (Loss) Attributable to Parent	us-gaap_NetIncomeLoss	$ 0