First Interstate BancSystem (FIBK) 10-K 27 Feb 20 2019 FY Annual report Financial data

Document and Entity Information

Document and Entity Information	12 Months Ended
	Dec. 31, 2019USD ($)	Jan. 31, 2020shares	Jun. 30, 2019USD ($)	Apr. 08, 2019USD ($)office$ / sharesshares	Dec. 31, 2018USD ($)
Entity Information [Line Items]
Assets	$ 14,644,200,000				$ 13,300,200,000
Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	10.50%				9.88%
Tier One Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	8.50%				7.88%
Tier One Common Equity Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	7.00%				6.38%
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2019
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	2019
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	No
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			$ 1,791,530,989
Documents Incorporated by Reference	The registrant intends to file a definitive Proxy Statement for the Annual Meeting of Shareholders scheduled to be held May 5, 2020 . The information required by Part III of this Form 10-K is incorporated by reference to such Proxy Statement.
Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets	6.50%				6.50%
Common Equity Tier One Risk Based Capital To Risk Weighted Assets	12.62%				11.40%
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	8.00%				8.00%
Tier One Risk Based Capital to Risk Weighted Assets	13.41%				12.26%
Capital Required to be Well Capitalized to Risk Weighted Assets	10.00%				10.00%
Capital to Risk Weighted Assets	14.10%				12.99%
Well capitalized, ratio (percent)	5.00%				5.00%
Tier One Leverage Capital to Average Assets	10.13%				9.47%
Deposits	$ 11,663,500,000				$ 10,680,700,000
Loans and Leases Receivable, Allowance	$ 9,031,600,000				$ 8,503,700,000
Class A Common Stock
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding | shares		43,173,724
Class B Common Stock
Entity Information [Line Items]
Entity Common Stock, Shares Outstanding | shares		22,110,620
Community 1st Bank [Member]
Entity Information [Line Items]
Number of banking offices | office				3
Community 1st Bank [Member] | Class A Common Stock
Entity Information [Line Items]
Business Acquisition, Share Price | $ / shares				$ 40.64
Business Acquisition Conversion Share Rate | shares				0.3784
Idaho Independent Bank [Member]
Entity Information [Line Items]
Number of banking offices | office				11
Class A common stock				$ 157,300,000
Idaho Independent Bank [Member] | Class A Common Stock
Entity Information [Line Items]
Business Acquisition, Share Price | $ / shares				$ 40.64
Business Acquisition Conversion Share Rate | shares				0.50

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets
Cash and due from banks	$ 241,500	$ 244,100
Interest bearing deposits in banks	100	100
Federal funds sold	835,200	577,800
Total cash and cash equivalents	1,076,800	822,000
Investment securities:
Available-for-sale	2,960,000	2,270,700
Held-to-maturity (estimated fair values of $94.5 and $400.7 at December 31, 2019 and 2018, respectively)	92,300	406,800
Marketable Securities	3,052,300	2,677,500
Loans held for investment	8,930,700	8,470,400
Mortgage loans held for sale	100,900	33,300
Total loans	9,031,600	8,503,700
Less allowance for loan losses	73,000	73,000
Net loans	8,958,600	8,430,700
Goodwill	621,600	546,700
Goodwill	306,000	245,200
Indefinite-lived Intangible Assets (Excluding Goodwill)	62,100	56,900
Premises and equipment, net of accumulated depreciation	293,800	275,100
Core deposit intangibles, net of accumulated amortization	8,500	14,400
Accrued interest receivable	46,700	44,900
Mortgage servicing rights, net of accumulated amortization and impairment reserve	30,200	27,700
Other assets	187,600	159,100
Total assets	14,644,200	13,300,200
Deposits:
Non-interest bearing	3,426,500	3,158,300
Interest bearing	8,237,000	7,522,400
Total deposits	11,663,500	10,680,700
Securities sold under repurchase agreements	697,600	712,400
Accounts payable and accrued expenses	129,600	94,100
Accrued interest payable	12,100	7,800
Deferred tax liability, net	26,700	8,600
Long-term debt	13,900	15,800
Other borrowed funds	0
Subordinated debentures held by subsidiary trusts	86,900	86,900
Total liabilities	12,630,300	11,606,300
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	1,049,300	866,700
Retained earnings	953,600	851,800
Accumulated other comprehensive income (loss), net	11,000	(24,600)
Total stockholders’ equity	2,013,900	1,693,900
Total liabilities and stockholders’ equity	$ 14,644,200	$ 13,300,200

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Assets:
Held-to-maturity investment securities, estimated fair value	$ 94.5	$ 275.1	$ 400.7

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Interest income:
Interest and fees on loans	$ 470.9	$ 404.3	$ 324.7
Interest and dividends on investment securities:
Taxable	62.3	55.4	42.8
Exempt from federal taxes	2	2.4	3.2
Interest on deposits in banks	18.8	11.3	7.1
Total interest income	554	473.4	377.8
Interest expense:
Interest on deposits	49.3	32.6	21.4
Interest on securities sold under repurchase agreements	3.9	2.7	1.3
Interest Expense, Short-term Borrowings	0	0.2
Interest on other borrowed funds	1.3	1.3	0.5
Interest on long-term debt			1.7
Interest on subordinated debentures held by subsidiary trusts	4.5	4.1	3.1
Total interest expense	59	40.9	28
Net interest income	495	432.5	349.8
Provision for loan losses	13.9	8.6	11
Net interest income after provision for loan losses	481.1	423.9	338.8
Non-interest income:
Fees and Commissions, Mortgage Banking and Servicing	30.4	24.9	28.9
Loss on termination of interest rate swap	0	0	(1.1)
Investment securities gains (losses), net	0.1	(0.1)	0.7
Other income	15.9	15.1	14.3
Total non-interest income	149.9	143.3	141.8
Non-interest expense:
Salaries and wages	155.3	146.4	122.7
Employee benefits	51.5	47.9	37.6
Information Technology and Data Processing	32.3	28.7	25.1
Occupancy, Net	28.3	25.4	22.4
Equipment Expense	13.2	12.7	11.5
FDIC insurance premiums	11.6	10.5	10.4
Federal Deposit Insurance Corporation Premium Expense	3.5	5.6	4.7
Mortgage servicing rights impairment (recovery)	4.3	3.1	3
OREO expense, net of income	0.4	0	(0.1)
Other Real Estate Owned Expense Net Of Income	(2.2)	0.3	0.4
Core deposit intangibles amortization	11.2	7.9	5.5
Other expenses	66.2	60	53.5
Acquisition related expenses	20.3	12.4	27.2
Total non-interest expense	395.9	360.9	323.9
Income before income tax expense	235.1	206.3	156.7
Income tax expense	54.1	46.1	50.2
Net income	$ 181	$ 160.2	$ 106.5
Basic earnings per common share (in dollars per share)	$ 2.84	$ 2.77	$ 2.07
Diluted earnings per common share (in dollars per share)	$ 2.83	$ 2.75	$ 2.05
Credit and Debit Card [Member]
Interest expense:
Revenue from Contract with Customer, Excluding Assessed Tax	$ 41.5	$ 43.3	$ 43.3
Financial Service, Other [Member]
Interest expense:
Revenue from Contract with Customer, Excluding Assessed Tax	17.1	15.1	13.3
Investment Advisory, Management and Administrative Service [Member]
Non-interest income:
Investment Banking, Advisory, Brokerage, and Underwriting Fees and Commissions	23.8	23.2	21.1
Deposit Account [Member]
Interest expense:
Revenue from Contract with Customer, Excluding Assessed Tax	$ 21.1	$ 21.8	$ 21.3

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 181	$ 160.2	$ 106.5
Investment securities available-for-sale:
Change in net unrealized gains (losses) during the period	54.9	(13.9)	(6.5)
Reclassification adjustment for net (gains) losses included in income	(0.1)	0.1	(0.7)
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity	(6)	0	0
Other Comprehensive Income (Loss), Transfers From Available-For-Sale To Held-To-Maturity Securities, Before Tax	281.1	1.6	1.9
Change in net unrealized loss on derivatives	0	0	(1.1)
Reclassification adjustment for derivative net (gains) losses included in net income	0	0	1.1
Defined benefit post-retirement benefit plans:
Change in net actuarial loss	(0.8)	(0.6)	(1.3)
Other comprehensive income (loss), before tax	48	(12.8)	(6.6)
Deferred tax benefit (expense) related to other comprehensive income (loss)	(12.4)	3.3	2.8
Other comprehensive income (loss), net of tax	35.6	(9.5)	(3.8)
Comprehensive income	$ 216.6	$ 150.7	$ 102.7

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Millions	Total	Common Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Adjustments for New Accounting Principle, Early Adoption [Member]	Restricted Stock [Member]
Beginning Balance at Dec. 31, 2016	$ 982.6	$ 296.1	$ 694.7	$ (8.2)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	106.5	0	106.5	0
Other comprehensive loss, net of tax	(3.8)	$ 0	0	(3.8)
Stock Repurchased and Retired During Period, Shares		33,063
Common stock transactions:
Common shares purchased and retired	(1.3)	$ (1.3)	0	0
Stock Issued During Period, Value, New Issues	386	386	0	0
Non-vested common shares forfeited or canceled	0		0
Stock Issued, Value, Stock Options Exercised, Net of Tax Benefit (Expense)	2.4	2.4	0	0
Stock compensation expense	3.8	3.8	0	0		$ 3.3
Stock-based compensation expense	(48.6)	0	(48.6)	0
Cash dividends declared:
Common (per share)	(1,427.6)	(687)	(752.6)	(12)
Common Stock, Value, Issued	0	0	0	0
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect	0		3.1	(3.1)
Net income	160.2
Other comprehensive loss, net of tax	(9.5)	$ 0	0	(9.5)
Stock Repurchased and Retired During Period, Shares		24,271
Common shares purchased and retired	(1)	$ (1)	0	0
Stock Issued During Period, Value, New Issues	173.3	173.3	0	0
Non-vested common shares forfeited or canceled	0	0	0	0
Stock Issued, Value, Stock Options Exercised, Net of Tax Benefit (Expense)	1.8
Stock compensation expense	5.6	5.6	0	0		5.6
Stock-based compensation expense	(64.1)		(64.1)	0		$ 0
Common (per share)	(1,693.9)	(866.7)	(851.8)	(24.6)
Ending Balance at Dec. 31, 2018	1,693.9			(24.6)
Accumulated Other Comprehensive Income (Loss), Net of Tax	(24.6)
Common stock transactions:
Common Stock, Value, Issued	0	0	0	0
Net income	181	0	181	0
Other comprehensive loss, net of tax	35.6	$ 0		35.6
Other comprehensive loss, net of tax | Accounting Standards Update 2018-02 [Member]					$ 0
Stock Repurchased and Retired During Period, Shares		43,560
Common shares purchased and retired	(2.5)	$ (2.5)	0	0
Stock Issued During Period, Value, New Issues	176.1	176.1	0	0
Non-vested common shares issued	0
Non-vested common shares forfeited or canceled	0			0
Stock Issued, Value, Stock Options Exercised, Net of Tax Benefit (Expense)	1	1	0	0
Stock compensation expense	8	8	0	0
Cash dividends declared:
Common (per share)	(79.2)	0	(79.2)	0
Ending Balance at Dec. 31, 2019	2,013.9	1,049.3	181	11
Accumulated Other Comprehensive Income (Loss), Net of Tax	11
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,013.9		$ 953.6
Common stock transactions:
Common Stock, Value, Issued	$ 0	$ 0		$ 0

Consolidated Statements of St_2

Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Stock-based compensation expense		$ (64.1)	$ (48.6)
Stockholders' Equity Attributable to Parent	$ 2,013.9	1,693.9
Net income	$ 181	$ 160.2	$ 106.5
Common stock transactions:
Stock options exercised (in shares)	191,193
Dividends declared (in usd per share)	$ 1.24	$ 1.12	$ 0.96
Common Stock
Stock-based compensation expense			$ 0
Stockholders' Equity Attributable to Parent	$ 1,049.3
Net income	$ 0		$ 0
Common stock transactions:
Common shares purchased and retired	43,560	24,271	33,063
Stock issued during period, new issues (in shares)	4,356,973	3,848,929	11,267,676
Stock issued during period, RSU (in shares)	212,587	214,892	140,246
Stock issued during period, RSU forfeited (in shares)	46,198	43,079	53,571
Stock options exercised (in shares)	143,222	161,217	218,095
Shares tendered in payment of option price and income tax withholding amounts, shares	47,971	38,450	67,792
Retained Earnings [Member]
Stock-based compensation expense		$ (64.1)	$ (48.6)
Stockholders' Equity Attributable to Parent	$ 181
Net income	181		106.5
Accumulated Other Comprehensive Income (Loss)
Stock-based compensation expense		0	0
Stockholders' Equity Attributable to Parent	11	(24.6)
Net income	$ 0		$ 0
Restricted Stock [Member]
Stock-based compensation expense		$ 0

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fees and Commissions, Mortgage Banking and Servicing	$ 30,400	$ 24,900	$ 28,900
Noninterest Income, Other Operating Income	15,900	15,100	14,300
Noninterest Income	149,900	143,300	141,800
Loss on termination of interest rate swap	0	0	(1,100)
Cash flows from operating activities:
Net income	181,000	160,200	106,500
Adjustments to reconcile net income from operations to net cash provided by operating activities:
Provision for loan losses	13,900	8,600	11,000
Net (gain) loss on disposal of property and equipment	(1,500)	(1,200)	200
Depreciation and amortization	38,700	27,600	18,400
Net premium amortization on investment securities	8,900	10,000	11,600
Net (gain) loss on investment securities transactions	(100)	100	(700)
Realized and unrealized net gains on mortgage banking activities	(30,500)	(23,000)	(25,200)
Net loss (gain) on sale of OREO	(3,600)	(800)	100
Write-downs of OREO and other assets pending disposal	900	100	400
Net (gain) on sale of health savings accounts	0	0	(3,100)
Mortgage servicing rights impairment (recovery)	400	0	(100)
Deferred income tax expense	5,400	15,800	20,900
Net increase in cash surrender value of company-owned life insurance policies	(6,700)	(5,000)	(5,400)
Stock-based compensation expense	8,000	5,600	3,900
Originations of mortgage loans held for sale	(1,015,600)	(768,100)	(815,000)
Proceeds from sales of mortgage loans held for sale	971,200	798,400	860,100
Changes in operating assets and liabilities:
Decrease (increase) in interest receivable	300	(3,300)	(600)
Increase in other assets	(22,100)	(8,000)	(6,100)
(Decrease) increase in interest payable	(13,500)	2,200	200
Decrease in accounts payable and accrued expenses	(7,800)	(200)	(22,500)
Net cash provided by operating activities	127,300	219,000	154,600
Purchases of investment securities:
Held-to-maturity	0	(2,000)	(12,800)
Payments to Acquire Debt Securities, Available-for-sale	1,270,000	541,000
Available-for-sale			(614,300)
Proceeds from maturities, pay-downs, calls and sales of investment securities:
Held-to-maturity	35,600	79,300	97,400
Available-for-sale	978,600	460,000	426,200
Proceeds from bank-owned life insurance settlements	3,200	0	0
Extensions of credit to clients, net of repayments	(81,400)	(221,700)	(99,700)
Payments For (Proceeds From) Recoveries of Loans Previously Charged off	(9,700)	12,000
Recoveries of loans charged-off			7,800
Proceeds from sales of OREO	25,400	9,100	5,900
Acquisition of intangible assets	0	0	(28,000)
Proceeds from the sale of health savings accounts	300	0	6,100
Acquisition of banks and bank holding companies, net of cash and cash equivalents acquired	(298,400)	28,100	(91,800)
Capital expenditures, net of proceeds from sales	(16,600)	(4,900)	(11,000)
Net cash used in investing activities	16,800	(181,100)	130,600
Cash flows from financing activities:
Net increase (decrease) in deposits	276,200	49,500	(110,200)
Net increase (decrease) in securities sold under repurchase agreements	(45,200)	69,400	105,400
Net increase (decrease) in other borrowed funds	(4,100)	(26,100)	100
Repayments of long-term debt	(2,000)	(7,100)	100
Advances on long-term debt	100	2,800	5,000
Proceeds from issuance of common stock	1,000	1,800	2,400
Purchase and retirement of common stock	(2,500)	(1,000)	(1,300)
Dividends paid to common stockholders	(79,200)	(64,100)	(48,600)
Net cash used in financing activities	144,300	25,200	(47,100)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect	254,800	63,100
Net change in cash and cash equivalents			(23,100)
Cash and cash equivalents at beginning of period		758,900	782,000
Cash and cash equivalents at end of period			758,900
Supplemental disclosures of cash flow information:
Cash paid during the period for income taxes	(51,200)	(25,300)	(28,800)
Interest Paid, Excluding Capitalized Interest, Operating Activities	54,700	38,700
Cash paid during the period for interest expense			(27,800)
Transfer of Portfolio Loans and Leases to Held-for-sale	0	1,600	3,000
Other Comprehensive Income (Loss), Transfers From Available-For-Sale To Held-To-Maturity Securities, Before Tax	281,100
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity	(6,000)	0	0
Right-of-Use Asset Obtained in Exchange for Finance Lease Liability	39,600	0	0
Supplemental disclosures of noncash investing and financing activities:
Transfer from long-term debt to other borrowed funds	0	0	20,000
Transfer of loans to other real estate owned	14,100	12,100	5,400
Capitalization of internally originated mortgage servicing rights	7,300	6,100	5,600
Supplemental schedule of noncash investing activities from acquisitions:
Investment securities available for sale	78,700	3,100	424,300
Investment securities held to maturity	0	0	57,300
Loans held for sale	500	0	10,300
Loans	416,600	713,100	2,079,300
Premises and equipment	24,600	14,000	47,700
Goodwill	75,300	101,100	231,900
Core deposit intangible	16,600	15,700	48,000
Mortgage servicing rights	0	0	3,500
Deferred tax assets	2,200	3,600	7,600
Interest receivable	15,200	9,500	57,000
Deferred tax assets	0	0	28,600
Other real estate owned	2,400	600	1,200
Other assets	6,500	6,200	31,600
Total noncash assets acquired	638,600	866,900	3,028,300
Deposits	706,700	696,300	2,669,000
Noncash Or Part Noncash Acquisition, Securities Sold Under Repurchase Agreements	30,400	0	0
Deferred tax liability	19,900	7,700	64,200
Noncash or Part Noncash Acquisition, Debt Assumed	4,100	7,000	0
Noncash or Part Noncash Acquisition, Other Liabilities Assumed	0	6,100	0
Noncash or Part Noncash Acquisition, Trust Preferred Securities Assumed	0	4,400	0
Noncash or Part Noncash Acquisition, Deferred Tax Liability	100	300	0
Total liabilities assumed	(761,200)	(721,800)	(2,733,200)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	1,076,800	822,000	758,900
Credit and Debit Card [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	41,500	43,300	43,300
Investment Advisory, Management and Administrative Service [Member]
Investment Banking, Advisory, Brokerage, and Underwriting Fees and Commissions	23,800	23,200	21,100
Deposit Account [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	21,100	21,800	21,300
Financial Service, Other [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 17,100	$ 15,100	$ 13,300

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2019
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, 2019 , 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 2018 and 2017 to conform to the 2019 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 loan 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, 2019 and 2018 , the Company had cash of $769.3 million and $574.4 million , respectively, on deposit with the Federal Reserve Bank. In addition, the Company maintained compensating balances with the Federal Reserve Bank of approximately $46.3 million and $28.1 million as of December 31, 2019 and 2018 , respectively, to reduce service charges for check clearing services. Investment Securities . 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. 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, 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 are included in investment securities gains. Declines in the fair value of securities below their cost that are judged to be other-than-temporary are included in other expenses if the decline is related to credit losses. Other-than-temporary impairment losses related to other factors are recognized in other comprehensive income, net of income taxes. In estimating other-than-temporary impairment losses, the Company considers, among other things, the length of time and the extent to which the fair value has been less than cost, the financial condition and near-term prospects of the issuer and the intent and ability of the Company to retain its investment in the issuer for a period of time sufficient to allow for any anticipated recovery in fair value. The cost of securities sold is based on the specific identification method. Loans . Loans are reported at the principal amount outstanding. Interest income on loans is calculated using the simple interest method on the daily balance of the principal amount outstanding. Loan origination fees and certain direct origination costs are deferred, and the net amount is amortized as an adjustment of the related loan’s yield using a level yield method over the expected lives of the related loans. The accrual of interest on loans is discontinued when, in management’s opinion, the borrower may be unable to meet payment obligations as they become due or when a loan becomes contractually past due ninety days or more with respect to interest or principal, unless such past due loan is well secured and in the process of collection. When interest accrual is discontinued, all unpaid accrued interest is reversed against current period interest income. Interest income is subsequently recognized only to the extent cash payments are received in excess of principal due. Loans are returned to accrual status when all 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. A loan is considered impaired when, based upon current information and events, it is probable that the Company will be unable to collect, on a timely basis, all amounts due according to the contractual terms of the loan’s original agreement. The amount of the impairment is measured using cash flows discounted at the loan’s effective interest rate, except when it is determined that the primary source of repayment for the loan is the operation or liquidation of the underlying collateral. In such cases, the current fair value of the collateral, reduced by anticipated selling costs, is used to measure impairment. The Company considers impaired loans to include all loans, except consumer loans, that are risk rated as doubtful or on which interest accrual has been discontinued or that have been renegotiated in a troubled debt restructuring. Interest payments received on impaired loans are applied based on whether they are on accrual or non-accrual status. Interest income recognized by the Company on impaired loans primarily relates to loans modified in troubled debt restructurings that remain on accrual status. Interest payments received on non-accrual impaired loans are applied to principal. Interest income is subsequently recognized only to the extent cash payments are received in excess of principal due. Loans acquired in a business combination are recorded and initially measured at their estimated fair value as of the acquisition date, with no carryover of the related allowance for credit losses. Credit risks are included in the determination of fair value. For loans with no significant evidence of credit deterioration since origination, the difference between the fair value and the unpaid principal balance of the loan at the acquisition date is amortized into interest income using the effective interest method over the remaining period to contractual maturity. The accounting for loans acquired with evidence of a deterioration of credit quality is described below. Loans acquired through the completion of a transfer, including loans acquired in business combinations, that have evidence of deterioration of credit quality since origination and for which it is probable, at acquisition, that the Company will be unable to collect all contractually required payments receivable are initially recorded at fair value (as determined by the present value of expected future cash flows) with no valuation allowance. The difference between the undiscounted cash flows expected at acquisition and the recorded fair value of the loan, or the “accretable yield,” is recognized as interest income on a level-yield method over the life of the loan. Contractually required payments for interest and principal that exceed the undiscounted cash flows expected at acquisition, or the “nonaccretable difference,” are not recognized as a yield adjustment, a loss accrual or a valuation allowance. Increases in expected cash flows subsequent to the initial measurement are recognized prospectively through adjustment of the yield on the loan over its remaining life. Decreases in expected cash flows are recognized as impairment. Valuation allowances on these impaired loans reflect only losses incurred after the acquisition. A loan is considered a troubled debt restructuring when a borrower is experiencing financial difficulties that leads to a restructuring of the loan and the Company grants concessions to the borrower in the restructuring that it would not otherwise consider. These concessions may include rate reductions, principal forgiveness, extension of maturity date and other actions to minimize potential losses. Certain troubled debt restructurings are on non-accrual status at the time of restructuring and are returned to accrual status only after considering the borrower’s sustained repayment performance in accordance with the restructuring agreement for a reasonable period of at least six months and management is reasonably assured of 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 no longer be disclosed as a troubled debt restructuring although they continue to be individually evaluated for impairment and disclosed as impaired loans. Loans held for sale include residential mortgage loans originated for immediate sale. The Company has elected to account for loans held for sale using the fair value option. Under the fair value option, net loan origination fees are recognized in non-interest income at the time of origination. Subsequent changes in the estimated fair values of loans held for sale are recorded as unrealized gains and losses in non-interest income. Estimated fair values of loans held for sale are determined based upon current secondary market prices for loans with similar coupons, maturities and credit quality, or in the case of committed loans, on current delivery prices. Gains and losses on loans held for sale are recognized based on the difference between the net sales proceeds, including the estimated value associated with servicing assets or liabilities, and the net carrying value of the loans sold. Adjustments to reflect unrealized gains and losses resulting from changes in fair value of loans held for sale, as well as realized gains and losses on the sale of loans, are included in non-interest income - mortgage banking revenues on the accompanying consolidated statements of income. Loans held for sale were $100.9 million and $33.3 million as of December 31, 2019 and 2018 , respectively. As of December 31, 2019 , the Company had $1.4 million recorded investments in consumer mortgage loans secured by residential real estate for which formal foreclosure proceedings were in process. Allowance for Loan Losses . The allowance for loan losses is established through a provision for loan losses which is charged to expense. Loans, or portions thereof, are charged against the allowance for loan losses when management believes that the collectability of the principal is unlikely or, with respect to consumer installment and credit card loans, according to established delinquency schedules. The allowance balance is an amount that management believes will be adequate to absorb known and inherent losses in the loan portfolio based upon quarterly analysis of the current risk characteristics of the loan portfolio, an assessment of individual problem loans and actual loss experience, industry concentrations and current economic factors and the estimated impact of current economic and environmental conditions on historical loss rates. Loans acquired in business combinations are recorded at their estimated fair values on the date of acquisition. Accordingly, no allowance for loan losses related to these loans is recorded at the date of transfer. An allowance for loan losses is recorded for credit deterioration occurring subsequent to the transfer date. 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 4 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 office space and have entered into various other agreements in conducting our business. Operating lease right-of-use assets are included within the Premises and Equipment line item and our operating lease liability is included within the Other Liabilities line item. Operating lease expense is recognized on a straight-line basis over the lease term, subject to any changes in the lease or expectations regarding the terms. Variable lease costs such as property taxes are expensed as incurred. Lease and non-lease components are accounted for separately as the amounts are readily determinable under our lease contracts. Leases with an initial term of 12 months or less are not recorded on the balance sheet. Upon adoption of ASU 2016-02, the Company elected to apply certain practical expedients whereby we did not reassess (i) whether any expired or existing contracts are or contain leases, (ii) the lease classification for any expired or existing leases, and (iii) initial direct costs for any existing leases. We elected the hindsight practical expedient to determine the lease term for existing leases. In recognizing lease right-of use assets and related lease liabilities, we determine whether an agreement represents a lease and at commencement of the lease we evaluate each agreement to determine whether the lease is an operating or financing lease. Some of our lease agreements have contained renewal options, tenant improvement allowances, rent holidays, and rent escalation clauses. We hold one financing lease with the remaining leases classified as operating leases. Right-of-use lease assets represent our right to use the underlying asset for the lease term and the lease obligation represents our commitment to make the lease payments arising from the lease. Right-of-use lease assets and obligations are recognized at the commencement date based on the present value of remaining lease payments over the lease term. For the Company’s leases that do not provide an implicit rate, we use an estimated incremental borrowing rate based on the information available at the commencement date in determining the present value of lease payments. The right-of-use lease asset includes any lease payments made prior to commencement and excludes any lease incentives. The estimated lease term may include options to extend or terminate the lease when it is reasonably certain that we will exercise that option. 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. 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) and profit sharing plans 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. As of December 31, 2019 and 2018 , deferred compensation plan assets were $18.2 million and $12.1 million , respectively. Corresponding deferred compensation plan liabilities were $18.2 million and $12.1 million as of December 31, 2019 and 2018 , respectively. 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. The amount of the impairment loss, if any, is based on the asset’s fair value. No impairment losses were recognized in 2019 , 2018 , or 2017 . 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 loan 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 OREO expense. Write-downs of $0.9 million , $0.1 million and $0.4 million were recorded in 2019 , 2018 and 2017 , respectively. The carrying value of foreclosed residential real estate properties included in other real estate owned was $2.3 million as of December 31, 2019 , and $2.0 million as of December 31, 2018 . 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 capital stock. As of December 31, 2019 , restricted equity securities of the Federal Reserve Bank and the FHLB of $42.8 million and $10.7 million , respectively, were included in other assets at cost. As of December 31, 2018 , restricted equity securities of the Federal Reserve Bank and the FHLB were $37.5 million and $10.7 million , respectively. 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, the length of time a decline has persisted, the impact of legislative and regulatory changes on the issuing organizations and the liquidity positions of the issuing organizations. Based on management’s assessment, no impairment losses were recorded on restricted equity securities during 2019 , 2018 or 2017 . 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 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 effective portion of 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 ineffective portion of the gain or loss on derivative instruments, if any, is recognized in earnings. The Company does not enter into interest rate swap agreements for trading or speculative purposes. As of December 31, 2019 , the Company does not have an existing agreement. 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. 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. Diluted earnings per common share is calculated by dividing net income available to common shareholders by the weighted average number of common shares outstanding determined for the basic earnings per share calculation plus the dilutive effect of stock compensation using the treasury stock 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. Positions taken in the Company’s tax returns may be subject to challenge by the taxing authorities upon examination. Uncertain tax positions are initially recognized in the financial statements when it is more likely than not the position will be sustained upon examination by the tax authorities. Such tax positions are both initially and subsequently measured as the largest amount of tax benefit that is greater than 50% likely of being realized upon settlement with the tax authority, assuming full knowledge of the position and all relevant facts. The Company provides for interest and, in some cases, penalties on tax positions that may be challenged by the taxing authorities. Interest expense is recognized beginning in the first period that such interest would begin accruing. Penalties are recognized in the period that the Company claims the position in the tax return. Interest and penalties on income tax uncertainties are classified within income tax expense in the consolidated statements of income. With few exceptions, the Company is no longer subject to U.S. federal and state examinations by tax authorities for years before 2016 . The Company had no material penalties as of December 31, 2019 ,

Acquisitions

Acquisitions	12 Months Ended
	Dec. 31, 2019
Business Combinations [Abstract]
Acquisitions	ACQUISITIONS Community 1 st Bank. On October 11, 2018 , the Company entered into a definitive agreement to acquire all of 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 is 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. Adjustments to the fair value marks for deferred taxes and accounts payable and accrued expenses were made since the prior quarter, none of which were material. The adjustments had no impact on 2019 earnings and resulted in a net increase to goodwill of $0.1 million from the third quarter reported balances. All amounts reported were finalized in 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 CMYF 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 of 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 conversion of the 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 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 in 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 losses (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. Adjustments to the fair value marks for deferred tax assets and accounts payable and accrued expenses were made since the prior quarter, none of which were material. The adjustments had no impact on 2019 earnings and a net decrease to goodwill of $1.1 million from the third quarter reported balances. (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 and to record the fair value of right-of-use asset leases. (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 and to record the lease liability. (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. The Company acquired certain loans that are 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 acquired loans that have evidence of deterioration in credit quality since origination for which it is probable, at acquisition, the Company will be unable to collect all contractual amounts owed. 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 underling 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. Northwest Bancorporation, Inc. On April 25, 2018 , the Company entered into a definitive agreement to acquire all of the outstanding stock of Northwest Bancorporation, Inc. (“Northwest”), the parent company of Inland Northwest Bank (“INB”), a Spokane, Washington based community bank with 20 banking offices across Idaho, Oregon and Washington. The acquisition was completed on August 16, 2018 , and the Company merged INB with its existing bank subsidiary, First Interstate Bank, on November 9, 2018 . Consideration for the acquisition was $176.3 million , consisting of the issuance of 3.84 million shares of the Company's Class A common stock valued at $45.15 per share, the closing price of the Company's Class A common stock as quoted on the NASDAQ stock market on the acquisition date. The Company paid approximately $3.0 million in cash related to Northwest warrants, which were included in the consideration paid. Holders of each share of Northwest common stock received 0.516 shares of First Interstate Class A common stock for each share of Northwest common stock. Additionally, all Northwest stock purchase warrants outstanding immediately prior to the close of the transaction were canceled in exchange for the right to receive a cash payment as provided in the Agreement. Previously unvested Northwest restricted stock awards outstanding immediately prior to the close of the transaction vested and were considered issued and outstanding at acquisition close. The assets and liabilities of Northwest 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 $100.7 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 Northwest and the Company. The following table summarizes the consideration paid, fair values of the Northwest assets acquired and liabilities assumed, and the resulting goodwill. All amounts reported were finalized in the second quarter of 2019. As Recorded Fair Value As Recorded As of August 16, 2018 by Northwest Adjustments by the Company Assets acquired: Cash and cash equivalents $ 31.2 $ — $ 31.2 Investment securities 3.1 — 3.1 Loans held for investment 727.9 (14.8 ) (1) 713.1 Allowance for loan loss (8.0 ) 8.0 (2) — Premises and equipment 14.5 — 14.5 Other real estate owned (“OREO”) 0.3 0.3 0.6 Core deposit intangible assets 2.4 13.3 (3) 15.7 Other assets 29.3 (10.0 ) (4) 19.3 Total assets acquired 800.7 (3.2 ) 797.5 Liabilities assumed: Deposits 696.1 0.2 (5) 696.3 Accounts payable and accrued expense 8.1 (0.4 ) (6) 7.7 Long term debt 13.0 0.1 13.1 Trust preferred securities 5.2 (0.8 ) (7) 4.4 Deferred tax liability, net (1.2 ) 1.6 (8) 0.4 Total liabilities assumed 721.2 0.7 721.9 Net assets acquired $ 79.5 $ (3.9 ) $ 75.6 Consideration paid: Cash $ 3.0 Class A common stock 173.3 Total consideration paid 176.3 Goodwill $ 100.7 Explanation of fair value adjustments and the removal of previously recorded fair value marks recorded by Northwest. (1) 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. (2) Adjustment to remove the Northwest allowance for loan losses at acquisition date, as the credit risk is included in the fair value adjustment for loans receivable described in (1) above. (3) Write down 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. (4) Adjustment represents the value of the core deposit base assumed in the acquisition based upon valuation from an independent accounting and advisory firm. (5) Adjustment consists of reductions to the fair value of other items, including the removal of Northwest previously recorded goodwill. (6) 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. (7) Decrease due to the write-off of off-balance sheet reserves. (8) Write down of the book value of debt to the estimated fair values on the date of acquisition based upon favorable interest rates in the market. (9) Adjustment consists of the write-off of pre-existing deferred tax assets and purchase accounting adjustments as a result of the acquisition. Core deposit intangible assets of $15.7 million are being amortized using an accelerated method over the estimated useful lives of the related deposits of 10 years. The Company acquired certain loans that are 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 acquired loans that have evidence of deterioration in credit quality since origination for which it is probable, at acquisition, the Company will be unable to collect all contractual amounts owed. 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 underling credit-impaired loans, (ii) assumptions regarding future principal and interest amounts collected, and (iii) indices used to fair value variable rate loans. Information regarding Northwest loans acquired deemed credit-impaired as of the August 16, 2018 acquisition date are as follows: Contractually required principal and interest payments $ 27.5 Contractual cash flows not expected to be collected (“non-accretable discount”) 4.4 Cash flows expected to be collected 23.1 Interest component of cash flows expected to be collected (“accretable discount”) 3.2 Fair value of acquired credit-impaired loans $ 19.9 Information regarding Northwest acquired loans not deemed credit-impaired at the August 16, 2018 acquisition date are as follows: Contractually required principal and interest payments $ 894.8 Contractual cash flows not expected to be collected 26.1 Fair value at acquisition $ 693.2 Unaudited pro forma consolidated revenues and net income as if the Northwest acquisition had occurred as of January 1, 2017, 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 August 16, 2018 acquisition date. The acquired entity continued to operate as INB until November 9, 2018 at which point INB’s operations were integrated with the Company’s operations, and INB merged with FIB. Standalone amounts for INB were no longer available after that date. Acquisition related expenses The Company recorded third party acquisition related costs of $20.3 million , $12.4 million and $27.2 million in 2019 , 2018 and 2017 , respectively. These costs are incorporated in non-interest expense in the Company’s consolidated statements of income and are summarized below. Dec 31, 2019 Dec 31, 2018 Dec 31, 2017 Legal and professional fees $ 1.0 $ 4.0 $ 9.6 Employee expenses 8.2 1.1 5.1 Technology conversion and contract termination 9.1 6.6 10.2 Other 2.0 0.7 2.3 Total acquisition related expenses $ 20.3 $ 12.4 $ 27.2

Goodwill and Core Deposit Intan

Goodwill and Core Deposit Intangibles	12 Months Ended
	Dec. 31, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Core Deposit Intangibles	GOODWILL AND CORE DEPOSIT INTANGIBLES Goodwill Year Ended December 31, 2019 2018 Net carrying value at beginning of period $ 546.7 $ 444.7 Acquisitions and measurement period adjustments 74.9 102.0 Net carrying value at end of period $ 621.6 $ 546.7 The Company performed its annual goodwill impairment qualitative assessment as of July 1, 2019 , 2018 , and 2017 and determined the Company’s goodwill was no t considered impaired. In addition, there were no events or circumstances that occurred during the second half of 2019 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, 2019 . Core deposit intangibles (“CDI”) The following table sets forth activity for identifiable core deposit intangibles subject to amortization: Year Ended December 31, 2019 2018 Gross CDI, beginning of period $ 89.7 $ 74.0 Established through acquisitions 16.6 15.7 Reductions due to sale of accounts (0.3 ) — Accumulated amortization (43.9 ) (32.8 ) Net CDI, end of period $ 62.1 $ 56.9 Amortization expense of CDI assets was $11.2 million , $7.9 million and $5.5 million for the fiscal years ended December 31, 2019 , 2018 and 2017 , 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, 2020 $ 10.9 2021 9.9 2022 9.0 2023 8.2 2024 7.3 Thereafter 16.8 Total $ 62.1

Investment Securities

Investment Securities	12 Months Ended
	Dec. 31, 2019
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, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value 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 & collateralized mortgage obligations 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 Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value 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 December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Available-for-Sale U.S. Treasury notes $ 2.6 $ — $ — $ 2.6 Obligations of U.S. government agencies 569.3 0.1 (10.2 ) 559.2 U.S. agency residential mortgage-backed securities & 1,566.4 2.5 (24.1 ) 1,544.8 Private mortgage-backed securities 72.0 — (1.8 ) 70.2 Corporate Securities 92.9 — (1.0 ) 91.9 Other investments 2.0 — — 2.0 Total $ 2,305.2 $ 2.6 $ (37.1 ) $ 2,270.7 December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Held-to Maturity State, county and municipal securities $ 150.9 $ 1.8 $ (0.9 ) $ 151.8 Obligations of U.S. government agencies 19.8 — (0.3 ) 19.5 U.S. agency residential mortgage-backed securities & collateralized mortgage obligations 189.7 0.3 (6.5 ) 183.5 Corporate securities 46.3 0.1 (0.6 ) 45.8 Other investments 0.1 — — 0.1 Total $ 406.8 $ 2.2 $ (8.3 ) $ 400.7 At December 31, 2018 , we had $406.8 million of investment securities classified as held to maturity. As a result of the adoption of ASU 2017-12 discussed in “Note 27 – Recent Authoritative Accounting Guidance”, the Company transferred investment securities classified as held-to-maturity to investment securities available-for-sale. At the time of transfer, the amortized cost and fair value of these securities totaled $281.1 million and $275.1 million , respectively. In addition, the unrealized loss of $6.0 million was recorded in the consolidated statement of comprehensive income. There were no material gross gains and no material gross losses realized on the disposition of available-for-sale securities in 2019 and 2018 . There were $1.1 million of gross gains realized and $0.4 million of gross losses realized on the disposition of available-for-sale securities in 2017 . As of December 31, 2019 , the Company had general obligation securities with amortized costs of $45.9 million included in state, county and municipal securities, of which $28.0 million 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, 2019 and 2018 . There were no held-to-maturity securities in a continuous unrealized loss position as of December 31, 2019 . Less than 12 Months 12 Months or More Total December 31, 2019 Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Available-for-Sale Obligations of U.S. government agencies $ 185.3 $ (0.8 ) $ — $ — $ 185.3 $ (0.8 ) U.S. agency residential mortgage-backed securities & collateralized mortgage obligations 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 ) Less than 12 Months 12 Months or More Total December 31, 2018 Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Available-for-Sale Obligations of U.S. government agencies $ 363.1 $ (7.9 ) $ 154.5 $ (2.3 ) $ 517.6 $ (10.2 ) U.S. agency residential mortgage-backed securities & collateralized mortgage obligations 735.2 (14.5 ) 503.7 (9.6 ) 1,238.9 (24.1 ) Private mortgage-backed securities — — 69.4 (1.8 ) 69.4 (1.8 ) Corporate securities 24.9 (0.2 ) 51.4 (0.8 ) 76.3 (1.0 ) Total $ 1,123.2 $ (22.6 ) $ 779.0 $ (14.5 ) $ 1,902.2 $ (37.1 ) Less than 12 Months 12 Months or More Total December 31, 2018 Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Held-to-Maturity State, county and municipal securities $ 25.9 $ (0.3 ) $ 57.1 $ (0.6 ) $ 83.0 $ (0.9 ) U.S. agency residential mortgage-backed 45.0 (2.2 ) 120.2 (4.3 ) 165.2 (6.5 ) Corporate securities — — 39.6 (0.6 ) 39.6 (0.6 ) Obligations of U.S. government agencies 19.5 (0.3 ) — — 19.5 (0.3 ) Total $ 90.4 $ (2.8 ) $ 216.9 $ (5.5 ) $ 307.3 $ (8.3 ) The investment portfolio is evaluated quarterly for other-than-temporary declines in the market value of each individual investment security. Consideration is given to the length of time and the extent to which the fair value has been less than cost; the financial condition and near term prospects of the issuer; and, the intent and ability of the Company to retain its investment in the issuer for a period of time sufficient to allow for any anticipated recovery in fair value. The Company had 338 and 760 individual investment securities that were in an unrealized loss position as of December 31, 2019 and 2018 , respectively. Unrealized losses as of December 31, 2019 and 2018 related primarily to fluctuations in the current interest rates. The fair value of these investment securities is expected to recover as the securities approach their maturity or repricing date or if market yields for such investments decline. As of December 31, 2019 , 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. No impairment losses were recorded during 2019 , 2018 or 2017 . Maturities of investment securities at December 31, 2019 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, 2019 Amortized Cost Estimated Fair Value Amortized Cost Estimated Fair Value Within one year $ 677.3 $ 661.4 $ 38.6 $ 38.6 After one year but within five years 1,486.2 1,470.9 29.7 30.1 After five years but within ten years 328.0 428.2 21.3 23.1 After ten years 454.0 399.5 2.7 2.7 Total $ 2,945.5 $ 2,960.0 $ 92.3 $ 94.5 At December 31, 2019 , the Company had investment securities callable within one year with amortized costs and estimated fair values of $181.2 million and $181.1 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. At December 31, 2019 , the Company had no callable structured notes. Maturities of securities do not reflect rate repricing opportunities present in adjustable rate mortgage-backed securities. At December 31, 2019 and 2018 , the Company had variable rate mortgage-backed securities with amortized costs of $298.1 million and $219.5 million , respectively, classified as available-for-sale in the table above. There are no significant concentrations of investments at December 31, 2019 , (greater than 10 percent of stockholders’ equity) in any individual security issuer, except for U.S. government or agency-backed securities. Investment securities with amortized cost of $2,132.0 million and $1,943.1 million at December 31, 2019 and 2018 , respectively, were pledged to secure public deposits and securities sold under repurchase agreements. The approximate fair value of securities pledged at December 31, 2019 and 2018 was $2,144.9 million and $1,908.4 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, 2019
Loans and Leases Receivable, Net Amount [Abstract]
Loans	LOANS The following table presents loans by class as of the dates indicated: December 31, 2019 2018 Real estate loans: Commercial $ 3,484.7 $ 3,235.4 Construction: Land acquisition & development 302.1 321.6 Residential 244.1 242.8 Commercial 431.5 274.3 Total construction loans 977.7 838.7 Residential 1,546.1 1,542.0 Agricultural 226.6 217.4 Total real estate loans 6,235.1 5,833.5 Consumer: Indirect consumer 784.6 787.8 Other consumer 179.0 200.6 Credit card 81.6 81.8 Total consumer loans 1,045.2 1,070.2 Commercial 1,371.3 1,310.3 Agricultural 279.1 254.8 Other, including overdrafts — 1.6 Loans held for investment 8,930.7 8,470.4 Mortgage loans held for sale 100.9 33.3 Total loans $ 9,031.6 $ 8,503.7 The Company has lending policies and procedures in place that are designed to maximize loan income within an acceptable level of risk. Management reviews and approves these policies and procedures on a regular basis. A reporting system supplements the review process by providing management with frequent reports related to loan production, loan quality, concentrations of credit, loan delinquencies and internally risk-classified loans. Real estate loans include construction and permanent financing for both single-family and multi-unit properties, term loans for commercial, agricultural and industrial property and/or buildings and home equity loans and lines of credit secured by real estate. Longer-term residential real estate loans are generally sold in the secondary market. Those residential real estate loans not sold are typically secured by first liens on the financed property and generally mature in less than fifteen years . Home equity loans and lines of credit are typically secured by first or second liens on residential real estate and generally do not exceed a loan to value ratio of 80% . The Company had home equity loans and lines of credit of $423.5 million and $409.5 million as of December 31, 2019 and 2018 , respectively, included in residential real estate loans. Commercial and agricultural real estate loans are generally secured by first liens on income-producing real estate and generally mature in less than 5 years . Construction 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. Consumer loans include direct personal loans, credit card loans, lines of credit and indirect dealer loans for the purchase of automobiles, recreational vehicles, boats and other consumer goods. Personal loans and indirect dealer loans are generally secured by automobiles, boats and other types of personal property and are made on an installment basis. Credit cards are offered to individuals in our market areas. Lines of credit are generally floating rate loans that are unsecured or secured by personal property. Commercial loans include a mix of variable and fixed rate loans 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. The loans are generally made with business operations as the primary source of repayment, but also include collateralization by inventory, accounts receivable, equipment and/or personal guarantees. 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 five years or less, with operating lines for one production season. Included in the loan table above, are loans acquired in business combinations including certain loans that had evidence of deterioration in credit quality since origination and for which it was probable, at acquisition, that all contractually required payments would not be collected. The following table displays the outstanding unpaid principal balance and accrual status of loans acquired with credit impairment as of December 31, 2019 and 2018 . December 31, 2019 2018 Outstanding principal $ 49.8 $ 43.4 Carrying value: Loans on accrual status 36.8 30.2 Total carrying value $ 36.8 $ 30.2 The following table summarizes changes in the accretable yield for loans acquired credit impaired for the years ended December 31, 2019 , 2018 , and 2017 : Year Ended December 31, 2019 2018 2017 Beginning balance $ 8.9 $ 7.3 $ 6.8 Acquisitions 3.4 3.2 1.9 Additions — 0.6 0.1 Accretion income (3.4 ) (3.1 ) (2.9 ) Reductions due to exit events (1.4 ) (1.1 ) (1.5 ) Reclassifications from nonaccretable differences 2.5 2.0 2.9 Ending balance $ 10.0 $ 8.9 $ 7.3 Loans are considered past due if the required principal and interest payments have not been received as of the date such payments were due. The following tables present the contractual aging of the Company’s recorded investment in past due loans by class as of the period indicated: 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 Loans Real estate Commercial $ 5.5 $ 1.1 $ 0.6 $ 7.2 $ 3,464.5 $ 13.0 $ 3,484.7 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 1,535.2 4.6 1,546.1 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 6,191.5 24.7 6,235.1 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 1,347.9 13.7 1,371.3 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 8,844.3 42.9 8,930.7 Mortgage loans originated for sale — — — — 100.9 — 100.9 Total loans $ 27.6 $ 10.2 $ 5.7 $ 43.5 $ 8,945.2 $ 42.9 $ 9,031.6 Total Loans 30 - 59 60 - 89 > 90 30 or More Days Days Days Days Current Non-accrual Total As of December 31, 2018 Past Due Past Due Past Due Past Due Loans Loans Loans Real estate Commercial $ 10.4 $ 1.0 $ 0.8 $ 12.2 $ 3,214.0 $ 9.2 $ 3,235.4 Construction: Land acquisition & development 1.6 0.1 0.2 1.9 316.0 3.7 321.6 Residential 1.0 0.4 — 1.4 240.4 1.0 242.8 Commercial 0.4 — — 0.4 273.7 0.2 274.3 Total construction loans 3.0 0.5 0.2 3.7 830.1 4.9 838.7 Residential 8.8 1.1 0.2 10.1 1,525.3 6.6 1,542.0 Agricultural 2.2 — — 2.2 202.6 12.6 217.4 Total real estate loans 24.4 2.6 1.2 28.2 5,772.0 33.3 5,833.5 Consumer: Indirect consumer 6.8 2.1 0.4 9.3 776.8 1.7 787.8 Other consumer 1.4 0.5 0.1 2.0 198.1 0.5 200.6 Credit card 0.9 0.4 0.8 2.1 79.7 — 81.8 Total consumer loans 9.1 3.0 1.3 13.4 1,054.6 2.2 1,070.2 Commercial 8.3 1.2 1.3 10.8 1,283.7 15.8 1,310.3 Agricultural 2.1 0.3 — 2.4 249.4 3.0 254.8 Other, including overdrafts — — — — 1.6 — 1.6 Loans held for investment 43.9 7.1 3.8 54.8 8,361.3 54.3 8,470.4 Mortgage loans originated for sale — — — — 33.3 — 33.3 Total loans $ 43.9 $ 7.1 $ 3.8 $ 54.8 $ 8,394.6 $ 54.3 $ 8,503.7 Acquired loans that meet the criteria for non-accrual of interest prior to the acquisition were considered performing upon acquisition. If interest on non-accrual loans had been accrued, such income would have approximated $2.5 million , $3.0 million and $3.5 million during the years ended December 31, 2019 , 2018 , and 2017 , respectively. The Company considers impaired loans to include all originated loans, except consumer loans, that are risk rated as doubtful, or have been placed on non-accrual status or renegotiated in troubled debt restructurings, and all loans acquired with evidence of deterioration in credit quality and for which it was probable, at the acquisition, that the Company would be unable to collect all contractual amounts owed. The following tables present information on the Company’s recorded investment in impaired loans as of dates indicated: December 31, 2019 Unpaid Total Principal Balance Recorded Investment With No Allowance Recorded Investment With Allowance Total Recorded Investment Related Allowance 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 Total Principal Balance Recorded Investment With No Allowance Recorded Investment With Allowance Total Recorded Investment Related Allowance 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 December 31, 2017 Unpaid Total Principal Balance Recorded Investment With No Allowance Recorded Investment With Allowance Total Recorded Investment Related Allowance Real estate: Commercial $ 45.6 $ 20.9 $ 14.1 $ 35.0 $ 3.9 Construction: Land acquisition & development 10.0 3.4 0.5 3.9 — Residential 1.8 1.7 — 1.7 — Commercial 4.7 0.4 3.5 3.9 2.2 Total construction loans 16.5 5.5 4.0 9.5 2.2 Residential 11.5 8.2 2.0 10.2 0.1 Agricultural 3.7 3.6 — 3.6 — Total real estate loans 77.3 38.2 20.1 58.3 6.2 Commercial 29.5 12.4 11.4 23.8 4.4 Agricultural 1.1 0.8 0.3 1.1 0.2 Total $ 107.9 $ 51.4 $ 31.8 $ 83.2 $ 10.8 The following tables present the average recorded investment in and income recognized on impaired loans for the periods indicated: Year Ended December 31, 2019 2018 2017 Average Recorded Investment Income Recognized Average Recorded Investment Income Recognized Average Recorded Investment Income Recognized Real estate $ 41.4 $ 0.1 $ 50.0 $ 0.1 $ 63.5 $ 0.3 Commercial 18.7 0.2 21.9 0.2 28.1 0.2 Agricultural 4.6 — 2.1 — 2.4 — Total $ 64.7 $ 0.3 $ 74.0 $ 0.3 $ 94.0 $ 0.5 The amount of interest income recognized by the Company within the period that the loans were impaired was primarily related to loans modified in troubled debt restructurings that remained on accrual status. Interest payments received on non-accrual impaired loans are applied to principal. Interest income is subsequently recognized only to the extent cash payments are received in excess of principal due. If interest on impaired loans had been accrued, interest income on impaired loans during 2019 , 2018 , and 2017 would have been approximately $2.5 million , $3.0 million and $3.5 million , respectively. Collateral dependent impaired loans are recorded at the fair value less selling costs of the underlying collateral determined using discounted cash flows, independent appraisals and management estimates based upon current market conditions. For loans measured under the present value of cash flows method, the change in present value attributable to the passage of time, if applicable, is recognized in the provision for loan losses and thus no interest income is recognized. 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. 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 after considering the borrower’s sustained repayment performance in accordance with the restructuring agreement for a period of at least six months and management is reasonably assured of 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, although they continue to be individually evaluated for impairment and disclosed as impaired loans. The Company had loans renegotiated in troubled debt restructurings 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. The Company had loans renegotiated in troubled debt restructurings of $23.4 million as of December 31, 2018 , of which $17.8 million were included in non-accrual loans and $5.6 million were on accrual status. The following table presents information on the Company’s troubled debt restructurings that occurred during the periods indicated: Number of Notes Type of Concession Principal Balance at Restructure Date Year Ended December 31, 2019 Interest only period Extension of terms or maturity Interest rate adjustment Other 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 11 $ 0.2 $ 0.2 $ — $ 10.0 $ 10.4 Number of Notes Type of Concession Principal Balance at Restructure Date Year Ended December 31, 2018 Interest only period Extension of terms or maturity Interest rate adjustment Other 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 5 $ 3.6 $ — $ — $ 0.5 $ 4.1 Number of Notes Type of Concession Principal Balance at Restructure Date Year Ended December 31, 2017 Interest only period Extension of terms or maturity Interest rate adjustment Other Commercial 17 $ 1.2 $ 2.0 $ — $ 6.0 $ 9.2 Agriculture 1 — 0.1 — — 0.1 Total loans restructured 24 $ 2.7 $ 3.3 $ — $ 6.9 $ 12.9 Other concessions include payment reductions or deferrals for a specified period of time or the extension of amortization schedules. A specific reserve may have been previously recorded for loans modified in troubled debt restructurings that were on non-accrual status or otherwise deemed impaired before the modification. In periods subsequent to modification, the Company continues to evaluate all loans modified in troubled debt restructurings for possible impairment, which is recognized through the allowance for loan losses. Financial effects of modifications may include principal loan forgiveness or other charge-offs directly related to the restructuring. The Company had no charge-offs directly related to loans modified in troubled debt restructurings taken at the time of restructuring during 2019 , 2018 , and 2017 . The Company considers a payment default to occur on loans modified in troubled debt restructurings when the loan is 90 days or more past due or was placed on non-accrual status after the modification. The Company’s loans modified in troubled debt restructurings within the previous 12 months for which there was a payment default during the period were no t significant as of December 31, 2019 and December 31, 2018 . As of December 31, 2017 , the Company had one $1.3 million commercial loan modified in troubled debt restructurings within the previous 12 months for which there was a payment default during the period. As of December 31, 2019 , 2018 , and 2017 all of the loans modified in troubled debt restructurings with payment defaults during the previous twelve months were on non-accrual status. At December 31, 2019 , there were no material commitments to lend additional funds to borrowers whose existing loans have been renegotiated or are classified as non-accrual. 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. The Company adheres to 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: Other Assets Especially Mentioned — includes loans that exhibit weaknesses in financial condition, loan structure or documentation, which if not promptly corrected, may lead to the development of abnormal risk elements. Substandard — includes loans that are inadequately protected by the current sound worth and paying capacity of the borrower. Although the primary source of repayment for a Substandard is not currently 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 to a point where collection or liquidation in full, on the basis of currently existing facts, conditions and values, is highly questionable and improbable. Doubtful loans are required to be placed on non-accrual status and are assigned specific loss exposure. The following tables present the Company’s recorded investment in criticized loans by class and credit quality indicator based on the most recent analysis performed as of the dates indicated: As of December 31, 2019 Other Assets Especially Mentioned Substandard Doubtful Total Criticized Loans Real estate: Commercial $ 84.7 $ 97.3 $ 0.8 $ 182.8 Construction: Land acquisition & development 3.8 1.9 1.0 6.7 Residential 0.9 2.2 — 3.1 Commercial 1.7 1.5 — 3.2 Total construction loans 6.4 5.6 1.0 13.0 Residential 2.6 7.8 0.3 10.7 Agricultural 14.3 26.6 — 40.9 Total real estate loans 108.0 137.3 2.1 247.4 Consumer: Indirect consumer 0.2 2.9 — 3.1 Direct consumer 0.4 0.8 0.1 1.3 Total consumer loans 0.6 3.7 0.1 4.4 Commercial 40.4 60.3 3.6 104.3 Agricultural 8.5 22.7 0.1 31.3 Total $ 157.5 $ 224.0 $ 5.9 $ 387.4 As of December 31, 2018 Other Assets Especially Mentioned Substandard Doubtful Total Criticized Loans Real estate: Commercial $ 102.5 $ 87.4 $ 2.9 $ 192.8 Construction: Land acquisition & development 5.0 7.0 3.3 15.3 Residential 2.8 2.0 0.4 5.2 Commercial 1.7 3.9 — 5.6 Total construction loans 9.5 12.9 3.7 26.1 Residential 3.0 10.8 0.7 14.5 Agricultural 9.0 24.0 0.1 33.1 Total real estate loans 124.0 135.1 7.4 266.5 Consumer: Indirect consumer 0.7 2.1 0.1 2.9 Direct consumer 0.3 0.8 0.1 1.2 Total consumer loans 1.0 2.9 0.2 4.1 Commercial 39.4 45.8 11.8 97.0 Agricultural 14.4 17.8 1.5 33.7 Total $ 178.8 $ 201.6 $ 20.9 $ 401.3 The Company maintains a credit review function, which is independent of the credit approval process, to assess assigned internal risk classifications and monitor compliance with internal lending policies and procedures. Written action plans with firm target dates for resolution of identified problems are maintained and reviewed on a quarterly basis for all categories of criticized loans.

Allowance for Loan Losses

Allowance for Loan Losses	12 Months Ended
	Dec. 31, 2019
Receivables [Abstract]
Allowance for Loan Losses	ALLOWANCE FOR LOAN LOSSES The following tables present a summary of changes in the allowance for loan losses by portfolio segment: 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 expense (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 6,194.0 1,045.2 1,354.0 272.8 — 8,866.0 Total loans held for investment $ 6,235.1 $ 1,045.2 $ 1,371.3 $ 279.1 $ — $ 8,930.7 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 expense (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 5,791.7 1,070.2 1,290.4 251.7 1.6 8,405.6 Total loans held for investment $ 5,833.5 $ 1,070.2 $ 1,310.3 $ 254.8 $ 1.6 $ 8,470.4 Year Ended December 31, 2017 Real Estate Consumer Commercial Agriculture Other Total Allowance for loan losses: Beginning balance $ 28.6 $ 7.7 $ 38.1 $ 1.8 $ — $ 76.2 Provision charged (credited) to operating expense 6.0 8.1 (2.9 ) (0.2 ) — 11.0 Less loans charged-off (4.3 ) (11.3 ) (6.8 ) (0.4 ) — (22.8 ) Add back recoveries of loans previously charged-off 1.4 4.2 2.1 — — 7.7 Ending balance $ 31.7 $ 8.7 $ 30.5 $ 1.2 $ — $ 72.1 Individually evaluated for impairment $ 6.2 $ — $ 4.4 $ 0.2 $ — $ 10.8 Collectively evaluated for impairment 25.5 8.7 26.1 1.0 — 61.3 Ending balance $ 31.7 $ 8.7 $ 30.5 $ 1.2 $ — $ 72.1 Total loans: Individually evaluated for impairment $ 58.3 $ — $ 23.8 $ 1.1 $ — $ 83.2 Collectively evaluated for impairment 5,118.5 1,034.4 1,191.6 135.1 4.9 7,484.5 Total loans held for investment $ 5,176.8 $ 1,034.4 $ 1,215.4 $ 136.2 $ 4.9 $ 7,567.7 The Company performs a quarterly assessment of the adequacy of its allowance for loan losses in accordance with GAAP. The methodology used to assess the adequacy is consistently applied to the Company’s loan portfolio and consists of three elements: (1) specific valuation allowances based on probable losses on impaired loans; (2) historical valuation allowances based on loan loss experience for similar loans with similar characteristics and trends; and (3) general valuation allowances determined based on changes in the nature of the loan portfolio, overall portfolio quality, industry concentrations, delinquency trends, general economic conditions and other qualitative risk factors both internal and external to the Company. Specific allowances are established for loans where management has determined that probability of a loss exists by analyzing the borrower’s ability to repay amounts owed, collateral deficiencies and any relevant qualitative or economic factors impacting the loan. Historical valuation allowances are determined by applying percentage loss factors to the credit exposures from outstanding loans. For commercial, agricultural and real estate loans, loss factors are applied based on the internal risk classifications of these loans. For consumer loans, loss factors are applied on a portfolio basis. For commercial, agriculture and real estate loans, loss factor percentages are based on a migration analysis of our historical loss experience, designed to account for credit deterioration. For consumer loans, the loss factor percentages are based on a three-year loss history for the 2018 and 2019 periods and on a one-year loss history for the 2017 comparable periods. The loan loss rates for 2018 and 2019 also incorporate the available loss history data from BOTC prior to the merger date to represent a consolidated institutional loss rate for both originated and acquired portfolios. General valuation allowances are determined by evaluating, on a quarterly basis, changes in the nature and volume of the loan portfolio, overall portfolio quality, industry concentrations, current economic and regulatory conditions and the estimated impact of these factors on historical loss rates. An allowance for loan losses is established for loans acquired deemed credit impaired and for which the Company projects a decrease in the expected cash flows in periods subsequent to the acquisition of such loans. As of December 31, 2019 and 2018 , the Company’s allowance for loan losses included $1.0 million and $0.8 million , respectively, related to loans acquired credit impaired.

Premises and Equipment

Premises and Equipment	12 Months Ended
	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Premises and Equipment	PREMISES AND EQUIPMENT Premises and equipment and related accumulated depreciation are as follows: December 31, 2019 2018 Land $ 50.8 $ 49.9 Buildings and improvements 330.1 268.1 Furniture and equipment 94.2 104.0 Total premises and equipment 475.1 422.0 Less accumulated depreciation (169.1 ) (176.8 ) Premises and equipment, net $ 306.0 $ 245.2 Depreciation expense was $23.3 million , $16.2 million , and $14.8 million for the years ended December 31, 2019 , 2018 , and 2017 , 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, 2019
Investments, All Other Investments [Abstract]
Company-Owned Life Insurance	COMPANY-OWNED LIFE INSURANCE Company-owned life insurance consists of the following: December 31, 2019 2018 Key executive, principal shareholder $ 4.4 $ 4.7 Key executive split dollar 7.0 9.4 Group life 282.4 261.0 Total $ 293.8 $ 275.1 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 $4.4 million and $4.7 million at December 31, 2019 and 2018 , 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 and $9.4 million at December 31, 2019 and 2018 , 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. The Company has group life insurance policies covering selected officers of FIB. The net cash surrender value of these policies was $282.4 million and $261.0 million at December 31, 2019 and 2018

Other Real Estate Owned

Other Real Estate Owned	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 2017 Balance at beginning of year $ 14.4 $ 10.1 $ 10.0 Acquisitions 2.4 0.6 1.2 Additions 14.1 12.1 5.4 Capitalized improvements 0.3 — — Valuation adjustments (0.9 ) (0.1 ) (0.4 ) Dispositions (21.8 ) (8.3 ) (6.1 ) Balance at end of year $ 8.5 $ 14.4 $ 10.1 Write-downs of $0.9 million , $0.1 million , and $0.4 million during 2019 , 2018 , and 2017 , 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.

Derivatives and Hedging Activit

Derivatives and Hedging Activities	12 Months Ended
	Dec. 31, 2019
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivatives and Hedging Activities	DERIVATIVES AND HEDGING ACTIVITIES 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, 2019 December 31, 2018 Notional Amount Estimated Fair Value Notional Amount Estimated Fair Value Derivative Assets (included in other assets on the consolidated balance sheets) Non-hedging interest rate derivatives: Interest rate swap contracts $ 503.2 $ 21.9 $ 403.3 $ 8.8 Interest rate lock commitments 67.8 1.3 51.0 1.3 Total derivative assets $ 571.0 $ 23.2 $ 454.3 $ 10.1 Derivative Liabilities (included in accounts payable and accrued expenses on the consolidated balance sheets) Non-hedging interest rate derivatives: Interest rate swap contracts $ 503.2 $ 21.9 $ 403.3 $ 8.8 Forward loan sales contracts 128.0 0.3 64.6 0.6 Total derivative liabilities $ 631.2 $ 22.2 $ 467.9 $ 9.4 The Company did no t hold derivatives designated as hedging instruments at December 31, 2019 and 2018 . 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 December 31, 2019 and December 31, 2018 : 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 December 31, 2018 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 $ 8.8 $ — $ 8.8 $ 2.7 $ 2.4 $ 3.7 Mortgage related derivatives 1.3 — 1.3 — — 1.3 Total derivatives 10.1 — 10.1 2.7 2.4 5.0 Total assets $ 10.1 $ — $ 10.1 $ 2.7 $ 2.4 $ 5.0 Financial Liabilities Interest rate swap contracts $ 8.8 $ — $ 8.8 $ 2.7 $ 4.1 $ 2.0 Mortgage related derivatives 0.6 — 0.6 — — 0.6 Total derivatives 9.4 — 9.4 2.7 4.1 2.6 Repurchase agreements 712.4 — 712.4 — 712.4 — Total liabilities $ 721.8 $ — $ 721.8 $ 2.7 $ 716.5 $ 2.6 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, 2019 2018 2017 Derivatives designated as hedges: Amount of loss recognized in other comprehensive income (effective portion) $ — $ — $ (1.1 ) Reclassification adjustment for derivative net (gains) losses included in income — — 1.1 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 2.5 1.3 0.8 Amount of net gains (losses) recognized in mortgage banking revenues $ 0.3 $ (0.5 ) $ (1.7 )

Mortgage Servicing Rights

Mortgage Servicing Rights	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 2017 Balance at beginning of year $ 27.7 $ 24.8 $ 18.7 Acquisitions of mortgage servicing rights — — 3.5 Originations of mortgage servicing rights 7.3 6.0 5.6 Amortization expense (4.4 ) (3.1 ) (3.0 ) Balance at end of year 30.6 27.7 24.8 Less valuation reserve (0.4 ) — — Balance at end of year, net of valuation reserve $ 30.2 $ 27.7 $ 24.8 Principal balance of serviced loans underlying mortgage servicing rights $ 3,710.1 $ 3,698.2 $ 3,636.7 Mortgage servicing rights as a percentage of serviced loans 0.81 % 0.75 % 0.68 % At December 31, 2019 , the estimated fair value and weighted average remaining life of the Company’s mortgage servicing rights were $34.8 million and 6.0 years, respectively. The fair value of mortgage servicing rights was determined using discount rates ranging from 9.3% to 11.0% and monthly prepayment speeds ranging from 0.6% to 1.5% depending upon the risk characteristics of the underlying loans. At December 31, 2018 , the estimated fair value and weighted average remaining life of the Company’s mortgage servicing rights were $42.4 million and 8.0 years, respectively. The fair value of mortgage servicing rights was determined using discount rates ranging from 10.4% to 12.1% and monthly prepayment speeds ranging from 0.4% to 1.5% depending upon the risk characteristics of the underlying loans. There were no material impairments reversed in 2019 , 2018 and 2017 , respectively. No permanent impairment was recorded in 2019 , 2018 , or 2017

Deposits

Deposits	12 Months Ended
	Dec. 31, 2019
Deposits [Abstract]
Deposits	DEPOSITS Deposits are summarized as follows: December 31, 2019 2018 Non-interest bearing demand $ 3,426.5 $ 3,158.3 Interest bearing: Demand 3,195.4 2,957.5 Savings 3,591.6 3,247.9 Time, $100 and over 651.1 547.6 Time, other 798.9 769.4 Total interest bearing 8,237.0 7,522.4 Total deposits $ 11,663.5 $ 10,680.7 Other time deposits include $2.9 million and $24.1 million brokered deposits as of December 31, 2019 and 2018 , respectively, and deposits obtained through the Company’s participation in the Certificate of Deposit Account Registry Service (“CDARS”). CDARS deposits totaled $117.7 million and $87.1 million as of December 31, 2019 and 2018 , respectively. As of December 31, 2019 and 2018 , the Company had time deposits of $278.4 million and $221.0 million , respectively, that met or exceeded the FDIC insurance limit of $250,000 . Maturities of time deposits at December 31, 2019 are as follows: Time, $100 and Over Total Time Due within 3 months or less $ 101.6 $ 323.6 Due after 3 months and within 6 months 140.8 301.4 Due after 6 months and within 12 months 273.7 515.9 Due within 2021 104.0 220.4 Due within 2022 23.6 64.0 Due within 2023 3.5 13.9 Due within 2024 and thereafter 3.9 10.8 Total $ 651.1 $ 1,450.0 Interest expense on time deposits of $100 and over was $11.5 million , $5.0 million , and $3.5 million for the years ended December 31, 2019 , 2018 , and 2017 , respectively.

Long-Term Debt and Other Borrow

Long-Term Debt and Other Borrowed Funds	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 Subsidiaries: 8.00% capital lease obligation with term ending October 25, 2029 $ 1.2 $ 1.3 6.24% note payable maturing September 6, 2032, principal due at maturity, interest payable monthly — 1.8 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 $ 13.9 $ 15.8 Maturities of long-term debt at December 31, 2019 are as follows: 2020 $ 0.1 2021 0.1 2022 5.1 2023 0.1 2024 0.1 Thereafter 8.4 Total $ 13.9 The Company has available lines of credit with the FHLB of approximately $1,616.0 million , subject to collateral availability. As of December 31, 2019 and 2018 , 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. The Company borrowed or assumed through acquisitions $12.6 million and $14.4 million as of December 31, 2019 and 2018 , respectively, related to New Market Tax Credits. During the third quarter of 2019, the Company redeemed the note payable maturing September 2032. The long-term debt obligations consists of fixed rate note payables with various interest rates from 1.00% to 6.24% 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, 2019 and 2018 , 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 $455.6 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, 2019
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 $84.3 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 as of December 31, Issuance Maturity Date 2019 2018 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 4.4 4.4 Total subordinated debentures held by subsidiary trusts $ 86.9 $ 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, 2019 , the interest rate on the Subordinated Debentures was 4.35% . 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, 2019 , the interest rate on the Subordinated Debentures was 4.64% . 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, 2019 , the interest rate on the Subordinated Debentures was 4.29% . 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, 2019 the interest rate on the Subordinated Debentures was 4.80% . 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, 2019 the interest rate on the Subordinated Debentures was 4.85% . 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, 2019 , the interest rate on the Subordinated Debentures was 4.85% . 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, 2019 the interest rate on the Subordinated Debentures was 3.66% . 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, 2019
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 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 . The Company had 38,169,575 shares of Class A common stock and 22,453,672 shares of Class B common stock outstanding as of December 31, 2018 . 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 . During 2018 , the Company issued 11,389 shares of its Class A common stock with an aggregate value of $0.5 million to directors for their service on the Company’s board of directors during 2018 . The aggregate value of the shares issued to directors of is included in stock-based compensation expense in the accompanying consolidated statements of changes in stockholders’ equity. During 2019 and 2018 , the Company did no t repurchase any shares of its Class A common stock other than stock repurchases which 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 June 11, 2019 , the company’s board of directors adopted a new stock repurchase program to replace the program that had been in place since 2015 and which had only 24,123 shares of Class A common stock remaining to be purchased thereunder. Under the new stock repurchase program, the Company may repurchase up to 2.5 million of its outstanding shares of Class A common stock. To date the Company has no t repurchased any shares under the current authorization. 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. On September 25, 2017, the Company filed a shelf registration statement on Form S-3, which was subsequently declared effective by the SEC. The registration statement permits us to offer and sell up to $ 250.0 million of our Class A common shares in one or more future public offerings. At the present time, we have no specific plans to offer any of the securities covered by the registration statement.

Earnings per Common Share

Earnings per Common Share	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 2017 Net income, basic and diluted $ 181.0 $ 160.2 $ 106.5 Weighted average common shares outstanding for basic earnings per share computation 63,645,029 57,778,857 51,429,366 Dilutive effects of stock-based compensation 239,839 438,266 473,843 Weighted average common shares outstanding for diluted earnings per common share computation 63,884,868 58,217,123 51,903,209 Basic earnings per common share $ 2.84 $ 2.77 $ 2.07 Diluted earnings per common share 2.83 2.75 2.05 The Company had 150 , 448 , and 83,635 unvested time restricted stock outstanding as of December 31, 2019 , 2018 , and 2017 respectively, that were not included in the computation of diluted earnings per common share because their effect would be anti-dilutive. The Company had 138,298 , 83,475 , and 113,874 shares of unvested restricted stock as of December 31, 2019 , 2018 , and 2017 , respectively, that were not included in the computation of diluted earnings per common share because performance conditions for vesting had not been met.

Regulatory Capital

Regulatory Capital	12 Months Ended
	Dec. 31, 2019
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, 2019 , the Company exceeded all capital adequacy requirements to which it is subject. As of December 31, 2019 , 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, 2019 and 2018 , the Bank met these requirements. On July 2, 2013, the Board of Governors of the Federal Reserve Bank issued a final rule implementing a revised regulatory capital framework for U.S. banks in accordance with the Basel III international accord and satisfying related mandates under the Dodd-Frank Wall Street Reform and Consumer Protection Act. The revised regulatory capital framework (the “Basel III Capital Rules”) substantially revised the risk-based capital requirements applicable to bank holding companies and depository institutions by defining the components of capital and addressing other issues affecting the numerator in banking institutions’ regulatory capital ratios, addressing risk weights and other issues affecting the denominator in banking institutions’ regulatory capital ratios and replacing the existing risk-weighting approach with a more risk-sensitive approach. The Basel III Capital Rules became effective for the Company on January 1, 2015, subject to a phase-in period for certain provisions. The capital conservation buffer required under Basel III began to phase in starting January 1, 2016 and became fully implemented on January 1, 2019. The Company’s actual capital amounts and ratios and selected minimum regulatory thresholds and prompt corrective action provisions as of December 31, 2019 and 2018 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) Amount Ratio Amount Ratio Amount Ratio Amount Ratio December 31, 2019 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 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) Amount Ratio Amount Ratio Amount Ratio Amount Ratio December 31, 2018 Total risk-based capital: Consolidated $ 1,285.0 12.99 % $ 791.2 8.00 % $ 976.6 9.88 % $ 989.0 10.00 % FIB 1,184.5 12.01 788.8 8.00 973.7 9.88 986.0 10.00 Tier 1 risk-based capital: Consolidated 1,212.0 12.26 593.4 6.00 778.8 7.88 791.2 8.00 FIB 1,111.6 11.27 591.6 6.00 776.5 7.88 788.8 8.00 Common equity tier 1 risk-based capital: Consolidated 1,127.8 11.40 445.0 4.50 630.5 6.38 642.8 6.50 FIB 1,111.6 11.27 443.7 4.50 628.6 6.38 640.9 6.50 Leverage capital ratio: Consolidated 1,212.0 9.47 511.9 4.00 511.9 4.00 639.9 5.00 FIB 1,111.6 8.97 495.9 4.00 495.9 4.00 619.8 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, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	COMMITMENTS AND CONTINGENCIES The Company had commitments under construction contracts of $2.1 million as of December 31, 2019 . 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 $4.8 million , $3.2 million , and $2.5 million , in 2019 , 2018 and 2017 , 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, 2019 , are as follows: Third Parties Related Entity Total For the year ending December 31: 2020 $ 5.0 $ 1.4 $ 6.4 2021 4.7 1.4 6.1 2022 4.4 1.2 5.6 2023 4.2 1.0 5.2 2024 4.1 1.0 5.1 Thereafter 20.5 1.6 22.1 Total $ 42.9 $ 7.6 $ 50.5 Residential mortgage loans sold to investors in the secondary market are sold with varying recourse provisions. Essentially all of 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; unmarketability; etc. 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 $0.9 million and $1.5 million of sold residential mortgage loans with recourse provisions still in effect as of December 31, 2019 and 2018 , 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, 2019 , 2018 and 2017 . 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 loan and lease 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, 2019
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. The Company evaluates each client’s creditworthiness on a case-by-case basis. The amount of collateral obtained is based on management’s credit evaluation of the client. Collateral held varies but may include accounts receivable, inventory, premises and equipment, and income-producing commercial properties. 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. Commitments generally have fixed expiration dates or other termination clauses and may require payment of a fee. Generally, commitments to extend credit are subject to annual renewal. Since many of the commitments are expected to expire without being drawn upon, the total commitment amounts do not necessarily represent future cash requirements. Commitments to extend credit to borrowers approximated $2,807.8 million at December 31, 2019 , which included $764.1 million on unused credit card lines and $1,212.7 million with commitment maturities beyond one year. Commitments to extend credit to borrowers approximated $2,620.4 million at December 31, 2018 , which included $726.7 million on unused credit card lines and $1,058.5 million with commitment maturities beyond one year. Standby letters of credit are conditional commitments issued by the Company to guarantee the performance of a client to a third party. Most commitments extend for no more than two years and are generally subject to annual renewal. 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. At December 31, 2019 and 2018 , the Company had outstanding stand-by letters of credit of $42.7 million and $46.7 million , respectively. 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.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Income Taxes	INCOME TAXES Income tax expense consists of the following: Year ended December 31, 2019 2018 2017 Current: Federal $ 40.5 $ 23.1 $ 24.3 State 8.2 7.2 5.0 Total current 48.7 30.3 29.3 Deferred: Federal 3.7 12.3 18.4 State 1.7 3.5 2.5 Total deferred 5.4 15.8 20.9 Total income tax expense $ 54.1 $ 46.1 $ 50.2 Total income tax provision differs from the amount of income tax determined by applying the statutory federal income tax rate of 21% for 2019 and 2018 and 35% for 2017 , respectively, to income before income taxes due to the following: Year ended December 31, 2019 2018 2017 Tax expense at the statutory tax rate $ 49.4 $ 43.3 $ 54.9 Increase (decrease) in tax resulting from: Tax-exempt income (2.8 ) (2.8 ) (4.5 ) State income tax, net of federal income tax benefit 9.9 8.5 4.9 Benefit of stock-based compensation plans (1.2 ) (1.1 ) (2.6 ) Federal tax credits (2.0 ) (2.6 ) (2.5 ) Benefit due to enactment of federal tax reform — — (2.2 ) Other, net 0.8 0.8 2.2 Tax expense at effective tax rate $ 54.1 $ 46.1 $ 50.2 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, 2019 2018 Deferred tax assets: Loans, principally due to allowance for loan losses $ 18.1 $ 18.4 Loan discount 7.2 8.3 Investment securities, unrealized losses — 8.9 Deferred compensation 18.1 17.0 Non-performing loan interest 1.0 1.2 Other real estate owned write-downs and carrying costs 0.1 0.3 Tax credit carryforwards (1) 0.2 0.1 Net operating loss carryforwards (2) 2.6 3.9 Lease liabilities 9.9 — Other 0.9 0.1 Deferred tax assets 58.1 58.2 Deferred tax liabilities: Fixed assets, principally differences in bases and depreciation (8.8 ) (6.9 ) Deferred loan costs (2.8 ) (2.6 ) Investment securities, unrealized gains (3.9 ) — Investment in joint venture partnership, principally due to differences in depreciation of partnership assets (0.7 ) (0.8 ) Right of use assets (9.7 ) — Prepaid amounts (0.5 ) (0.6 ) Government agency stock dividends (1.2 ) (1.5 ) Goodwill and core deposit intangibles (49.2 ) (44.2 ) Mortgage servicing rights (7.2 ) (6.4 ) Other (0.8 ) (3.8 ) Deferred tax liabilities (84.8 ) (66.8 ) Net deferred tax assets (liabilities) $ (26.7 ) $ (8.6 ) (1) Based on filed tax returns and amounts expected to be reported in current year tax returns ( December 31, 2019 ), we had remaining federal tax credit carryforwards of $0.1 million from acquired companies. The remaining federal tax credits were primarily generated from AMT tax credit carryforwards, and their use is subject to annual limitations. (2) As of December 31, 2019 , we had remaining federal net operating loss carryforwards of $3.8 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 2029 and ending in 2037 and the state net operating losses will expire beginning in 2020 and ending in 2034. The use of these carryforwards is subject to annual limitations. The Company had current net income tax receivables of $6.1 million and $2.0 million at December 31, 2019 and 2018 , respectively.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2019
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, 2019 , there were 1,492,762 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 2019 or 2018 . 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 2019 or 2018 . 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, 2019 Number of Shares Weighted-Average Exercise Price Weighted-Average Remaining Contract Life Outstanding options, beginning of year 428,176 $ 15.61 Granted — — Exercised (191,193 ) 15.76 Forfeited (15,786 ) 17.05 Expired — — Outstanding options, end of year 221,197 $ 15.33 1.03 Outstanding options exercisable, end of year 221,197 $ 15.33 1.03 The total intrinsic value of fully-vested stock options outstanding as of December 31, 2019 was $5.8 million . The total intrinsic value of options exercised was $4.9 million , $4.9 million and $6.3 million during the years ended December 31, 2019 , 2018 and 2017 , respectively. The actual tax benefit realized for the tax deduction from option exercises totaled $0.9 million , $0.9 million and $2.0 million for the years ended December 31, 2019 , 2018 and 2017 , respectively. The Company received cash of $1.0 million , $1.8 million and $2.4 million from stock option exercises during the years ended December 31, 2019 , 2018 and 2017 , respectively. The Company redeemed common stock with aggregate values of $2.0 million , $1.6 million and $2.8 million tendered in payment for stock option exercises during the years ended December 31, 2019 , 2018 and 2017 , 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 $8.0 million , $5.6 million and $3.3 million was included in benefits on the Company’s consolidated statements of income for the years ended December 31, 2019 , 2018 and 2017 , respectively. Related income tax benefits recognized for the years ended December 31, 2019 , 2018 and 2017 were $0.4 million , $0.2 million and $0.6 million , respectively. The following table presents information regarding the Company’s restricted stock: As of December 31, 2019 Number of Shares Weighted-Average Measurement Date Fair Value Restricted stock, beginning of year 380,584 $ 37.46 Granted 230,922 41.29 Vested (202,286 ) 33.24 Forfeited (46,198 ) 41.18 Canceled — — Restricted stock, end of year 363,022 $ 41.47 During 2019 , the Company issued 230,922 restricted common shares. The 2019 restricted share awards included 18,335 additional shares related to the 2016 performance restricted stock grants and 72,926 performance restricted shares, of which 36,463 vest in varying percentages upon achievement of defined return on equity performance goals, and 36,463 vest in varying percentages upon achievement of defined total return to shareholder goals. Vesting of the 2019 performance restricted shares is also contingent on employment as of December 31, 2021 . Additionally, 139,661 time-restricted shares were issued during 2019 that vest one-third on each annual anniversary of the grant date through February 15, 2022 , contingent on continued employment through the vesting date. As of December 31, 2019 , there was $8.1 million of unrecognized compensation cost related to non-vested, restricted stock awards expected to be recognized over a period of 1.10 years .

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2019
Compensation Related Costs [Abstract]
Employee Benefit Plans	EMPLOYEE BENEFIT PLANS Profit Sharing Plan. The Company has a noncontributory profit sharing plan. All employees, other than temporary employees, working 20 hours or more per week are eligible to participate in the profit sharing plan. The Company’s Board of Directors authorizes all contributions to the profit sharing plan. Participants become 100% vested upon the completion of two years of vesting service. Accrued contribution expense for this plan of $2.1 million , $2.4 million and $1.6 million in 2019 , 2018 and 2017 , 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 contributes 100% of the first 5% of the participating employee’s eligible compensation. Contribution expense for this plan of $7.0 million , $6.3 million and $5.5 million in 2019 , 2018 and 2017 , 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 Company recorded a $2.8 million gain in conjunction with the Plan amendment, which was recorded in other comprehensive income and is being amortized as a reduction in net periodic benefit cost over the weighted average remaining service period of active employees expected to receive post-retirement healthcare benefits under the Plan of approximately four years . The Plan amendment triggered a curtailment, which immediately reduced the Company’s accumulated post-retirement benefit obligation and net periodic benefit cost by $2.8 million and $0.3 million , respectively. The Plan’s unfunded benefit obligation of $0.4 million and $0.5 million as of December 31, 2019 and 2018 , respectively, is included in accounts payable and accrued expenses in the Company’s consolidated balance sheets. Net periodic benefit costs of $0.7 million , $0.8 million and $0.4 million for the years ended December 31, 2019 , 2018 and 2017 , respectively, are included in employee benefits expense in the Company’s consolidated statements of income. Weighted average actuarial assumptions used to determine the post-retirement benefit obligation at December 31, 2019 , and the net periodic benefit costs for the year then ended, included a discount rate of 3.6% and a 5.5% annual increase in the per capita cost of covered healthcare benefits. Weighted average actuarial assumptions used to determine the post-retirement benefit obligation at December 31, 2018 , and the net periodic benefit costs for the year then ended, included a discount rate of 3.6% and a 6.0% annual increase in the per capita cost of covered healthcare benefits. The estimated effect of a one percent increase or a one percent decrease in the assumed healthcare cost trend rate would not significantly impact the service and interest cost components of the net periodic benefit cost or the accumulated post-retirement benefit obligation. Future benefit payments are expected to be $0.12 million , $0.09 million , $0.06 million , $0.05 million , $0.04 million and $0.08 million for 2020 , 2021 , 2022 , 2023 , 2024 , and 2028 through 2029 , respectively. At December 31, 2019 , the Company had accumulated other comprehensive gain related to the plan of $0.5 million , or $0.4 million net of related income tax benefit, comprised primarily of an unamortized transition asset of $0.7 million

Other Comprehensive Income

Other Comprehensive Income	12 Months Ended
	Dec. 31, 2019
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, 2019 Before Tax Amount Tax Expense (Benefit) Net of Tax Amount Investment securities available-for sale: Change in net unrealized gain during period $ 54.9 $ 14.1 $ 40.8 Reclassification adjustment for net gain 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 gains (0.6 ) (0.1 ) (0.5 ) Total other comprehensive loss $ (12.8 ) $ (3.3 ) $ (9.5 ) Year Ended December 31, 2017 Before Tax Amount Tax Expense (Benefit) Net of Tax Amount Investment securities available-for sale: Change in net unrealized loss during period $ (6.5 ) $ (2.7 ) $ (3.8 ) Reclassification adjustment for net gain included in net income (0.7 ) (0.3 ) (0.4 ) Change in unamortized loss on available-for-sale securities transferred into held-to-maturity 1.9 0.7 1.2 Change in net unrealized loss on derivatives (1.1 ) (0.4 ) (0.7 ) Reclassification adjustment for derivative net loss included in income 1.1 0.4 0.7 Defined benefits post-retirement benefit plan: Change in net actuarial loss (1.3 ) (0.5 ) (0.8 ) Total other comprehensive loss $ (6.6 ) $ (2.8 ) $ (3.8 ) The components of accumulated other comprehensive income (loss), net of income taxes, are as follows: Year ended December 31, 2019 2018 Net unrealized gain (loss) on investment securities available-for-sale $ 10.6 $ (25.5 ) Net actuarial gain on defined benefit post-retirement benefit plans 0.4 0.9 Net accumulated other comprehensive income (loss) $ 11.0 $ (24.6 )

Condensed Financial Information

Condensed Financial Information (Parent Company Only)	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 Condensed balance sheets: Cash and cash equivalents $ 147.1 $ 72.9 Investment in bank subsidiary 1,899.3 1,648.9 Advances to subsidiaries, net 48.9 46.0 Other assets 61.7 59.0 Total assets $ 2,157.0 $ 1,826.8 Other liabilities $ 56.2 $ 46.0 Subordinated debentures held by subsidiary trusts 86.9 86.9 Total liabilities 143.1 132.9 Stockholders’ equity 2,013.9 1,693.9 Total liabilities and stockholders’ equity $ 2,157.0 $ 1,826.8 Years Ended December 31, 2019 2018 2017 Condensed statements of income: Dividends from subsidiaries $ 178.0 $ 148.5 $ 150.0 Other interest income 0.3 0.1 0.1 Other income, primarily management fees from subsidiaries 25.9 17.0 18.0 Total income 204.2 165.6 168.1 Salaries and benefits 34.2 25.3 21.8 Interest expense 4.7 4.5 4.7 Acquisition expenses 17.0 8.1 25.3 Other operating expenses, net 14.8 14.5 13.0 Total expenses 70.7 52.4 64.8 Earnings before income tax benefit 133.5 113.2 103.3 Income tax benefit (11.9 ) (9.3 ) (14.2 ) Income before undistributed earnings of subsidiaries 145.4 122.5 117.5 Undistributed earnings of subsidiaries 35.6 37.7 (11.0 ) Net income $ 181.0 $ 160.2 $ 106.5 Years Ended December 31, 2019 2018 2017 Condensed statements of cash flows: Cash flows from operating activities: Net income $ 181.0 $ 160.2 $ 106.5 Adjustments to reconcile net income to cash provided by operating activities: Undistributed earnings of subsidiaries (35.6 ) (37.7 ) 11.0 Stock-based compensation expense 8.0 5.6 3.9 Other, net 8.1 16.4 14.7 Net cash provided by operating activities 161.5 144.5 136.1 Cash flows from investing activities: Capital distributions from nonbank subsidiaries — — 18.0 Acquisition of intangible assets — — (28.0 ) Acquisition of bank holding company, net of cash and cash equivalents received — (14.7 ) (128.3 ) Investment in subsidiary — — (18.0 ) Net cash used in investing activities — (14.7 ) (156.3 ) Cash flows from financing activities: Net (decrease) increase in advances from subsidiaries (6.6 ) (9.9 ) (28.4 ) Repayment of long-term debt — (26.0 ) — Proceeds from issuance of common stock, net of stock issuance costs 1.0 1.8 2.4 Purchase and retirement of common stock (2.5 ) (1.0 ) (1.3 ) Dividends paid to common stockholders (79.2 ) (64.1 ) (48.6 ) Net cash used in financing activities (87.3 ) (99.2 ) (75.9 ) Net change in cash and cash equivalents 74.2 30.6 (96.1 ) Cash and cash equivalents, beginning of year 72.9 42.3 138.4 Cash and cash equivalents, end of year $ 147.1 $ 72.9 $ 42.3 There was $176.1 million , $173.3 million , and $386.0 million in 2019 , 2018 , and 2017 , respectively, of noncash financing activities for the issuance of common stock were related to the CMYF and IIBK acquisitions in 2019 , INB acquisition in 2018 , and BOTC acquisition in 2017 .

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2019
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, 2019 and 2018 . 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, 2019 and 2018 . 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. The Company has documented and evaluated the pricing methodologies used by the vendors and maintains internal processes that regularly test valuations. These internal processes include obtaining and reviewing available reports on internal controls, evaluating the prices for reasonableness given market changes, obtaining and evaluating the inputs used in the model for a sample of securities, investigating anomalies and confirming determinations through discussions with the vendor. 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 3 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, 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 — Fair Value Measurements at Reporting Date Using As of December 31, 2018 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 $ 2.6 $ — $ 2.6 $ — Obligations of U.S. government agencies 559.2 — 559.2 — U.S. agency mortgage-backed securities & collateralized mortgage obligations 1,544.8 — 1,544.8 — Private mortgage-backed securities 70.2 — 70.2 — Corporate securities 91.9 — 91.9 — Other investments 2.0 — 2.0 — Loans held for sale 33.3 — 33.3 — Derivative assets: Interest rate swap contracts 8.8 — 8.8 — Interest rate lock commitments 1.3 — 1.3 — Derivative liabilities: Interest rate swap contracts 8.8 — 8.8 — Forward loan sales contracts 0.6 — 0.6 — Deferred compensation plan assets 12.1 — 12.1 — Deferred compensation plan liabilities 12.1 — 12.1 — 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 impairment. 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, 2019 Total Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Gains (Losses) 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 ) Fair Value Measurements at Reporting Date Using As of December 31, 2018 Total Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Gains (Losses) Impaired loans $ 24.1 $ — $ — $ 24.1 $ (12.2 ) Other real estate owned 0.6 — — 0.6 (0.6 ) Long-lived assets to be disposed of by sale 4.9 — — 4.9 (0.5 ) Impaired Loans. Collateralized impaired loans are reported at the fair value of the underlying collateral if repayment is expected solely from collateral. The impaired loans are reported at fair value through specific valuation allowance allocations. In addition, when it is determined that the fair value of an impaired 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 loan losses. Collateral values are estimated using independent appraisals and management estimates of current market conditions. 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 . As of December 31, 2018 , certain impaired loans with a carrying value of $36.3 million were reduced by specific valuation allowance allocations of $6.8 million and partial loan charge-offs of $5.4 million resulting in a reported fair value of $24.1 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 loan 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, 2019 , the Company had long-lived assets to be disposed of by sale with carrying values aggregating $6.4 million , which was reduced by write-downs of $0.2 million charged to other expense, resulting in a reported fair value of $6.2 million . As of December 31, 2018 , the Company had long-lived assets to be disposed of by sale with carrying values of $5.4 million , had $0.5 million write-downs, resulting in a reported fair value of $4.9 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, 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%) As of December 31, 2018 Fair Value Valuation Technique Unobservable Inputs Range (Weighted Average) Impaired loans $ 24.1 Appraisal Appraisal adjustment 0% - 26% (13%) Other real estate owned 0.6 Appraisal Appraisal adjustment 8% - 96% (39%) Long-lived assets to be disposed of by sale 4.9 Appraisal Appraisal adjustment 0% - 43% (10%) 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. 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, 2019 Carrying Amount Estimated Fair Value Quoted Prices in Significant Other Significant 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 $ — Fair Value Measurements at Reporting Date Using As of December 31, 2018 Carrying Amount Estimated Fair Value Quoted Prices in Significant Other Significant Financial assets: Cash and cash equivalents $ 822.0 $ 822.0 $ 822.0 $ — $ — Investment debt securities available-for-sale 2,270.7 2,270.7 — 2,270.7 — Investment debt securities held-to-maturity 406.8 400.7 — 400.7 — Accrued interest receivable 44.9 44.9 — 44.9 — Mortgage servicing rights, net 27.7 42.4 — 42.4 — Loans held for sale 33.3 33.3 — 33.3 — Net loans held for investment 8,397.4 8,439.7 — 8,415.6 24.1 Derivative assets 10.1 10.1 — 10.1 — Deferred compensation plan assets 12.1 12.1 — 12.1 — Total financial assets $ 12,025.0 $ 12,075.9 $ 822.0 $ 11,229.8 $ 24.1 Financial liabilities: Total deposits, excluding time deposits $ 9,363.7 $ 9,363.7 $ 9,363.7 $ — $ — Time deposits 1,317.0 1,299.0 — 1,299.0 — Securities sold under repurchase agreements 712.4 712.4 — 712.4 — Accrued interest payable 7.8 7.8 — 7.8 — Long-term debt 15.8 13.0 — 13.0 — Subordinated debentures held by subsidiary trusts 86.9 84.9 — 84.9 — Derivative liabilities 9.4 9.4 — 9.4 — Deferred compensation plan liabilities 12.1 12.1 — 12.1 — Total financial liabilities $ 11,525.1 $ 11,502.3 $ 9,363.7 $ 2,138.6 $ —

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2019
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 $40.3 million and $43.2 million at December 31, 2019 and 2018 , respectively. During 2019 , new loans and advances on existing loans of $16.5 million were funded and loan repayments totaled $18.3 million . In addition, $1.1 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. The Company leases an aircraft from an entity wholly-owned by the chairman of the Board of Directors of the Company. Under the terms of the lease, we pay a fee for each flight hour plus certain third party operating expenses related to the aircraft. During 2019 , 2018 and 2017 , the Company paid total fees and operating expenses of $22 thousand , $53 thousand and $45 thousand respectively, for its use of the aircraft. In addition, we lease a portion or our hanger and provide pilot services to the related entity. During 2019 , 2018 and 2017 , the Company received payments from the related entity of $30 thousand , $25 thousand and $17 thousand , respectively, for hangar use, pilot fees, and reimbursement of certain third party operating expenses related to the chairman’s personal use of the aircraft. The Company purchases services from an entity which includes certain members of the Company’s control group. Services provided for the Company’s benefit include shareholder education and communication, strategic enterprise planning and corporate governance consultation. During 2019 , 2018 and 2017 , the Company paid $85 thousand , $80 thousand and $73 thousand

Authoritative Accounting Guidan

Authoritative Accounting Guidance	12 Months Ended
	Dec. 31, 2019
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Recent Authoritative Accounting Guidance	RECENT AUTHORITATIVE ACCOUNTING GUIDANCE ASU 2016-02, “Leases (Topic 842).” In February 2016, the FASB issued ASU 2016-02, “Leases (Topic 842).” Under the new guidance, lessees will be required to recognize a lease liability and a right of use asset for all leases (with the exception of short-term leases) at the commencement date of the lease and disclose key information about leasing arrangements. Accounting by lessors is largely unchanged. In July 2018, the FASB issued ASU 2018-11 to provide entities with an additional (and optional) transition method to adopt the new leases standard. Under this new transition method, an entity initially applies the new leases standard at the adoption date and recognizes a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. Additionally, in December 2018, the FASB issued ASU 2018-20, “Leases (Topic 842) - Narrow-Scope Improvements for Lessors,” which provides for certain policy elections and changes lessor accounting for sales and similar taxes and certain lessor costs. Upon adoption of ASU 2016-02, ASU 2018-11 and ASU 2018-20 on January 1, 2019, the Company recognized right-of-use assets and related lease liabilities totaling $39.6 million and $39.6 million, respectively, with an immaterial impact on its consolidated results of operations and liquidity. The Company elected to apply certain practical expedients provided under ASU 2016-02 whereby we did not reassess (i) whether any expired or existing contracts are or contain leases, (ii) the lease classification for any expired or existing leases, and (iii) initial direct costs for any existing leases. The Company elected the hindsight practical expedient to determine the lease term for existing leases. We also did not apply the recognition requirements of ASU 2016-02 to any short-term leases (as defined by related accounting guidance). Lease and non-lease components are accounted for separately as the amounts are readily determinable under our lease contracts. 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. The amendments will be applied through a cumulative-effect adjustment to retained earnings as of the beginning of the first reporting period. A prospective transition approach is required for debt securities for which other-than-temporary impairment was recognized before the effective date. Amounts previously recognized in accumulated other comprehensive income as of the date of adoption that relate to improvement in cash flows expected to be collected will continue to be accreted into income over the remaining life of the asset. Recoveries of amounts previously written off relating to improvements in cash flows after the date of adoption will be recorded in earnings when received. We are currently evaluating the potential impact of ASU 2016-13 on our financial statements, which will be effective on January 1, 2020. We have formed a cross-functional working group comprised of individuals from various functional areas including credit, risk management, finance, and information technology, among others. We are currently working through our implementation plan which includes assessment and documentation of processes, internal controls and data sources; model development and documentation; and system configuration, among other things. We are in the process of implementing a third-party vendor solution. The adoption of ASU 2016-13 could result in an increase in the allowance for loan losses as a result of changing from an “incurred loss” model, which encompasses allowances for current known and inherent losses within the portfolio, to an “expected loss” model, which encompasses allowances for losses expected to be incurred over the life of the portfolio. The adoption will also necessitate that we establish an allowance for expected credit losses for certain debt securities and other financial assets. The impact of the adoption of ASU 2016-13 is influenced by the composition, characteristics, and quality of our loan and securities portfolios as well as the prevailing economic conditions and forecasts as of the adoption date and is currently expected to result in a 30 to 45 percent increase in our allowance for loan losses, however, this range is subject to change as we complete our related implementation and validation efforts. ASU 2017-08, “Receivables – Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities.” The amendments in ASU 2017-08 shorten the amortization period for the premium on certain purchased callable debt securities to the earliest call date. The new guidance does not change the accounting for purchased callable debt securities held at a discount; the discount continues to be amortized to maturity. ASU No. 2017-08 is effective for interim and annual reporting periods beginning after December 15, 2018. As the Company amortized premiums on callable debt securities to the earliest call date, the amendments in ASU 2017-08 became effective for the Company on January 1, 2019, and did not have a significant impact on the Company’s consolidated financial statements, results of operations, or liquidity. ASU 2017-12, “Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities.” In August 2017, the FASB issued ASU 2017-12, Derivatives and Hedging: Targeted Improvements to Accounting for Hedging Activities (ASU 2017-12). The purpose of this updated guidance is to better align a company’s financial reporting for hedging activities with the economic objectives of those activities. In addition, this ASU makes certain targeted improvements to simplify the application of the hedge accounting, including to derivative instruments as well as allow a one-time election to reclassify fixed-rate, prepayable debt securities from a held-to-maturity classification to an available-for-sale classification. ASU 2017-12 is effective for public business entities for fiscal years beginning after December 15, 2018. Guidance related to existing cash flow hedges and, if elected, fair value hedges is to be applied under a modified retrospective approach and guidance related to amended presentation and disclosures is to be applied under a prospective approach. Upon adoption of ASU 2017-12 on January 1, 2019, the guidance did not have an impact on the Company's derivatives and, thus, no adjustments were made related to derivatives. In conjunction with the adoption of ASU 2017-12, the Company made the transition election to reclassify $281.1 million in book value of securities, that qualified, from held-to-maturity to available-for-sale. 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. Early adoption is permitted. An entity is permitted to early adopt any removed or modified disclosures upon issuance of this Update and delay adoption of the additional disclosures until their effective date. 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 2018-14, “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. 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 2018-16, “Derivatives and Hedging (Topic 815) - Inclusion of the Secured Overnight Financing Rate (SOFR) Overnight Index Swap (OIS) Rate as a Benchmark Interest Rate for Hedge Accounting Purposes.” In October 2018, the FASB issued ASU 2018-16, Inclusion of the Secured Overnight Financing Rate (SOFR) Overnight Index Swap (OIS) Rate as a Benchmark Interest Rate for Hedge Accounting Purposes. The amendments in this Update permit use of the OIS rate based on SOFR as a U.S. benchmark interest rate for hedge accounting purposes under Topic 815 in addition to the interest rates on direct U.S. Treasury obligations, the LIBOR swap rate, the OIS rate based on the Fed Funds Effective Rate and the Securities Industry and Financial Markets Association (SIFMA) Municipal Swap Rate. The amendments in ASU 2018-16 became effective for the Company in conjunction with the adoption of ASU 2017-12 on January 1, 2019, and did not have a significant impact on the Company’s consolidated financial statements, results of operations or liquidity. ASU 2019-04, “Codification Improvements to Topic 326, Financial Instruments—Credit Losses, Topic 815, Derivatives and Hedging, and Topic 825, Financial Instruments”

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2019
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 February 19, 2020 , we declared a special dividend to common stockholders of $0.60 per share, which is payable on March 12, 2020 to shareholders of record as of March 2, 2020 . On January 28, 2020 , the Company declared a quarterly dividend to common shareholders of $0.34 per share, which was paid on February 20, 2020 to shareholders of record as of February 10, 2020 . 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, 2019
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, 2019 , 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 2018 and 2017 to conform to the 2019 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.
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 loan 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
Investment Securities	Investment Securities . 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. 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, 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 are included in investment securities gains. Declines in the fair value of securities below their cost that are judged to be other-than-temporary are included in other expenses if the decline is related to credit losses. Other-than-temporary impairment losses related to other factors are recognized in other comprehensive income, net of income taxes. In estimating other-than-temporary impairment losses, the Company considers, among other things, the length of time and the extent to which the fair value has been less than cost, the financial condition and near-term prospects of the issuer and the intent and ability of the Company to retain its investment in the issuer for a period of time sufficient to allow for any anticipated recovery in fair value. The cost of securities sold is based on the specific identification method.
Loans	Loans . Loans are reported at the principal amount outstanding. Interest income on loans is calculated using the simple interest method on the daily balance of the principal amount outstanding. Loan origination fees and certain direct origination costs are deferred, and the net amount is amortized as an adjustment of the related loan’s yield using a level yield method over the expected lives of the related loans. The accrual of interest on loans is discontinued when, in management’s opinion, the borrower may be unable to meet payment obligations as they become due or when a loan becomes contractually past due ninety days or more with respect to interest or principal, unless such past due loan is well secured and in the process of collection. When interest accrual is discontinued, all unpaid accrued interest is reversed against current period interest income. Interest income is subsequently recognized only to the extent cash payments are received in excess of principal due. Loans are returned to accrual status when all 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. A loan is considered impaired when, based upon current information and events, it is probable that the Company will be unable to collect, on a timely basis, all amounts due according to the contractual terms of the loan’s original agreement. The amount of the impairment is measured using cash flows discounted at the loan’s effective interest rate, except when it is determined that the primary source of repayment for the loan is the operation or liquidation of the underlying collateral. In such cases, the current fair value of the collateral, reduced by anticipated selling costs, is used to measure impairment. The Company considers impaired loans to include all loans, except consumer loans, that are risk rated as doubtful or on which interest accrual has been discontinued or that have been renegotiated in a troubled debt restructuring. Interest payments received on impaired loans are applied based on whether they are on accrual or non-accrual status. Interest income recognized by the Company on impaired loans primarily relates to loans modified in troubled debt restructurings that remain on accrual status. Interest payments received on non-accrual impaired loans are applied to principal. Interest income is subsequently recognized only to the extent cash payments are received in excess of principal due. Loans acquired in a business combination are recorded and initially measured at their estimated fair value as of the acquisition date, with no carryover of the related allowance for credit losses. Credit risks are included in the determination of fair value. For loans with no significant evidence of credit deterioration since origination, the difference between the fair value and the unpaid principal balance of the loan at the acquisition date is amortized into interest income using the effective interest method over the remaining period to contractual maturity. The accounting for loans acquired with evidence of a deterioration of credit quality is described below. Loans acquired through the completion of a transfer, including loans acquired in business combinations, that have evidence of deterioration of credit quality since origination and for which it is probable, at acquisition, that the Company will be unable to collect all contractually required payments receivable are initially recorded at fair value (as determined by the present value of expected future cash flows) with no valuation allowance. The difference between the undiscounted cash flows expected at acquisition and the recorded fair value of the loan, or the “accretable yield,” is recognized as interest income on a level-yield method over the life of the loan. Contractually required payments for interest and principal that exceed the undiscounted cash flows expected at acquisition, or the “nonaccretable difference,” are not recognized as a yield adjustment, a loss accrual or a valuation allowance. Increases in expected cash flows subsequent to the initial measurement are recognized prospectively through adjustment of the yield on the loan over its remaining life. Decreases in expected cash flows are recognized as impairment. Valuation allowances on these impaired loans reflect only losses incurred after the acquisition. A loan is considered a troubled debt restructuring when a borrower is experiencing financial difficulties that leads to a restructuring of the loan and the Company grants concessions to the borrower in the restructuring that it would not otherwise consider. These concessions may include rate reductions, principal forgiveness, extension of maturity date and other actions to minimize potential losses. Certain troubled debt restructurings are on non-accrual status at the time of restructuring and are returned to accrual status only after considering the borrower’s sustained repayment performance in accordance with the restructuring agreement for a reasonable period of at least six months and management is reasonably assured of 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 no longer be disclosed as a troubled debt restructuring although they continue to be individually evaluated for impairment and disclosed as impaired loans.
Allowance for Loan Losses	Allowance for Loan Losses . The allowance for loan losses is established through a provision for loan losses which is charged to expense. Loans, or portions thereof, are charged against the allowance for loan losses when management believes that the collectability of the principal is unlikely or, with respect to consumer installment and credit card loans, according to established delinquency schedules. The allowance balance is an amount that management believes will be adequate to absorb known and inherent losses in the loan portfolio based upon quarterly analysis of the current risk characteristics of the loan portfolio, an assessment of individual problem loans and actual loss experience, industry concentrations and current economic factors and the estimated impact of current economic and environmental conditions on historical loss rates. Loans acquired in business combinations are recorded at their estimated fair values on the date of acquisition. Accordingly, no allowance for loan losses related to these loans is recorded at the date of transfer. An allowance for loan losses is recorded for credit deterioration occurring subsequent to the transfer date.
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 4 to 15
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) and profit sharing plans 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.
Other Real Estate Owned	Other Real Estate Owned
Restricted Equity Securities	. Restricted Equity Securities.
Derivatives and Hedging Activities	or 2017 . 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 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 effective portion of 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 ineffective portion of the gain or loss on derivative instruments, if any, is recognized in earnings. The Company does not enter into interest rate swap agreements for trading or speculative purposes. As of December 31, 2019 , the Company does not have an existing agreement. 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. Earnings Per Common Share
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. For additional information concerning community banking, see “Business—Community Banking,” included in Part I, Item 1 of this report. Advertising Costs.
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
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, 2019
Business Combinations [Abstract]
Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed	The following table summarizes the consideration paid, fair values of the Northwest assets acquired and liabilities assumed, and the resulting goodwill. All amounts reported were finalized in the second quarter of 2019. As Recorded Fair Value As Recorded As of August 16, 2018 by Northwest Adjustments by the Company Assets acquired: Cash and cash equivalents $ 31.2 $ — $ 31.2 Investment securities 3.1 — 3.1 Loans held for investment 727.9 (14.8 ) (1) 713.1 Allowance for loan loss (8.0 ) 8.0 (2) — Premises and equipment 14.5 — 14.5 Other real estate owned (“OREO”) 0.3 0.3 0.6 Core deposit intangible assets 2.4 13.3 (3) 15.7 Other assets 29.3 (10.0 ) (4) 19.3 Total assets acquired 800.7 (3.2 ) 797.5 Liabilities assumed: Deposits 696.1 0.2 (5) 696.3 Accounts payable and accrued expense 8.1 (0.4 ) (6) 7.7 Long term debt 13.0 0.1 13.1 Trust preferred securities 5.2 (0.8 ) (7) 4.4 Deferred tax liability, net (1.2 ) 1.6 (8) 0.4 Total liabilities assumed 721.2 0.7 721.9 Net assets acquired $ 79.5 $ (3.9 ) $ 75.6 Consideration paid: Cash $ 3.0 Class A common stock 173.3 Total consideration paid 176.3 Goodwill $ 100.7 Explanation of fair value adjustments and the removal of previously recorded fair value marks recorded by Northwest. (1) 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. (2) Adjustment to remove the Northwest allowance for loan losses at acquisition date, as the credit risk is included in the fair value adjustment for loans receivable described in (1) above. (3) Write down 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. (4) Adjustment represents the value of the core deposit base assumed in the acquisition based upon valuation from an independent accounting and advisory firm. (5) Adjustment consists of reductions to the fair value of other items, including the removal of Northwest previously recorded goodwill. (6) 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. (7) Decrease due to the write-off of off-balance sheet reserves. (8) Write down of the book value of debt to the estimated fair values on the date of acquisition based upon favorable interest rates in the market. (9) Adjustment consists of the write-off of pre-existing deferred tax assets and purchase accounting adjustments as a result of the acquisition. 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 in 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 losses (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. Adjustments to the fair value marks for deferred tax assets and accounts payable and accrued expenses were made since the prior quarter, none of which were material. The adjustments had no impact on 2019 earnings and a net decrease to goodwill of $1.1 million from the third quarter reported balances. (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 and to record the fair value of right-of-use asset leases. (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 and to record the lease liability. (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 and are summarized below. Dec 31, 2019 Dec 31, 2018 Dec 31, 2017 Legal and professional fees $ 1.0 $ 4.0 $ 9.6 Employee expenses 8.2 1.1 5.1 Technology conversion and contract termination 9.1 6.6 10.2 Other 2.0 0.7 2.3 Total acquisition related expenses $ 20.3 $ 12.4 $ 27.2
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities Acquired During Period	Information regarding Northwest loans acquired deemed credit-impaired as of the August 16, 2018 acquisition date are as follows: Contractually required principal and interest payments $ 27.5 Contractual cash flows not expected to be collected (“non-accretable discount”) 4.4 Cash flows expected to be collected 23.1 Interest component of cash flows expected to be collected (“accretable discount”) 3.2 Fair value of acquired credit-impaired loans $ 19.9 Information regarding Northwest acquired loans not deemed credit-impaired at the August 16, 2018 acquisition date are as follows: Contractually required principal and interest payments $ 894.8 Contractual cash flows not expected to be collected 26.1 Fair value at acquisition $ 693.2 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, 2019
Goodwill and Intangible Assets Disclosure [Abstract]
Change in Carrying Amount of Goodwill	Goodwill Year Ended December 31, 2019 2018 Net carrying value at beginning of period $ 546.7 $ 444.7 Acquisitions and measurement period adjustments 74.9 102.0 Net carrying value at end of period $ 621.6 $ 546.7
Schedule of Activity of Identifiable Core Deposit Intangibles	Core deposit intangibles (“CDI”) The following table sets forth activity for identifiable core deposit intangibles subject to amortization: Year Ended December 31, 2019 2018 Gross CDI, beginning of period $ 89.7 $ 74.0 Established through acquisitions 16.6 15.7 Reductions due to sale of accounts (0.3 ) — Accumulated amortization (43.9 ) (32.8 ) Net CDI, end of period $ 62.1 $ 56.9
Schedule of Finite-Lived Intangible Assets, Future Amortization Expense	The following table provides estimated future CDI amortization expense: Years ending December 31, 2020 $ 10.9 2021 9.9 2022 9.0 2023 8.2 2024 7.3 Thereafter 16.8 Total $ 62.1

Investment Securities (Tables)

Investment Securities (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value 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 & collateralized mortgage obligations 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 Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value 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 December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Available-for-Sale U.S. Treasury notes $ 2.6 $ — $ — $ 2.6 Obligations of U.S. government agencies 569.3 0.1 (10.2 ) 559.2 U.S. agency residential mortgage-backed securities & 1,566.4 2.5 (24.1 ) 1,544.8 Private mortgage-backed securities 72.0 — (1.8 ) 70.2 Corporate Securities 92.9 — (1.0 ) 91.9 Other investments 2.0 — — 2.0 Total $ 2,305.2 $ 2.6 $ (37.1 ) $ 2,270.7 December 31, 2018 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Held-to Maturity State, county and municipal securities $ 150.9 $ 1.8 $ (0.9 ) $ 151.8 Obligations of U.S. government agencies 19.8 — (0.3 ) 19.5 U.S. agency residential mortgage-backed securities & collateralized mortgage obligations 189.7 0.3 (6.5 ) 183.5 Corporate securities 46.3 0.1 (0.6 ) 45.8 Other investments 0.1 — — 0.1 Total $ 406.8 $ 2.2 $ (8.3 ) $ 400.7
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, 2019 and 2018 . There were no held-to-maturity securities in a continuous unrealized loss position as of December 31, 2019 . Less than 12 Months 12 Months or More Total December 31, 2019 Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Available-for-Sale Obligations of U.S. government agencies $ 185.3 $ (0.8 ) $ — $ — $ 185.3 $ (0.8 ) U.S. agency residential mortgage-backed securities & collateralized mortgage obligations 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 ) Less than 12 Months 12 Months or More Total December 31, 2018 Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Available-for-Sale Obligations of U.S. government agencies $ 363.1 $ (7.9 ) $ 154.5 $ (2.3 ) $ 517.6 $ (10.2 ) U.S. agency residential mortgage-backed securities & collateralized mortgage obligations 735.2 (14.5 ) 503.7 (9.6 ) 1,238.9 (24.1 ) Private mortgage-backed securities — — 69.4 (1.8 ) 69.4 (1.8 ) Corporate securities 24.9 (0.2 ) 51.4 (0.8 ) 76.3 (1.0 ) Total $ 1,123.2 $ (22.6 ) $ 779.0 $ (14.5 ) $ 1,902.2 $ (37.1 ) Less than 12 Months 12 Months or More Total December 31, 2018 Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Held-to-Maturity State, county and municipal securities $ 25.9 $ (0.3 ) $ 57.1 $ (0.6 ) $ 83.0 $ (0.9 ) U.S. agency residential mortgage-backed 45.0 (2.2 ) 120.2 (4.3 ) 165.2 (6.5 ) Corporate securities — — 39.6 (0.6 ) 39.6 (0.6 ) Obligations of U.S. government agencies 19.5 (0.3 ) — — 19.5 (0.3 ) Total $ 90.4 $ (2.8 ) $ 216.9 $ (5.5 ) $ 307.3 $ (8.3 )
Maturities of Investment Securities	All other investment securities maturities are shown at contractual maturity dates. Available-for-Sale Held-to-Maturity December 31, 2019 Amortized Cost Estimated Fair Value Amortized Cost Estimated Fair Value Within one year $ 677.3 $ 661.4 $ 38.6 $ 38.6 After one year but within five years 1,486.2 1,470.9 29.7 30.1 After five years but within ten years 328.0 428.2 21.3 23.1 After ten years 454.0 399.5 2.7 2.7 Total $ 2,945.5 $ 2,960.0 $ 92.3 $ 94.5

Loans (Tables)

Loans (Tables)	12 Months Ended
	Dec. 31, 2019
Loans and Leases Receivable, Net Amount [Abstract]
Schedule of Loans by Class	The following table presents loans by class as of the dates indicated: December 31, 2019 2018 Real estate loans: Commercial $ 3,484.7 $ 3,235.4 Construction: Land acquisition & development 302.1 321.6 Residential 244.1 242.8 Commercial 431.5 274.3 Total construction loans 977.7 838.7 Residential 1,546.1 1,542.0 Agricultural 226.6 217.4 Total real estate loans 6,235.1 5,833.5 Consumer: Indirect consumer 784.6 787.8 Other consumer 179.0 200.6 Credit card 81.6 81.8 Total consumer loans 1,045.2 1,070.2 Commercial 1,371.3 1,310.3 Agricultural 279.1 254.8 Other, including overdrafts — 1.6 Loans held for investment 8,930.7 8,470.4 Mortgage loans held for sale 100.9 33.3 Total loans $ 9,031.6 $ 8,503.7
Deteriorated Loans Transferred in	The following table displays the outstanding unpaid principal balance and accrual status of loans acquired with credit impairment as of December 31, 2019 and 2018 . December 31, 2019 2018 Outstanding principal $ 49.8 $ 43.4 Carrying value: Loans on accrual status 36.8 30.2 Total carrying value $ 36.8 $ 30.2 The following table summarizes changes in the accretable yield for loans acquired credit impaired for the years ended December 31, 2019 , 2018 , and 2017 : Year Ended December 31, 2019 2018 2017 Beginning balance $ 8.9 $ 7.3 $ 6.8 Acquisitions 3.4 3.2 1.9 Additions — 0.6 0.1 Accretion income (3.4 ) (3.1 ) (2.9 ) Reductions due to exit events (1.4 ) (1.1 ) (1.5 ) Reclassifications from nonaccretable differences 2.5 2.0 2.9 Ending balance $ 10.0 $ 8.9 $ 7.3
Schedule of Recorded Investment in Past Due Loans by Class	The following tables present the contractual aging of the Company’s recorded investment in past due loans by class as of the period indicated: 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 Loans Real estate Commercial $ 5.5 $ 1.1 $ 0.6 $ 7.2 $ 3,464.5 $ 13.0 $ 3,484.7 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 1,535.2 4.6 1,546.1 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 6,191.5 24.7 6,235.1 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 1,347.9 13.7 1,371.3 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 8,844.3 42.9 8,930.7 Mortgage loans originated for sale — — — — 100.9 — 100.9 Total loans $ 27.6 $ 10.2 $ 5.7 $ 43.5 $ 8,945.2 $ 42.9 $ 9,031.6 Total Loans 30 - 59 60 - 89 > 90 30 or More Days Days Days Days Current Non-accrual Total As of December 31, 2018 Past Due Past Due Past Due Past Due Loans Loans Loans Real estate Commercial $ 10.4 $ 1.0 $ 0.8 $ 12.2 $ 3,214.0 $ 9.2 $ 3,235.4 Construction: Land acquisition & development 1.6 0.1 0.2 1.9 316.0 3.7 321.6 Residential 1.0 0.4 — 1.4 240.4 1.0 242.8 Commercial 0.4 — — 0.4 273.7 0.2 274.3 Total construction loans 3.0 0.5 0.2 3.7 830.1 4.9 838.7 Residential 8.8 1.1 0.2 10.1 1,525.3 6.6 1,542.0 Agricultural 2.2 — — 2.2 202.6 12.6 217.4 Total real estate loans 24.4 2.6 1.2 28.2 5,772.0 33.3 5,833.5 Consumer: Indirect consumer 6.8 2.1 0.4 9.3 776.8 1.7 787.8 Other consumer 1.4 0.5 0.1 2.0 198.1 0.5 200.6 Credit card 0.9 0.4 0.8 2.1 79.7 — 81.8 Total consumer loans 9.1 3.0 1.3 13.4 1,054.6 2.2 1,070.2 Commercial 8.3 1.2 1.3 10.8 1,283.7 15.8 1,310.3 Agricultural 2.1 0.3 — 2.4 249.4 3.0 254.8 Other, including overdrafts — — — — 1.6 — 1.6 Loans held for investment 43.9 7.1 3.8 54.8 8,361.3 54.3 8,470.4 Mortgage loans originated for sale — — — — 33.3 — 33.3 Total loans $ 43.9 $ 7.1 $ 3.8 $ 54.8 $ 8,394.6 $ 54.3 $ 8,503.7
Schedule of Recorded Investment in Impaired Loans	The following tables present information on the Company’s recorded investment in impaired loans as of dates indicated: December 31, 2019 Unpaid Total Principal Balance Recorded Investment With No Allowance Recorded Investment With Allowance Total Recorded Investment Related Allowance 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 Total Principal Balance Recorded Investment With No Allowance Recorded Investment With Allowance Total Recorded Investment Related Allowance 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 December 31, 2017 Unpaid Total Principal Balance Recorded Investment With No Allowance Recorded Investment With Allowance Total Recorded Investment Related Allowance Real estate: Commercial $ 45.6 $ 20.9 $ 14.1 $ 35.0 $ 3.9 Construction: Land acquisition & development 10.0 3.4 0.5 3.9 — Residential 1.8 1.7 — 1.7 — Commercial 4.7 0.4 3.5 3.9 2.2 Total construction loans 16.5 5.5 4.0 9.5 2.2 Residential 11.5 8.2 2.0 10.2 0.1 Agricultural 3.7 3.6 — 3.6 — Total real estate loans 77.3 38.2 20.1 58.3 6.2 Commercial 29.5 12.4 11.4 23.8 4.4 Agricultural 1.1 0.8 0.3 1.1 0.2 Total $ 107.9 $ 51.4 $ 31.8 $ 83.2 $ 10.8 The following tables present the average recorded investment in and income recognized on impaired loans for the periods indicated: Year Ended December 31, 2019 2018 2017 Average Recorded Investment Income Recognized Average Recorded Investment Income Recognized Average Recorded Investment Income Recognized Real estate $ 41.4 $ 0.1 $ 50.0 $ 0.1 $ 63.5 $ 0.3 Commercial 18.7 0.2 21.9 0.2 28.1 0.2 Agricultural 4.6 — 2.1 — 2.4 — Total $ 64.7 $ 0.3 $ 74.0 $ 0.3 $ 94.0 $ 0.5
Schedule of Loans Renegotiated in Troubled Debt Restructurings	The following table presents information on the Company’s troubled debt restructurings that occurred during the periods indicated: Number of Notes Type of Concession Principal Balance at Restructure Date Year Ended December 31, 2019 Interest only period Extension of terms or maturity Interest rate adjustment Other 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 11 $ 0.2 $ 0.2 $ — $ 10.0 $ 10.4 Number of Notes Type of Concession Principal Balance at Restructure Date Year Ended December 31, 2018 Interest only period Extension of terms or maturity Interest rate adjustment Other 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 5 $ 3.6 $ — $ — $ 0.5 $ 4.1 Number of Notes Type of Concession Principal Balance at Restructure Date Year Ended December 31, 2017 Interest only period Extension of terms or maturity Interest rate adjustment Other Commercial 17 $ 1.2 $ 2.0 $ — $ 6.0 $ 9.2 Agriculture 1 — 0.1 — — 0.1 Total loans restructured 24 $ 2.7 $ 3.3 $ — $ 6.9 $ 12.9
Schedule of Recorded Investment in Criticized Loans by Class and Credit Quality Indicator	The following tables present the Company’s recorded investment in criticized loans by class and credit quality indicator based on the most recent analysis performed as of the dates indicated: As of December 31, 2019 Other Assets Especially Mentioned Substandard Doubtful Total Criticized Loans Real estate: Commercial $ 84.7 $ 97.3 $ 0.8 $ 182.8 Construction: Land acquisition & development 3.8 1.9 1.0 6.7 Residential 0.9 2.2 — 3.1 Commercial 1.7 1.5 — 3.2 Total construction loans 6.4 5.6 1.0 13.0 Residential 2.6 7.8 0.3 10.7 Agricultural 14.3 26.6 — 40.9 Total real estate loans 108.0 137.3 2.1 247.4 Consumer: Indirect consumer 0.2 2.9 — 3.1 Direct consumer 0.4 0.8 0.1 1.3 Total consumer loans 0.6 3.7 0.1 4.4 Commercial 40.4 60.3 3.6 104.3 Agricultural 8.5 22.7 0.1 31.3 Total $ 157.5 $ 224.0 $ 5.9 $ 387.4 As of December 31, 2018 Other Assets Especially Mentioned Substandard Doubtful Total Criticized Loans Real estate: Commercial $ 102.5 $ 87.4 $ 2.9 $ 192.8 Construction: Land acquisition & development 5.0 7.0 3.3 15.3 Residential 2.8 2.0 0.4 5.2 Commercial 1.7 3.9 — 5.6 Total construction loans 9.5 12.9 3.7 26.1 Residential 3.0 10.8 0.7 14.5 Agricultural 9.0 24.0 0.1 33.1 Total real estate loans 124.0 135.1 7.4 266.5 Consumer: Indirect consumer 0.7 2.1 0.1 2.9 Direct consumer 0.3 0.8 0.1 1.2 Total consumer loans 1.0 2.9 0.2 4.1 Commercial 39.4 45.8 11.8 97.0 Agricultural 14.4 17.8 1.5 33.7 Total $ 178.8 $ 201.6 $ 20.9 $ 401.3

Allowance for Loan Losses (Tabl

Allowance for Loan Losses (Tables)	12 Months Ended
	Dec. 31, 2019
Receivables [Abstract]
Schedule of Allowance for Loan Losses by Portfolio Segment	The following tables present a summary of changes in the allowance for loan losses by portfolio segment: 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 expense (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 6,194.0 1,045.2 1,354.0 272.8 — 8,866.0 Total loans held for investment $ 6,235.1 $ 1,045.2 $ 1,371.3 $ 279.1 $ — $ 8,930.7 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 expense (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 5,791.7 1,070.2 1,290.4 251.7 1.6 8,405.6 Total loans held for investment $ 5,833.5 $ 1,070.2 $ 1,310.3 $ 254.8 $ 1.6 $ 8,470.4 Year Ended December 31, 2017 Real Estate Consumer Commercial Agriculture Other Total Allowance for loan losses: Beginning balance $ 28.6 $ 7.7 $ 38.1 $ 1.8 $ — $ 76.2 Provision charged (credited) to operating expense 6.0 8.1 (2.9 ) (0.2 ) — 11.0 Less loans charged-off (4.3 ) (11.3 ) (6.8 ) (0.4 ) — (22.8 ) Add back recoveries of loans previously charged-off 1.4 4.2 2.1 — — 7.7 Ending balance $ 31.7 $ 8.7 $ 30.5 $ 1.2 $ — $ 72.1 Individually evaluated for impairment $ 6.2 $ — $ 4.4 $ 0.2 $ — $ 10.8 Collectively evaluated for impairment 25.5 8.7 26.1 1.0 — 61.3 Ending balance $ 31.7 $ 8.7 $ 30.5 $ 1.2 $ — $ 72.1 Total loans: Individually evaluated for impairment $ 58.3 $ — $ 23.8 $ 1.1 $ — $ 83.2 Collectively evaluated for impairment 5,118.5 1,034.4 1,191.6 135.1 4.9 7,484.5 Total loans held for investment $ 5,176.8 $ 1,034.4 $ 1,215.4 $ 136.2 $ 4.9 $ 7,567.7

Premises and Equipment (Tables)

Premises and Equipment (Tables)	12 Months Ended
	Dec. 31, 2019
Property, Plant and Equipment [Abstract]
Premises and Equipment and Related Accumulated Depreciation	Premises and equipment and related accumulated depreciation are as follows: December 31, 2019 2018 Land $ 50.8 $ 49.9 Buildings and improvements 330.1 268.1 Furniture and equipment 94.2 104.0 Total premises and equipment 475.1 422.0 Less accumulated depreciation (169.1 ) (176.8 ) Premises and equipment, net $ 306.0 $ 245.2

Company-Owned Life Insurance (T

Company-Owned Life Insurance (Tables)	12 Months Ended
	Dec. 31, 2019
Investments, All Other Investments [Abstract]
Schedule of Life Insurance Corporate or Bank Owned	Company-owned life insurance consists of the following: December 31, 2019 2018 Key executive, principal shareholder $ 4.4 $ 4.7 Key executive split dollar 7.0 9.4 Group life 282.4 261.0 Total $ 293.8 $ 275.1

Other Real Estate Owned (Tables

Other Real Estate Owned (Tables)	12 Months Ended
	Dec. 31, 2019
Repossessed Assets [Abstract]
Other Real Estate Owned	Information with respect to the Company’s other real estate owned follows: Year Ended December 31, 2019 2018 2017 Balance at beginning of year $ 14.4 $ 10.1 $ 10.0 Acquisitions 2.4 0.6 1.2 Additions 14.1 12.1 5.4 Capitalized improvements 0.3 — — Valuation adjustments (0.9 ) (0.1 ) (0.4 ) Dispositions (21.8 ) (8.3 ) (6.1 ) Balance at end of year $ 8.5 $ 14.4 $ 10.1

Derivatives and Hedging Activ_2

Derivatives and Hedging Activities (Tables)	12 Months Ended
	Dec. 31, 2019
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 December 31, 2019 and December 31, 2018 : 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 December 31, 2018 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 $ 8.8 $ — $ 8.8 $ 2.7 $ 2.4 $ 3.7 Mortgage related derivatives 1.3 — 1.3 — — 1.3 Total derivatives 10.1 — 10.1 2.7 2.4 5.0 Total assets $ 10.1 $ — $ 10.1 $ 2.7 $ 2.4 $ 5.0 Financial Liabilities Interest rate swap contracts $ 8.8 $ — $ 8.8 $ 2.7 $ 4.1 $ 2.0 Mortgage related derivatives 0.6 — 0.6 — — 0.6 Total derivatives 9.4 — 9.4 2.7 4.1 2.6 Repurchase agreements 712.4 — 712.4 — 712.4 — Total liabilities $ 721.8 $ — $ 721.8 $ 2.7 $ 716.5 $ 2.6
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 December 31, 2019 and December 31, 2018 : 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 December 31, 2018 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 $ 8.8 $ — $ 8.8 $ 2.7 $ 2.4 $ 3.7 Mortgage related derivatives 1.3 — 1.3 — — 1.3 Total derivatives 10.1 — 10.1 2.7 2.4 5.0 Total assets $ 10.1 $ — $ 10.1 $ 2.7 $ 2.4 $ 5.0 Financial Liabilities Interest rate swap contracts $ 8.8 $ — $ 8.8 $ 2.7 $ 4.1 $ 2.0 Mortgage related derivatives 0.6 — 0.6 — — 0.6 Total derivatives 9.4 — 9.4 2.7 4.1 2.6 Repurchase agreements 712.4 — 712.4 — 712.4 — Total liabilities $ 721.8 $ — $ 721.8 $ 2.7 $ 716.5 $ 2.6
Schedule of Derivative Instruments in Statement of Financial Position, Fair Value	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, 2019 December 31, 2018 Notional Amount Estimated Fair Value Notional Amount Estimated Fair Value Derivative Assets (included in other assets on the consolidated balance sheets) Non-hedging interest rate derivatives: Interest rate swap contracts $ 503.2 $ 21.9 $ 403.3 $ 8.8 Interest rate lock commitments 67.8 1.3 51.0 1.3 Total derivative assets $ 571.0 $ 23.2 $ 454.3 $ 10.1 Derivative Liabilities (included in accounts payable and accrued expenses on the consolidated balance sheets) Non-hedging interest rate derivatives: Interest rate swap contracts $ 503.2 $ 21.9 $ 403.3 $ 8.8 Forward loan sales contracts 128.0 0.3 64.6 0.6 Total derivative liabilities $ 631.2 $ 22.2 $ 467.9 $ 9.4
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, 2019 2018 2017 Derivatives designated as hedges: Amount of loss recognized in other comprehensive income (effective portion) $ — $ — $ (1.1 ) Reclassification adjustment for derivative net (gains) losses included in income — — 1.1 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 2.5 1.3 0.8 Amount of net gains (losses) recognized in mortgage banking revenues $ 0.3 $ (0.5 ) $ (1.7 )

Mortgage Servicing Rights (Tabl

Mortgage Servicing Rights (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 2017 Balance at beginning of year $ 27.7 $ 24.8 $ 18.7 Acquisitions of mortgage servicing rights — — 3.5 Originations of mortgage servicing rights 7.3 6.0 5.6 Amortization expense (4.4 ) (3.1 ) (3.0 ) Balance at end of year 30.6 27.7 24.8 Less valuation reserve (0.4 ) — — Balance at end of year, net of valuation reserve $ 30.2 $ 27.7 $ 24.8 Principal balance of serviced loans underlying mortgage servicing rights $ 3,710.1 $ 3,698.2 $ 3,636.7 Mortgage servicing rights as a percentage of serviced loans 0.81 % 0.75 % 0.68 %

Deposits (Tables)

Deposits (Tables)	12 Months Ended
	Dec. 31, 2019
Deposits [Abstract]
Schedule of Deposits, By Type	Deposits are summarized as follows: December 31, 2019 2018 Non-interest bearing demand $ 3,426.5 $ 3,158.3 Interest bearing: Demand 3,195.4 2,957.5 Savings 3,591.6 3,247.9 Time, $100 and over 651.1 547.6 Time, other 798.9 769.4 Total interest bearing 8,237.0 7,522.4 Total deposits $ 11,663.5 $ 10,680.7
Schedule of Maturities of Time Deposits	Maturities of time deposits at December 31, 2019 are as follows: Time, $100 and Over Total Time Due within 3 months or less $ 101.6 $ 323.6 Due after 3 months and within 6 months 140.8 301.4 Due after 6 months and within 12 months 273.7 515.9 Due within 2021 104.0 220.4 Due within 2022 23.6 64.0 Due within 2023 3.5 13.9 Due within 2024 and thereafter 3.9 10.8 Total $ 651.1 $ 1,450.0

Long-Term Debt and Other Borr_2

Long-Term Debt and Other Borrowed Funds (Tables)	12 Months Ended
	Dec. 31, 2019
Debt Disclosure [Abstract]
Summary of Long-Term Debt	A summary of long-term debt follows: December 31, 2019 2018 Subsidiaries: 8.00% capital lease obligation with term ending October 25, 2029 $ 1.2 $ 1.3 6.24% note payable maturing September 6, 2032, principal due at maturity, interest payable monthly — 1.8 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 $ 13.9 $ 15.8
Schedule of Maturities of Long-term Debt	Maturities of long-term debt at December 31, 2019 are as follows: 2020 $ 0.1 2021 0.1 2022 5.1 2023 0.1 2024 0.1 Thereafter 8.4 Total $ 13.9

Subordinated Debentures Held _2

Subordinated Debentures Held by Subsidiary Trusts (Tables)	12 Months Ended
	Dec. 31, 2019
Subordinated Borrowings [Abstract]
Schedule of Subordinated Borrowings	A summary of Subordinated Debenture issuances follows: Principal Amount Outstanding as of December 31, Issuance Maturity Date 2019 2018 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 4.4 4.4 Total subordinated debentures held by subsidiary trusts $ 86.9 $ 86.9

Earnings per Common Share (Tabl

Earnings per Common Share (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 2018 2017 Net income, basic and diluted $ 181.0 $ 160.2 $ 106.5 Weighted average common shares outstanding for basic earnings per share computation 63,645,029 57,778,857 51,429,366 Dilutive effects of stock-based compensation 239,839 438,266 473,843 Weighted average common shares outstanding for diluted earnings per common share computation 63,884,868 58,217,123 51,903,209 Basic earnings per common share $ 2.84 $ 2.77 $ 2.07 Diluted earnings per common share 2.83 2.75 2.05

Regulatory Capital (Tables)

Regulatory Capital (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 and 2018 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) Amount Ratio Amount Ratio Amount Ratio Amount Ratio December 31, 2019 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 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) Amount Ratio Amount Ratio Amount Ratio Amount Ratio December 31, 2018 Total risk-based capital: Consolidated $ 1,285.0 12.99 % $ 791.2 8.00 % $ 976.6 9.88 % $ 989.0 10.00 % FIB 1,184.5 12.01 788.8 8.00 973.7 9.88 986.0 10.00 Tier 1 risk-based capital: Consolidated 1,212.0 12.26 593.4 6.00 778.8 7.88 791.2 8.00 FIB 1,111.6 11.27 591.6 6.00 776.5 7.88 788.8 8.00 Common equity tier 1 risk-based capital: Consolidated 1,127.8 11.40 445.0 4.50 630.5 6.38 642.8 6.50 FIB 1,111.6 11.27 443.7 4.50 628.6 6.38 640.9 6.50 Leverage capital ratio: Consolidated 1,212.0 9.47 511.9 4.00 511.9 4.00 639.9 5.00 FIB 1,111.6 8.97 495.9 4.00 495.9 4.00 619.8 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, 2019
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, 2019 , are as follows: Third Parties Related Entity Total For the year ending December 31: 2020 $ 5.0 $ 1.4 $ 6.4 2021 4.7 1.4 6.1 2022 4.4 1.2 5.6 2023 4.2 1.0 5.2 2024 4.1 1.0 5.1 Thereafter 20.5 1.6 22.1 Total $ 42.9 $ 7.6 $ 50.5

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Schedule of Income Tax Expense	Income tax expense consists of the following: Year ended December 31, 2019 2018 2017 Current: Federal $ 40.5 $ 23.1 $ 24.3 State 8.2 7.2 5.0 Total current 48.7 30.3 29.3 Deferred: Federal 3.7 12.3 18.4 State 1.7 3.5 2.5 Total deferred 5.4 15.8 20.9 Total income tax expense $ 54.1 $ 46.1 $ 50.2
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 2019 and 2018 and 35% for 2017 , respectively, to income before income taxes due to the following: Year ended December 31, 2019 2018 2017 Tax expense at the statutory tax rate $ 49.4 $ 43.3 $ 54.9 Increase (decrease) in tax resulting from: Tax-exempt income (2.8 ) (2.8 ) (4.5 ) State income tax, net of federal income tax benefit 9.9 8.5 4.9 Benefit of stock-based compensation plans (1.2 ) (1.1 ) (2.6 ) Federal tax credits (2.0 ) (2.6 ) (2.5 ) Benefit due to enactment of federal tax reform — — (2.2 ) Other, net 0.8 0.8 2.2 Tax expense at effective tax rate $ 54.1 $ 46.1 $ 50.2
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, 2019 2018 Deferred tax assets: Loans, principally due to allowance for loan losses $ 18.1 $ 18.4 Loan discount 7.2 8.3 Investment securities, unrealized losses — 8.9 Deferred compensation 18.1 17.0 Non-performing loan interest 1.0 1.2 Other real estate owned write-downs and carrying costs 0.1 0.3 Tax credit carryforwards (1) 0.2 0.1 Net operating loss carryforwards (2) 2.6 3.9 Lease liabilities 9.9 — Other 0.9 0.1 Deferred tax assets 58.1 58.2 Deferred tax liabilities: Fixed assets, principally differences in bases and depreciation (8.8 ) (6.9 ) Deferred loan costs (2.8 ) (2.6 ) Investment securities, unrealized gains (3.9 ) — Investment in joint venture partnership, principally due to differences in depreciation of partnership assets (0.7 ) (0.8 ) Right of use assets (9.7 ) — Prepaid amounts (0.5 ) (0.6 ) Government agency stock dividends (1.2 ) (1.5 ) Goodwill and core deposit intangibles (49.2 ) (44.2 ) Mortgage servicing rights (7.2 ) (6.4 ) Other (0.8 ) (3.8 ) Deferred tax liabilities (84.8 ) (66.8 ) Net deferred tax assets (liabilities) $ (26.7 ) $ (8.6 ) (1) Based on filed tax returns and amounts expected to be reported in current year tax returns ( December 31, 2019 ), we had remaining federal tax credit carryforwards of $0.1 million from acquired companies. The remaining federal tax credits were primarily generated from AMT tax credit carryforwards, and their use is subject to annual limitations. (2) As of December 31, 2019 , we had remaining federal net operating loss carryforwards of $3.8 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 2029 and ending in 2037 and the state net operating losses will expire beginning in 2020 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, 2019
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, 2019 Number of Shares Weighted-Average Measurement Date Fair Value Restricted stock, beginning of year 380,584 $ 37.46 Granted 230,922 41.29 Vested (202,286 ) 33.24 Forfeited (46,198 ) 41.18 Canceled — — Restricted stock, end of year 363,022 $ 41.47

Other Comprehensive Income (Tab

Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2019
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, 2019 Before Tax Amount Tax Expense (Benefit) Net of Tax Amount Investment securities available-for sale: Change in net unrealized gain during period $ 54.9 $ 14.1 $ 40.8 Reclassification adjustment for net gain 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 gains (0.6 ) (0.1 ) (0.5 ) Total other comprehensive loss $ (12.8 ) $ (3.3 ) $ (9.5 ) Year Ended December 31, 2017 Before Tax Amount Tax Expense (Benefit) Net of Tax Amount Investment securities available-for sale: Change in net unrealized loss during period $ (6.5 ) $ (2.7 ) $ (3.8 ) Reclassification adjustment for net gain included in net income (0.7 ) (0.3 ) (0.4 ) Change in unamortized loss on available-for-sale securities transferred into held-to-maturity 1.9 0.7 1.2 Change in net unrealized loss on derivatives (1.1 ) (0.4 ) (0.7 ) Reclassification adjustment for derivative net loss included in income 1.1 0.4 0.7 Defined benefits post-retirement benefit plan: Change in net actuarial loss (1.3 ) (0.5 ) (0.8 ) Total other comprehensive loss $ (6.6 ) $ (2.8 ) $ (3.8 )
Schedule of Accumulated Other Comprehensive Income (Loss)	The components of accumulated other comprehensive income (loss), net of income taxes, are as follows: Year ended December 31, 2019 2018 Net unrealized gain (loss) on investment securities available-for-sale $ 10.6 $ (25.5 ) Net actuarial gain on defined benefit post-retirement benefit plans 0.4 0.9 Net accumulated other comprehensive income (loss) $ 11.0 $ (24.6 )

Condensed Financial Informati_2

Condensed Financial Information (Parent Company Only) (Tables)	12 Months Ended
	Dec. 31, 2019
Condensed Financial Information Disclosure [Abstract]
Schedule of Condensed Financial Statements	Following is condensed financial information of First Interstate BancSystem, Inc. December 31, 2019 2018 Condensed balance sheets: Cash and cash equivalents $ 147.1 $ 72.9 Investment in bank subsidiary 1,899.3 1,648.9 Advances to subsidiaries, net 48.9 46.0 Other assets 61.7 59.0 Total assets $ 2,157.0 $ 1,826.8 Other liabilities $ 56.2 $ 46.0 Subordinated debentures held by subsidiary trusts 86.9 86.9 Total liabilities 143.1 132.9 Stockholders’ equity 2,013.9 1,693.9 Total liabilities and stockholders’ equity $ 2,157.0 $ 1,826.8 Years Ended December 31, 2019 2018 2017 Condensed statements of income: Dividends from subsidiaries $ 178.0 $ 148.5 $ 150.0 Other interest income 0.3 0.1 0.1 Other income, primarily management fees from subsidiaries 25.9 17.0 18.0 Total income 204.2 165.6 168.1 Salaries and benefits 34.2 25.3 21.8 Interest expense 4.7 4.5 4.7 Acquisition expenses 17.0 8.1 25.3 Other operating expenses, net 14.8 14.5 13.0 Total expenses 70.7 52.4 64.8 Earnings before income tax benefit 133.5 113.2 103.3 Income tax benefit (11.9 ) (9.3 ) (14.2 ) Income before undistributed earnings of subsidiaries 145.4 122.5 117.5 Undistributed earnings of subsidiaries 35.6 37.7 (11.0 ) Net income $ 181.0 $ 160.2 $ 106.5 Years Ended December 31, 2019 2018 2017 Condensed statements of cash flows: Cash flows from operating activities: Net income $ 181.0 $ 160.2 $ 106.5 Adjustments to reconcile net income to cash provided by operating activities: Undistributed earnings of subsidiaries (35.6 ) (37.7 ) 11.0 Stock-based compensation expense 8.0 5.6 3.9 Other, net 8.1 16.4 14.7 Net cash provided by operating activities 161.5 144.5 136.1 Cash flows from investing activities: Capital distributions from nonbank subsidiaries — — 18.0 Acquisition of intangible assets — — (28.0 ) Acquisition of bank holding company, net of cash and cash equivalents received — (14.7 ) (128.3 ) Investment in subsidiary — — (18.0 ) Net cash used in investing activities — (14.7 ) (156.3 ) Cash flows from financing activities: Net (decrease) increase in advances from subsidiaries (6.6 ) (9.9 ) (28.4 ) Repayment of long-term debt — (26.0 ) — Proceeds from issuance of common stock, net of stock issuance costs 1.0 1.8 2.4 Purchase and retirement of common stock (2.5 ) (1.0 ) (1.3 ) Dividends paid to common stockholders (79.2 ) (64.1 ) (48.6 ) Net cash used in financing activities (87.3 ) (99.2 ) (75.9 ) Net change in cash and cash equivalents 74.2 30.6 (96.1 ) Cash and cash equivalents, beginning of year 72.9 42.3 138.4 Cash and cash equivalents, end of year $ 147.1 $ 72.9 $ 42.3

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2019
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, 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 — Fair Value Measurements at Reporting Date Using As of December 31, 2018 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 $ 2.6 $ — $ 2.6 $ — Obligations of U.S. government agencies 559.2 — 559.2 — U.S. agency mortgage-backed securities & collateralized mortgage obligations 1,544.8 — 1,544.8 — Private mortgage-backed securities 70.2 — 70.2 — Corporate securities 91.9 — 91.9 — Other investments 2.0 — 2.0 — Loans held for sale 33.3 — 33.3 — Derivative assets: Interest rate swap contracts 8.8 — 8.8 — Interest rate lock commitments 1.3 — 1.3 — Derivative liabilities: Interest rate swap contracts 8.8 — 8.8 — Forward loan sales contracts 0.6 — 0.6 — Deferred compensation plan assets 12.1 — 12.1 — Deferred compensation plan liabilities 12.1 — 12.1 —
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, 2019 Total Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Gains (Losses) 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 ) Fair Value Measurements at Reporting Date Using As of December 31, 2018 Total Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Total Gains (Losses) Impaired loans $ 24.1 $ — $ — $ 24.1 $ (12.2 ) Other real estate owned 0.6 — — 0.6 (0.6 ) Long-lived assets to be disposed of by sale 4.9 — — 4.9 (0.5 )
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, 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%) As of December 31, 2018 Fair Value Valuation Technique Unobservable Inputs Range (Weighted Average) Impaired loans $ 24.1 Appraisal Appraisal adjustment 0% - 26% (13%) Other real estate owned 0.6 Appraisal Appraisal adjustment 8% - 96% (39%) Long-lived assets to be disposed of by sale 4.9 Appraisal Appraisal adjustment 0% - 43% (10%)
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, 2019 Carrying Amount Estimated Fair Value Quoted Prices in Significant Other Significant 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 $ — Fair Value Measurements at Reporting Date Using As of December 31, 2018 Carrying Amount Estimated Fair Value Quoted Prices in Significant Other Significant Financial assets: Cash and cash equivalents $ 822.0 $ 822.0 $ 822.0 $ — $ — Investment debt securities available-for-sale 2,270.7 2,270.7 — 2,270.7 — Investment debt securities held-to-maturity 406.8 400.7 — 400.7 — Accrued interest receivable 44.9 44.9 — 44.9 — Mortgage servicing rights, net 27.7 42.4 — 42.4 — Loans held for sale 33.3 33.3 — 33.3 — Net loans held for investment 8,397.4 8,439.7 — 8,415.6 24.1 Derivative assets 10.1 10.1 — 10.1 — Deferred compensation plan assets 12.1 12.1 — 12.1 — Total financial assets $ 12,025.0 $ 12,075.9 $ 822.0 $ 11,229.8 $ 24.1 Financial liabilities: Total deposits, excluding time deposits $ 9,363.7 $ 9,363.7 $ 9,363.7 $ — $ — Time deposits 1,317.0 1,299.0 — 1,299.0 — Securities sold under repurchase agreements 712.4 712.4 — 712.4 — Accrued interest payable 7.8 7.8 — 7.8 — Long-term debt 15.8 13.0 — 13.0 — Subordinated debentures held by subsidiary trusts 86.9 84.9 — 84.9 — Derivative liabilities 9.4 9.4 — 9.4 — Deferred compensation plan liabilities 12.1 12.1 — 12.1 — Total financial liabilities $ 11,525.1 $ 11,502.3 $ 9,363.7 $ 2,138.6 $ —

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2019USD ($)segmentsubsidiary	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Property, Plant and Equipment
Loans and Leases Receivable, Past Due Status, Policy [Policy Text Block]	ninety days
Cash and Cash Equivalents	Cash and Cash Equivalents
Finite-Lived Intangible Asset, Useful Life	10 years
Real Estate Acquired Through Foreclosure, Valuation Adjustments	$ 900,000	$ 100,000
Number of significant subsidiaries | subsidiary	1
Cash on deposit with Federal Reserve Bank	$ 769,300,000	574,400,000
Compensating balances with Federal Reserve Bank	46,300,000	28,100,000
Deferred compensation plan assets	18,200,000	12,100,000
Deferred compensation plan liabilities	18,200,000	12,100,000
Impairment losses	0	0
Write-downs of OREO	(900,000)	(100,000)	$ (400,000)
Carrying value of foreclosed OREO	2,300,000	2,000,000
Federal Reserve Bank stock	42,800,000	37,500,000
Federal Home Loan Bank stock	10,700,000	10,700,000
Income tax examination, penalties	$ 0	0		$ 0
Number of operating segments | segment	1
Advertising expense	$ 4,200,000	3,200,000	3,500,000
Stock compensation expense	8,000,000	5,600,000	3,800,000
Stock-based compensation expense		(64,100,000)	(48,600,000)
Mortgage Loans in Process of Foreclosure, Amount	$ 1,400,000
Benefit of stock-based compensation plans			(2,600,000)
Short-term Leases [Policy Text Block]	12 months
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	4 years
Furniture and equipment | Maximum [Member]
Property, Plant and Equipment
Estimated useful lives	15 years
Restricted Stock [Member]
Property, Plant and Equipment
Stock compensation expense		5,600,000	3,300,000
Tax benefits from stock-based compensation	$ 400,000	200,000	600,000
Stock-based compensation expense		0
Common Stock Including Additional Paid in Capital [Member]
Property, Plant and Equipment
Stock compensation expense	8,000,000	5,600,000	3,800,000
Stock-based compensation expense			$ 0
Benefit of stock-based compensation plans	$ (1,200,000)	$ (1,100,000)

Acquisitions - Narrative (Deta

Acquisitions - Narrative (Details) $ / shares in Units, $ in Millions	Apr. 08, 2019USD ($)segmentoffice$ / sharesshares	Aug. 16, 2018USD ($)office$ / sharesshares	Dec. 31, 2019USD ($)	Sep. 30, 2018USD ($)	Dec. 31, 2019USD ($)segment	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Business Acquisition [Line Items]
Change in goodwill			$ 0.1
Goodwill			621.6		$ 621.6	$ 546.7	$ 444.7
Number of operating segments | segment					1
Acquisition related expenses				$ 12.4	$ 20.3	12.4	$ 27.2
Core Deposits Intangibles
Business Acquisition [Line Items]
Finite-Lived Intangible Assets, Net	$ 3		$ 62.1		62.1	56.9
Finite-lived intangible assets acquired					16.6	$ 15.7
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
Inland Northwest Bank [Member]
Business Acquisition [Line Items]
Change in goodwill					$ 0.3
Core deposit intangible assets		$ 15.7
Number of banking offices | office		20
Cash paid as consideration		$ 3
Total consideration paid		176.3
Goodwill		$ 100.7
Number of operating segments | segment	1
Inland Northwest Bank [Member] | Class A Common Stock
Business Acquisition [Line Items]
Business Acquisition Conversion Share Rate | shares		0.516
Common stock issued (in shares) | shares		3,840,000
Business Acquisition, Share Price | $ / shares		$ 45.15
Inland Northwest 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 Rec

Acquisitions - Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed (Details) $ / shares in Units, $ in Millions	Apr. 08, 2019USD ($)segmentoffice$ / sharesshares	Aug. 16, 2018USD ($)office$ / sharesshares	Dec. 31, 2019USD ($)segment	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Business Acquisition [Line Items]
Goodwill			$ 621.6	$ 546.7	$ 444.7
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	$ 31.2
Investment securities	62.7	3.1
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held for Investment	$ 347.6	727.9
Allowance for loan losses		(8)
Premises and equipment		14.5
Other real estate owned (“OREO”)		0.3
Core deposit intangible assets		2.4
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Assets		29.3
Total assets acquired		800.7
Deposits		696.1
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities		8.1
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt		13
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Trust Preferred Securities		5.2
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Deferred Tax Liability		(1.2)
Total liabilities assumed		721.2
Net assets acquired		79.5
Northwest Bank Acquisition, Fair Value Adjustments [Member]
Business Acquisition [Line Items]
Cash and cash equivalents		0
Investment securities		0
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held for Investment		(14.8)
Allowance for loan losses		8
Premises and equipment		0
Other real estate owned (“OREO”)		0.3
Core deposit intangible assets		13.3
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Assets		(10)
Total assets acquired		(3.2)
Deposits		0.2
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities		(0.4)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt		0.1
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Trust Preferred Securities		(0.8)
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Deferred Tax Liability		1.6
Total liabilities assumed		0.7
Net assets acquired		$ (3.9)
Inland Northwest Bank [Member]
Business Acquisition [Line Items]
Number of banking offices | office		20
Cash and cash equivalents		$ 31.2
Investment securities		3.1
Business Combinations, Recognized Identifiable Assets Acquired And Liabilities Assumed, Loans Held for Investment		713.1
Allowance for loan losses		0
Premises and equipment		14.5
Other real estate owned (“OREO”)		0.6
Core deposit intangible assets		15.7
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Assets		19.3
Total assets acquired		797.5
Deposits		696.3
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed, Other Liabilities		7.7
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt		13.1
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Trust Preferred Securities		4.4
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Deferred Tax Liability		0.4
Total liabilities assumed		721.9
Net assets acquired		75.6
Cash		3
Class A common stock		173.3
Total consideration paid		176.3
Goodwill		$ 100.7
Number of operating segments | segment	1
Inland Northwest Bank [Member] | Class A Common Stock
Business Acquisition [Line Items]
Business Acquisition Conversion Share Rate | shares		0.516
Common stock issued (in shares) | shares		3,840,000
Business Acquisition, Share Price | $ / shares		$ 45.15
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
Core Deposits Intangibles | Inland Northwest Bank [Member]
Business Acquisition [Line Items]
Estimated useful lives of related deposits		10 years

Acquisitions - Schedule of Acq

Acquisitions - Schedule of Acquisition Related Expenses (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Business Combination, Acquisition Related Costs, Legal Fees	$ 4	$ 1		$ 9.6
Business Combination, Acquisition Related Costs, Employee Expense	1.1	8.2		5.1
Business Combination, Acquisition Related Costs, Technology and Contract Termination	6.6	9.1		10.2
Business Combination, Acquisition Related Costs, Other Expenses	0.7	2		2.3
Total acquisition related expenses	$ 12.4	$ 20.3	$ 12.4	$ 27.2

Acquisitions - Schedules of Lo

Acquisitions - Schedules of Loans Acquired (Details) - USD ($) $ in Millions	Apr. 08, 2019	Aug. 16, 2018
Idaho Independent Bank [Member]
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
Inland Northwest Bank [Member]
Business Acquisition [Line Items]
Contractually required principal and interest payments		$ 27.5
Contractual cash flows not expected to be collected (“non-accretable discount”)		4.4
Cash flows expected to be collected		23.1
Interest component of cash flows expected to be collected (“accretable discount”)		3.2
Fair value of acquired credit-impaired loans		19.9
Contractually required principal and interest payments		894.8
Contractual cash flows not expected to be collected		26.1
Fair value at acquisition		$ 693.2

Acquisitions - Schedule of Una

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	3 Months Ended	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Line Items]
Goodwill, Acquired During Period		$ 74.9	$ 102
Change in goodwill	$ 0.1
Property, Plant and Equipment, Net	306	306	245.2
Goodwill [Roll Forward]
Net carrying value at beginning of period		546.7	444.7
Net carrying value at end of period	$ 621.6	621.6	$ 546.7
Inland Northwest Bank [Member]
Goodwill [Line Items]
Change in goodwill		$ 0.3

Goodwill and Core Deposit Int_4

Goodwill and Core Deposit Intangibles - Goodwill Narrative (Details) - USD ($)	Jul. 01, 2019	Jul. 01, 2017	Jul. 01, 2015	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Line Items]
Finite-Lived Intangible Asset, Useful Life					10 years
Change in goodwill				$ 100,000
Goodwill				$ 621,600,000	$ 621,600,000	$ 546,700,000	$ 444,700,000
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, 2019	Dec. 31, 2018	Dec. 31, 2017
Finite-lived Intangible Assets [Roll Forward]
Accumulated amortization	$ 11.2	$ 7.9	$ 5.5
Finite-Lived Intangible Asset, Useful Life	10 years
Core Deposits Intangibles
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross		89.7	$ 74
Finite-lived Intangible Assets [Roll Forward]
Gross CDI, beginning of period	$ 56.9
Established through acquisitions	16.6	15.7
Reductions due to sale of accounts	(0.3)	0
Finite-Lived Intangible Assets, Accumulated Amortization	43.9	32.8
Net CDI, end of period	$ 62.1	$ 56.9

Goodwill and Core Deposit Int_6

Goodwill and Core Deposit Intangibles - Intangible Assets Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
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	$ (0.3)	$ 0

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, 2019	Apr. 08, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	10 years
Core Deposits Intangibles
Finite-Lived Intangible Assets [Line Items]
2020	$ 10.9
2021	9.9
2022	9
2023	8.2
2024	7.3
Thereafter	16.8
Total	$ 62.1	$ 3	$ 56.9

Investment Securities - Amorti

Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 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	$ 677.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	7.2	$ 37.1
Available-for-sale Securities, Gross Realized Gains	0		$ 1.1
Available-for-sale Securities, Gross Realized Losses		0	$ (0.4)
Available-for-Sale:
Amortized Cost	2,945.5	2,305.2
Gross Unrealized Gains	21.7	2.6
Gross Unrealized Losses	(7.2)	(37.1)
Estimated Fair Value	2,960	2,270.7
Held-to-Maturity:
Amortized Cost	92.3	406.8		$ 281.1
Gross Unrealized Gains	2.2	2.2
Gross Unrealized Losses	0	(8.3)
Estimated Fair Value	94.5	400.7		$ 275.1
Debt Securities, Unrealized Gain (Loss)		6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	661.4
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, within One Year, Amortized Cost	38.6
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling within One Year, Fair Value	38.6
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost	1,486.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	1,470.9
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	29.7
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	30.1
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost	328
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	428.2
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	21.3
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	23.1
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost	454
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	399.5
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost	2.7
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	2.7
US Treasury notes
Available-for-Sale:
Amortized Cost	9	2.6
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	9	2.6
State, county and municipal securities
Available-for-Sale:
Amortized Cost	80.1
Gross Unrealized Gains	0.8
Gross Unrealized Losses	0
Estimated Fair Value	80.9
Held-to-Maturity:
Amortized Cost	57.3	150.9
Gross Unrealized Gains	2.1	1.8
Gross Unrealized Losses	0	(0.9)
Estimated Fair Value	59.4	151.8
Available-for-sale securities, amortized cost basis	45.9
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	0.8	10.2
Available-for-Sale:
Amortized Cost	367.5	569.3
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	(0.8)	(10.2)
Estimated Fair Value	366.8	559.2
Held-to-Maturity:
Amortized Cost	19.8	19.8
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	(0.3)
Estimated Fair Value	19.8	19.5
Corporate Debt Securities [Member]
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	134.5	92.9
Gross Unrealized Gains	1.2	0
Gross Unrealized Losses	0	(1)
Estimated Fair Value	135.7	91.9
Held-to-Maturity:
Amortized Cost	13.9	46.3
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	0	(0.6)
Estimated Fair Value	14	45.8
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	6	24.1
Available-for-Sale:
Amortized Cost	2,303.6	1,566.4
Gross Unrealized Gains	19.6	2.5
Gross Unrealized Losses	(6)	(24.1)
Estimated Fair Value	2,317.2	1,544.8
Held-to-Maturity:
Amortized Cost	1.2	189.7
Gross Unrealized Gains	0	0.3
Gross Unrealized Losses	0	(6.5)
Estimated Fair Value	1.2	183.5
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	0.4	1.8
Available-for-Sale:
Amortized Cost	47.6	72
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(0.4)	(1.8)
Estimated Fair Value	47.2	70.2
Other Debt Obligations [Member]
Available-for-Sale:
Amortized Cost	3.2	2
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	3.2	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	28
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	181.2
Investment Securities, Primarily Classified As Available-For-Sale, Debt Maturities, Rolling Year Two Through Five, Fair Value	181.1
Held-to-Maturity:
Investment Securities, Callable Structured, Amortized Cost Basis	$ 0

Investment Securities - Realiz

Investment Securities - Realized Gains (Losses) on Investments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Gross realized gains	$ 0		$ 1.1
Gross realized losses		$ 0	$ 0.4
Schedule of Available-for-sale Securities
Less than 12 Months	925.4	1,123.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	5.4	22.6
12 Months or More	202.5	779
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1.8	14.5
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,127.9	1,902.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	7.2	37.1
US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities
Less than 12 Months	185.3	363.1
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0.8	7.9
12 Months or More	0	154.5
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	2.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	185.3	517.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0.8	10.2
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities
Less than 12 Months	740.1	735.2
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	4.6	14.5
12 Months or More	155.9	503.7
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1.4	9.6
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	896	1,238.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	6	24.1
State, county and municipal securities
Schedule of Available-for-sale Securities
General obligation securities, amortized cost	45.9
State, county and municipal securities | Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming
Schedule of Available-for-sale Securities
General obligation securities, amortized cost	28
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	46.6	69.4
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0.4	1.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	46.6	69.4
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 0.4	1.8
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities
Less than 12 Months		24.9
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		0.2
12 Months or More		51.4
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		0.8
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		76.3
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss		$ 1

Investment Securities - Gross

Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details)	12 Months Ended
	Dec. 31, 2019USD ($)Investments	Jan. 01, 2019USD ($)	Dec. 31, 2018USD ($)Investments
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	$ 92,300,000	$ 281,100,000	$ 406,800,000
Fair Value
Less than 12 Months			90,400,000
12 Months or More			216,900,000
Gross Unrealized Losses
Less than 12 Months			(2,800,000)
12 Months or More			(5,500,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value			307,300,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss			$ (8,300,000)
Available-for-Sale and Held-to-Maturity:
Investment securities in an unrealized loss position (number of securities) | Investments	338		760
Impairment losses	$ 0
Gross Unrealized Losses
Less than 12 Months	(5,400,000)		$ (22,600,000)
12 Months or More	202,500,000		779,000,000
12 Months or More	(1,800,000)		(14,500,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,127,900,000		1,902,200,000
Total	(7,200,000)		(37,100,000)
Less than 12 Months	925,400,000		1,123,200,000
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	1,200,000		189,700,000
Fair Value
Less than 12 Months			45,000,000
12 Months or More			120,200,000
Gross Unrealized Losses
Less than 12 Months			(2,200,000)
12 Months or More			(4,300,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value			165,200,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss			(6,500,000)
Gross Unrealized Losses
Less than 12 Months	(4,600,000)		(14,500,000)
12 Months or More	155,900,000		503,700,000
12 Months or More	(1,400,000)		(9,600,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	896,000,000		1,238,900,000
Total	(6,000,000)		(24,100,000)
Less than 12 Months	740,100,000		735,200,000
US Government Agencies Debt Securities [Member]
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	19,800,000		19,800,000
Fair Value
Less than 12 Months			19,500,000
12 Months or More			0
Gross Unrealized Losses
Less than 12 Months			(300,000)
12 Months or More			0
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value			19,500,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss			(300,000)
Gross Unrealized Losses
Less than 12 Months	(800,000)		(7,900,000)
12 Months or More	0		154,500,000
12 Months or More	0		(2,300,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	185,300,000		517,600,000
Total	(800,000)		(10,200,000)
Less than 12 Months	185,300,000		363,100,000
Corporate Debt Securities [Member]
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	13,900,000		46,300,000
Fair Value
Less than 12 Months			0
12 Months or More			39,600,000
Gross Unrealized Losses
Less than 12 Months			0
12 Months or More			(600,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value			39,600,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss			(600,000)
Gross Unrealized Losses
Less than 12 Months			(200,000)
12 Months or More			51,400,000
12 Months or More			(800,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value			76,300,000
Total			(1,000,000)
Less than 12 Months			24,900,000
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Gross Unrealized Losses
Less than 12 Months	0		0
12 Months or More	46,600,000		69,400,000
12 Months or More	(400,000)		(1,800,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	46,600,000		69,400,000
Total	(400,000)		(1,800,000)
Less than 12 Months	0		0
State, county and municipal securities
Investments, Debt and Equity Securities [Abstract]
Debt Securities, Held-to-maturity	$ 57,300,000		150,900,000
Fair Value
Less than 12 Months			25,900,000
12 Months or More			57,100,000
Gross Unrealized Losses
Less than 12 Months			(300,000)
12 Months or More			(600,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value			83,000,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss			$ (900,000)

Investment Securities - Maturi

Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
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	$ 677.3
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Amortized Cost	1,486.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Amortized Cost	328
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Amortized Cost	454
Amortized Cost	2,945.5		$ 2,305.2
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	661.4
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	1,470.9
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	428.2
Debt Securities, Available-for-sale, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	399.5
Estimated Fair Value	2,960		2,270.7
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	38.6
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after One Through Five Years, Amortized Cost	29.7
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after Five Through Ten Years, Amortized Cost	21.3
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, after 10 Years, Amortized Cost	2.7
Amortized Cost	92.3	$ 281.1	406.8
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	38.6
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after One Through Five Years, Fair Value	30.1
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after Five Through Ten Years, Fair Value	23.1
Debt Securities, Held-to-maturity, Maturity, Allocated and Single Maturity Date, Rolling after 10 Years, Fair Value	2.7
Estimated Fair Value	94.5	$ 275.1	400.7
Available-for-sale Securities and Held-to-maturity Securities
Securities sold under agreements to repurchase	2,132		1,943.1
Pledged assets separately reported, securities pledged for repurchase agreements, at fair value	2,144.9		1,908.4
Mortgage Backed Securities, Variable Rate
Available-for-sale Securities and Held-to-maturity Securities
Available-for-sale securities, amortized cost basis	$ 298.1		$ 219.5

Loans - Schedule of Loans by C

Loans - Schedule of Loans by Class (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Loans and Leases Receivable
Loans held for investment	$ 8,930.7	$ 8,470.4
Mortgage loans held for sale	100.9	33.3
Total loans	9,031.6	8,503.7
Commercial
Loans and Leases Receivable
Loans held for investment	$ 3,484.7	3,235.4
Loans receivable, maturity	5 years
Residential
Loans and Leases Receivable
Loans held for investment	$ 244.1	242.8
Commercial
Loans and Leases Receivable
Loans held for investment	431.5	274.3
Total construction loans
Loans and Leases Receivable
Loans held for investment	977.7	838.7
Residential
Loans and Leases Receivable
Loans held for investment	$ 1,546.1	1,542
Loans receivable, maturity	15 years
Agricultural
Loans and Leases Receivable
Loans held for investment	$ 226.6	217.4
Total real estate loans
Loans and Leases Receivable
Loans held for investment	6,235.1	5,833.5
Indirect consumer
Loans and Leases Receivable
Loans held for investment	784.6	787.8
Other consumer
Loans and Leases Receivable
Loans held for investment	179	200.6
Credit card consumer
Loans and Leases Receivable
Loans held for investment	81.6	81.8
Total consumer loans
Loans and Leases Receivable
Loans held for investment	1,045.2	1,070.2
Commercial
Loans and Leases Receivable
Loans held for investment	1,371.3	1,310.3
Agricultural
Loans and Leases Receivable
Loans held for investment	279.1	254.8
Other, including overdrafts
Loans and Leases Receivable
Loans held for investment	0	1.6
Loans held for investment
Loans and Leases Receivable
Loans held for investment	8,930.7	8,470.4
Mortgage loans originated for sale
Loans and Leases Receivable
Mortgage loans held for sale	100.9	33.3
Total loans
Loans and Leases Receivable
Total loans	9,031.6	8,503.7
Home equity line of credit
Loans and Leases Receivable
Loans held for investment	$ 423.5	$ 409.5
Loans receivable, loan to value limit	80.00%

Loans - Schedule of Outstandin

Loans - Schedule of Outstanding Unpaid Principal Balance and Accrual Status of Loans Acquired With Credit Impairment (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Deteriorated Loans Transferred in [Abstract]
Outstanding principal	$ 49.8	$ 43.4
Loans on accrual status	36.8	30.2
Total carrying value	$ 36.8	$ 30.2

Loans - Schedule of Changes in

Loans - Schedule of Changes in Accretable Yield for Loans Acquired with Credit Impairment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Rollforward [Abstract]
Beginning Balance	$ 8.9	$ 7.3	$ 6.8
Acquisitions	3.4	3.2	1.9
Additions	(3.4)	(3.1)	(2.9)
Accretion income	0	0.6	0.1
Reductions due to exit events	(1.4)	(1.1)	(1.5)
Reclassifications from nonaccretable differences	2.5	2	2.9
Ending Balance	$ 10	$ 8.9	$ 7.3

Loans - Schedule of Recorded I

Loans - Schedule of Recorded Investment in Past Due Loans by Class (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Loans and Leases Receivable Recorded Investment, Past Due
Impaired Loans, interest lost on nonaccrual loans	$ 3.5
Loans held for investment		$ 8,930.7	$ 8,470.4
Total loans		9,031.6	8,503.7
Commercial
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		7.2	12.2
Current Loans		3,464.5	3,214
Non-accrual Loans		13	9.2
Loans held for investment		3,484.7	3,235.4
Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		1.8	1.9
Current Loans		298.9	316
Non-accrual Loans		1.4	3.7
Loans held for investment		302.1	321.6
Residential
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		2.3	1.4
Current Loans		241.8	240.4
Non-accrual Loans		0	1
Loans held for investment		244.1	242.8
Commercial
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		0	0.4
Current Loans		431	273.7
Non-accrual Loans		0.5	0.2
Loans held for investment		431.5	274.3
Total construction loans
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		4.1	3.7
Current Loans		971.7	830.1
Non-accrual Loans		1.9	4.9
Loans held for investment		977.7	838.7
Residential
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		6.3	10.1
Current Loans		1,535.2	1,525.3
Non-accrual Loans		4.6	6.6
Loans held for investment		1,546.1	1,542
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		1.3	2.2
Current Loans		220.1	202.6
Non-accrual Loans		5.2	12.6
Loans held for investment		226.6	217.4
Total real estate loans
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		18.9	28.2
Current Loans		6,191.5	5,772
Non-accrual Loans		24.7	33.3
Loans held for investment		6,235.1	5,833.5
Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		10	9.3
Current Loans		773	776.8
Non-accrual Loans		1.6	1.7
Loans held for investment		784.6	787.8
Other consumer
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		1.8	2
Current Loans		176.7	198.1
Non-accrual Loans		0.5	0.5
Loans held for investment		179	200.6
Credit card consumer
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		2.1	2.1
Current Loans		79.5	79.7
Non-accrual Loans		0	0
Loans held for investment		81.6	81.8
Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		13.9	13.4
Current Loans		1,029.2	1,054.6
Non-accrual Loans		2.1	2.2
Loans held for investment		1,045.2	1,070.2
Commercial
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		9.7	10.8
Current Loans		1,347.9	1,283.7
Non-accrual Loans		13.7	15.8
Loans held for investment		1,371.3	1,310.3
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		1	2.4
Current Loans		275.7	249.4
Non-accrual Loans		2.4	3
Loans held for investment		279.1	254.8
Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		0	0
Current Loans		0	1.6
Non-accrual Loans		0	0
Loans held for investment		0	1.6
Loans held for investment
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		43.5	54.8
Current Loans		8,844.3	8,361.3
Non-accrual Loans		42.9	54.3
Loans held for investment		8,930.7	8,470.4
Mortgage loans originated for sale
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		0	0
Current Loans		100.9	33.3
Non-accrual Loans		0	0
Total loans
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		43.5	54.8
Current Loans		8,945.2	8,394.6
Non-accrual Loans		42.9	54.3
Total loans		9,031.6	8,503.7
Financing Receivables, 30 to 59 Days Past Due | Commercial
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		5.5	10.4
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		0.7	1.6
Financing Receivables, 30 to 59 Days Past Due | Residential
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		1.5	1
Financing Receivables, 30 to 59 Days Past Due | Commercial
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		0	0.4
Financing Receivables, 30 to 59 Days Past Due | Total construction loans
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		2.2	3
Financing Receivables, 30 to 59 Days Past Due | Residential
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		3.8	8.8
Financing Receivables, 30 to 59 Days Past Due | Agricultural
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		0.8	2.2
Financing Receivables, 30 to 59 Days Past Due | Total real estate loans
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		12.3	24.4
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		7.6	6.8
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		1.2	1.4
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		0.8	0.9
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		9.6	9.1
Financing Receivables, 30 to 59 Days Past Due | Commercial
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		4.8	8.3
Financing Receivables, 30 to 59 Days Past Due | Agricultural
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		0.9	2.1
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		27.6	43.9
Financing Receivables, 30 to 59 Days Past Due | Mortgage loans originated for sale
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 | Total loans
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		27.6	43.9
Financing Receivables, 60 to 89 Days Past Due | Commercial
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		1.1	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		0.8	0.1
Financing Receivables, 60 to 89 Days Past Due | Residential
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		0.8	0.4
Financing Receivables, 60 to 89 Days Past Due | Commercial
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 | Total construction loans
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		1.6	0.5
Financing Receivables, 60 to 89 Days Past Due | Residential
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		1.4	1.1
Financing Receivables, 60 to 89 Days Past Due | Agricultural
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		0.5	0
Financing Receivables, 60 to 89 Days Past Due | Total real estate loans
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		4.6	2.6
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		1.9	2.1
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		0.5	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		0.5	0.4
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		2.9	3
Financing Receivables, 60 to 89 Days Past Due | Commercial
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		2.6	1.2
Financing Receivables, 60 to 89 Days Past Due | Agricultural
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		0.1	0.3
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		10.2	7.1
Financing Receivables, 60 to 89 Days Past Due | Mortgage loans originated for sale
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 | Total loans
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		10.2	7.1
Financing Receivables, Equal to Greater than 90 Days Past Due | Commercial
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 | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		0.3	0.2
Financing Receivables, Equal to Greater than 90 Days Past Due | Residential
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
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 | Total construction loans
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		0.3	0.2
Financing Receivables, Equal to Greater than 90 Days Past Due | Residential
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		1.1	0.2
Financing Receivables, Equal to Greater than 90 Days Past Due | Agricultural
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 | Total real estate loans
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		2	1.2
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	0.4
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		0.1	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		0.8	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		1.4	1.3
Financing Receivables, Equal to Greater than 90 Days Past Due | Commercial
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		2.3	1.3
Financing Receivables, Equal to Greater than 90 Days Past Due | Agricultural
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 | 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		5.7	3.8
Financing Receivables, Equal to Greater than 90 Days Past Due | Mortgage loans originated for sale
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 | Total loans
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due		$ 5.7	$ 3.8

Loans - Schedule of Recorded_2

Loans - Schedule of Recorded Investment in Impaired Loans (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Loans and Leases Receivable, Impaired, Troubled Debt, Interest Income	$ 3	$ 2.5
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance		87.4	$ 107.9		$ 83
Recorded Investment With No Allowance		37.2	51.4		36
Recorded Investment With Allowance		27.5	31.8		28.8
Total Recorded Investment		64.7	83.2		64.8
Related Allowance		3.6	10.8		6.8
Average Recorded Investment	74	64.7		$ 94
Income Recognized	0.3	0.3		0.5
Impaired Loans, interest lost on nonaccrual loans			3.5
Commercial
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance		29.2	45.6		22.2
Recorded Investment With No Allowance		12.9	20.9		8.6
Recorded Investment With Allowance		10.8	14.1		7.7
Total Recorded Investment		23.7	35		16.3
Related Allowance		0.7	3.9		0.7
Land Acquisition & Development Construction Financing Receivable [Member]
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance			10
Recorded Investment With No Allowance			3.4
Recorded Investment With Allowance			0.5
Total Recorded Investment			3.9
Related Allowance			0
Land Acquisition And Development Construction Financing Receivable [Member]
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance		9.2			10
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
Residential
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance		0.1	1.8		1.1
Recorded Investment With No Allowance		0	1.7		0.6
Recorded Investment With Allowance		0	0		0.4
Total Recorded Investment		0	1.7		1
Related Allowance		0	0		0.1
Commercial
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance		1	4.7		0.7
Recorded Investment With No Allowance		0.5	0.4		0.2
Recorded Investment With Allowance		0	3.5		0
Total Recorded Investment		0.5	3.9		0.2
Related Allowance		0.1	2.2		0
Total construction loans
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance		10.3	16.5		11.8
Recorded Investment With No Allowance		0.9	5.5		1.2
Recorded Investment With Allowance		2.6	4		3.9
Total Recorded Investment		3.5	9.5		5.1
Related Allowance		0.6	2.2		0.3
Residential
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance		6.9	11.5		8.8
Recorded Investment With No Allowance		3.9	8.2		5.7
Recorded Investment With Allowance		1.8	2		2
Total Recorded Investment		5.7	10.2		7.7
Related Allowance		0.2	0.1		0.3
Agricultural
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance		8.6	3.7		12.9
Recorded Investment With No Allowance		5.2	3.6		12.5
Recorded Investment With Allowance		3	0		0.2
Total Recorded Investment		8.2	3.6		12.7
Related Allowance		0.2	0		0
Total real estate loans
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance		55	77.3		55.7
Recorded Investment With No Allowance		22.9	38.2		28
Recorded Investment With Allowance		18.2	20.1		13.8
Total Recorded Investment		41.1	58.3		41.8
Related Allowance		1.7	6.2		1.3
Average Recorded Investment	50	41.4		63.5
Income Recognized	0.1	0.1		0.3
Commercial
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance		25.5	29.5		24.1
Recorded Investment With No Allowance		12	12.4		5.5
Recorded Investment With Allowance		5.3	11.4		14.4
Total Recorded Investment		17.3	23.8		19.9
Related Allowance		1.7	4.4		5.2
Average Recorded Investment	21.9	18.7		28.1
Income Recognized	0.2	0.2		0.2
Agricultural
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance		6.9	1.1		3.2
Recorded Investment With No Allowance		2.3	0.8		2.5
Recorded Investment With Allowance		4	0.3		0.6
Total Recorded Investment		6.3	1.1		3.1
Related Allowance		0.2	$ 0.2		$ 0.3
Average Recorded Investment	2.1	4.6		2.4
Income Recognized	$ 0	$ 0		$ 0

Loans - Schedule of Loans Rene

Loans - Schedule of Loans Renegotiated in Troubled Debt Restructurings (Details)	3 Months Ended	12 Months Ended
	Dec. 31, 2019USD ($)	Dec. 31, 2019USD ($)note	Dec. 31, 2018USD ($)note	Dec. 31, 2017USD ($)note	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Troubled Debt Restructuring, Commitment to Lend	$ 0	$ 0		$ 0
Loans renegotiated in troubled debt restructurings	24,900,000	24,900,000	$ 23,400,000
Loans renegotiated in troubled debt restructurings, non-accrual loans	19,400,000	19,400,000	17,800,000
Loans renegotiated in troubled debt restructurings, accrual loans	5,500,000	$ 5,500,000	$ 5,600,000
Number of notes under troubled debt restructurings | note		11	5	24
Concessions in troubled debt restructurings, amount			$ 4,100,000	$ 12,900,000
Charge-offs directly related to modified loans	$ 0				$ 0	$ 0
Schedule of Recorded Investment in Past Due Loans by Class		The following tables present the contractual aging of the Company’s recorded investment in past due loans by class as of the period indicated: 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 Loans Real estate Commercial $ 5.5 $ 1.1 $ 0.6 $ 7.2 $ 3,464.5 $ 13.0 $ 3,484.7 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 1,535.2 4.6 1,546.1 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 6,191.5 24.7 6,235.1 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 1,347.9 13.7 1,371.3 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 8,844.3 42.9 8,930.7 Mortgage loans originated for sale — — — — 100.9 — 100.9 Total loans $ 27.6 $ 10.2 $ 5.7 $ 43.5 $ 8,945.2 $ 42.9 $ 9,031.6 Total Loans 30 - 59 60 - 89 > 90 30 or More Days Days Days Days Current Non-accrual Total As of December 31, 2018 Past Due Past Due Past Due Past Due Loans Loans Loans Real estate Commercial $ 10.4 $ 1.0 $ 0.8 $ 12.2 $ 3,214.0 $ 9.2 $ 3,235.4 Construction: Land acquisition & development 1.6 0.1 0.2 1.9 316.0 3.7 321.6 Residential 1.0 0.4 — 1.4 240.4 1.0 242.8 Commercial 0.4 — — 0.4 273.7 0.2 274.3 Total construction loans 3.0 0.5 0.2 3.7 830.1 4.9 838.7 Residential 8.8 1.1 0.2 10.1 1,525.3 6.6 1,542.0 Agricultural 2.2 — — 2.2 202.6 12.6 217.4 Total real estate loans 24.4 2.6 1.2 28.2 5,772.0 33.3 5,833.5 Consumer: Indirect consumer 6.8 2.1 0.4 9.3 776.8 1.7 787.8 Other consumer 1.4 0.5 0.1 2.0 198.1 0.5 200.6 Credit card 0.9 0.4 0.8 2.1 79.7 — 81.8 Total consumer loans 9.1 3.0 1.3 13.4 1,054.6 2.2 1,070.2 Commercial 8.3 1.2 1.3 10.8 1,283.7 15.8 1,310.3 Agricultural 2.1 0.3 — 2.4 249.4 3.0 254.8 Other, including overdrafts — — — — 1.6 — 1.6 Loans held for investment 43.9 7.1 3.8 54.8 8,361.3 54.3 8,470.4 Mortgage loans originated for sale — — — — 33.3 — 33.3 Total loans $ 43.9 $ 7.1 $ 3.8 $ 54.8 $ 8,394.6 $ 54.3 $ 8,503.7
Schedule of Loans Renegotiated in Troubled Debt Restructurings		The following table presents information on the Company’s troubled debt restructurings that occurred during the periods indicated: Number of Notes Type of Concession Principal Balance at Restructure Date Year Ended December 31, 2019 Interest only period Extension of terms or maturity Interest rate adjustment Other 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 11 $ 0.2 $ 0.2 $ — $ 10.0 $ 10.4 Number of Notes Type of Concession Principal Balance at Restructure Date Year Ended December 31, 2018 Interest only period Extension of terms or maturity Interest rate adjustment Other 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 5 $ 3.6 $ — $ — $ 0.5 $ 4.1 Number of Notes Type of Concession Principal Balance at Restructure Date Year Ended December 31, 2017 Interest only period Extension of terms or maturity Interest rate adjustment Other Commercial 17 $ 1.2 $ 2.0 $ — $ 6.0 $ 9.2 Agriculture 1 — 0.1 — — 0.1 Total loans restructured 24 $ 2.7 $ 3.3 $ — $ 6.9 $ 12.9
Financing Receivable, Troubled Debt Restructuring, Postmodification		$ 10,400,000
Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note		4	3
Concessions in troubled debt restructurings, amount			$ 3,600,000
Financing Receivable, Troubled Debt Restructuring, Postmodification		$ 3,300,000
Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note			1
Concessions in troubled debt restructurings, amount			$ 200,000
Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note			1
Concessions in troubled debt restructurings, amount			$ 300,000
Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note		1		17
Concessions in troubled debt restructurings, amount		$ 5,000,000		$ 9,200,000
Balance of notes for which there was a payment default				$ 1,300,000
Agriculture
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note		6
Concessions in troubled debt restructurings, amount		$ 2,100,000
Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note				1
Concessions in troubled debt restructurings, amount				$ 100,000
Interest Only Period Concession
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			3,600,000	2,700,000
Financing Receivable, Troubled Debt Restructuring, Postmodification		200,000
Interest Only Period Concession | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			3,600,000
Financing Receivable, Troubled Debt Restructuring, Postmodification		200,000
Interest Only Period Concession | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Interest Only Period Concession | Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Interest Only Period Concession | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		0		1,200,000
Interest Only Period Concession | Agriculture
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		0
Interest Only Period Concession | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount				0
Extension of terms or maturity
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0	3,300,000
Financing Receivable, Troubled Debt Restructuring, Postmodification		200,000
Extension of terms or maturity | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Financing Receivable, Troubled Debt Restructuring, Postmodification		200,000
Extension of terms or maturity | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Extension of terms or maturity | Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Extension of terms or maturity | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		0		2,000,000
Extension of terms or maturity | Agriculture
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		0
Extension of terms or maturity | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount				100,000
Interest rate adjustment
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0	0
Financing Receivable, Troubled Debt Restructuring, Postmodification		0
Interest rate adjustment | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Financing Receivable, Troubled Debt Restructuring, Postmodification		0
Interest rate adjustment | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Interest rate adjustment | Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Interest rate adjustment | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		0		0
Interest rate adjustment | Agriculture
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		0
Interest rate adjustment | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount				0
Other
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			500,000	6,900,000
Financing Receivable, Troubled Debt Restructuring, Postmodification		10,000,000
Other | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Financing Receivable, Troubled Debt Restructuring, Postmodification		2,900,000
Other | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			200,000
Other | Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			$ 300,000
Other | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		5,000,000		6,000,000
Other | Agriculture
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		$ 2,100,000
Other | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount				$ 0

Loans - Schedule of Recorded_3

Loans - Schedule of Recorded Investment in Criticized Loans by Class and Credit Quality Indicator (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	$ 8,930.7	$ 8,470.4
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3,484.7	3,235.4
Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	302.1	321.6
Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	244.1	242.8
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	431.5	274.3
Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	977.7	838.7
Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,546.1	1,542
Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	226.6	217.4
Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	6,235.1	5,833.5
Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	784.6	787.8
Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	179	200.6
Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,045.2	1,070.2
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,371.3	1,310.3
Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	279.1	254.8
Other Assets Especial Mentioned
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	157.5	178.8
Other Assets Especial Mentioned | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	84.7	102.5
Other Assets Especial Mentioned | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3.8	5
Other Assets Especial Mentioned | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.9	2.8
Other Assets Especial Mentioned | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1.7	1.7
Other Assets Especial Mentioned | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	6.4	9.5
Other Assets Especial Mentioned | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2.6	3
Other Assets Especial Mentioned | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	14.3	9
Other Assets Especial Mentioned | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	108	124
Other Assets Especial Mentioned | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.2	0.7
Other Assets Especial Mentioned | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.4	0.3
Other Assets Especial Mentioned | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.6	1
Other Assets Especial Mentioned | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	40.4	39.4
Other Assets Especial Mentioned | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	8.5	14.4
Substandard
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	224	201.6
Substandard | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	97.3	87.4
Substandard | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1.9	7
Substandard | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2.2	2
Substandard | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1.5	3.9
Substandard | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	5.6	12.9
Substandard | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	7.8	10.8
Substandard | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	26.6	24
Substandard | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	137.3	135.1
Substandard | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2.9	2.1
Substandard | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.8	0.8
Substandard | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3.7	2.9
Substandard | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	60.3	45.8
Substandard | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	22.7	17.8
Doubtful
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	5.9	20.9
Doubtful | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.8	2.9
Doubtful | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1	3.3
Doubtful | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0	0.4
Doubtful | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0	0
Doubtful | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1	3.7
Doubtful | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.3	0.7
Doubtful | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0	0.1
Doubtful | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2.1	7.4
Doubtful | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0	0.1
Doubtful | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.1	0.1
Doubtful | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.1	0.2
Doubtful | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3.6	11.8
Doubtful | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0.1	1.5
Total Criticized Loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	387.4	401.3
Total Criticized Loans | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	182.8	192.8
Total Criticized Loans | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	6.7	15.3
Total Criticized Loans | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3.1	5.2
Total Criticized Loans | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3.2	5.6
Total Criticized Loans | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	13	26.1
Total Criticized Loans | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	10.7	14.5
Total Criticized Loans | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	40.9	33.1
Total Criticized Loans | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	247.4	266.5
Total Criticized Loans | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3.1	2.9
Total Criticized Loans | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1.3	1.2
Total Criticized Loans | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	4.4	4.1
Total Criticized Loans | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	104.3	97
Total Criticized Loans | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	$ 31.3	$ 33.7

Allowance for Loan Losses - Sc

Allowance for Loan Losses - Schedule of Allowance for Loan Losses by Portfolio Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 73	$ 72.1	$ 76.2
Provision charged (credited) to operating expense	(13.9)	(8.6)	(11)
Less loans charged-off	(23.6)	(19.7)	(22.8)
Add back recoveries of loans previously charged-off	9.7	12	7.7
Ending balance	73	73	72.1
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				$ 3.6	$ 6.8	$ 10.8
Collectively evaluated for impairment				69.4	66.2	61.3
Ending balance	73	73	76.2	73	73	72.1
Total loans:
Individually evaluated for impairment				64.7	64.8	83.2
Collectively evaluated for impairment				8,866	8,405.6	7,484.5
Total loans held for investment				8,930.7	8,470.4	7,567.7
Allowance for loans losses				1	0.8
Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	31	31.7	28.6
Provision charged (credited) to operating expense	(1.3)	(0.7)	(6)
Less loans charged-off	(3.5)	(3.7)	(4.3)
Add back recoveries of loans previously charged-off	2.7	3.7	1.4
Ending balance	28.9	31	31.7
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				1.7	1.3	6.2
Collectively evaluated for impairment				27.2	29.7	25.5
Ending balance	31	31.7	31.7	28.9	31	31.7
Total loans:
Individually evaluated for impairment				41.1	41.8	58.3
Collectively evaluated for impairment				6,194	5,791.7	5,118.5
Total loans held for investment				6,235.1	5,833.5	5,176.8
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	8.7	8.7	7.7
Provision charged (credited) to operating expense	(10.6)	(6.8)	(8.1)
Less loans charged-off	(13)	(11.3)	(11.3)
Add back recoveries of loans previously charged-off	3.6	4.5	4.2
Ending balance	9.9	8.7	8.7
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				0	0	0
Collectively evaluated for impairment				9.9	8.7	8.7
Ending balance	9.9	8.7	8.7	9.9	8.7	8.7
Total loans:
Individually evaluated for impairment				0	0	0
Collectively evaluated for impairment				1,045.2	1,070.2	1,034.4
Total loans held for investment				1,045.2	1,070.2	1,034.4
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	31.3	30.5	38.1
Provision charged (credited) to operating expense	(4.5)	(1.9)	(2.9)
Less loans charged-off	(6.6)	(4.7)	(6.8)
Add back recoveries of loans previously charged-off	3.4	3.6	2.1
Ending balance	32.6	31.3	30.5
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				1.7	5.2	4.4
Collectively evaluated for impairment				30.9	26.1	26.1
Ending balance	31.3	31.3	38.1	32.6	31.3	30.5
Total loans:
Individually evaluated for impairment				17.3	19.9	23.8
Collectively evaluated for impairment				1,354	1,290.4	1,191.6
Total loans held for investment				1,371.3	1,310.3	1,215.4
Agriculture
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	2	1.2	1.8
Provision charged (credited) to operating expense	(0.1)	(0.6)	(0.2)
Less loans charged-off	(0.5)	0	(0.4)
Add back recoveries of loans previously charged-off	0	0.2	0
Ending balance	1.6	2	1.2
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				0.2	0.3	0.2
Collectively evaluated for impairment				1.4	1.7	1
Ending balance	2	1.2	1.2	1.6	2	1.2
Total loans:
Individually evaluated for impairment				6.3	3.1	1.1
Collectively evaluated for impairment				272.8	251.7	135.1
Total loans held for investment				279.1	254.8	136.2
Other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	0	0	0
Provision charged (credited) to operating expense	0	0	0
Less loans charged-off	0	0	0
Add back recoveries of loans previously charged-off	0	0	0
Ending balance	0	0	0
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				0	0	0
Collectively evaluated for impairment				0	0	0
Ending balance	$ 0	$ 0	$ 0	0	0	0
Total loans:
Individually evaluated for impairment				0	0	0
Collectively evaluated for impairment				0	1.6	4.9
Total loans held for investment				$ 0	$ 1.6	$ 4.9

Premises and Equipment (Details

Premises and Equipment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment
Depreciation	$ 23.3	$ 16.2	$ 14.8
Premises and equipment, gross	475.1	422
Less accumulated depreciation	(169.1)	(176.8)
Premises and equipment, net	306	245.2
Land
Property, Plant and Equipment
Premises and equipment, gross	50.8	49.9
Buildings and improvements
Property, Plant and Equipment
Premises and equipment, gross	330.1	268.1
Furniture and equipment
Property, Plant and Equipment
Premises and equipment, gross	$ 94.2	$ 104

Company-Owned Life Insurance (D

Company-Owned Life Insurance (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Schedule of Company - Owned Life Insurance, By Type
Total	$ 293.8	$ 275.1
Key executive, principal shareholder
Schedule of Company - Owned Life Insurance, By Type
Cash surrender value of life insurance	4.4	4.7
Key executive split dollar
Schedule of Company - Owned Life Insurance, By Type
Cash surrender value of life insurance	7	9.4
FIB | Group life
Schedule of Company - Owned Life Insurance, By Type
Cash surrender value of life insurance	$ 282.4	$ 261

Other Real Estate Owned (Detail

Other Real Estate Owned (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Repossessed Assets [Abstract]
Real Estate Acquired Through Foreclosure, Valuation Adjustments	$ 0.9	$ 0.1
Real Estate Acquired Through Foreclosure [Roll Forward]
Balance at beginning of year	14.4	10.1	$ 10
Acquisitions	2.4	0.6	1.2
Additions	14.1	12.1	5.4
Valuation adjustments	(0.9)	(0.1)	(0.4)
Dispositions	(21.8)	(8.3)	(6.1)
Balance at end of year	8.5	14.4	10.1
Real Estate Acquired Through Foreclosure, Improvements	0.3		0
Write-downs of OREO	$ (0.9)	$ (0.1)	$ (0.4)

Derivatives and Hedging Activ_3

Derivatives and Hedging Activities - Notional Amounts and Estimated Fair Values of Derivatives (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative Asset, Notional Amount	$ 571	$ 454.3
Derivative Assets, Estimated Fair Value	23.2	10.1
Derivative Liability, Notional Amount	631.2	467.9
Derivative Liabilities, Estimated Fair Value	22.2	9.4
Not Designated as Hedging Instrument | Interest rate swap contracts
Derivative [Line Items]
Derivative Asset, Notional Amount	503.2	403.3
Derivative Assets, Estimated Fair Value	21.9	8.8
Derivative Liability, Notional Amount	503.2	403.3
Derivative Liabilities, Estimated Fair Value	21.9	8.8
Not Designated as Hedging Instrument | Interest rate lock commitments
Derivative [Line Items]
Derivative Asset, Notional Amount	67.8	51
Derivative Assets, Estimated Fair Value	1.3	1.3
Not Designated as Hedging Instrument | Forward loan sales contracts
Derivative [Line Items]
Derivative Liability, Notional Amount	128	64.6
Derivative Liabilities, Estimated Fair Value	$ 0.3	$ 0.6

Derivatives and Hedging Activ_4

Derivatives and Hedging Activities - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative liability, notional amount	$ 631,200,000	$ 467,900,000
Hedge ineffectiveness	$ 0

Derivatives and Hedging Activ_5

Derivatives and Hedging Activities - Schedule of Master Netting Arrangements (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Financial Assets
Gross Amounts Recognized	$ 23.2	$ 10.1
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	23.2	10.1
Financial Instruments	0.1	2.7
Fair Value of Financial Collateral in the Balance Sheet	18	2.4
Net Amount	5.1	5
Offsetting Derivative Liabilities [Abstract]
Gross Amounts Recognized	22.2	9.4
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	22.2	9.4
Financial Instruments	0.1	2.7
Fair Value of Financial Collateral in the Balance Sheet	0	4.1
Net Amount	22.1	2.6
Repurchase agreements
Gross Amounts Recognized	697.6	712.4
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	697.6	712.4
Financial Instruments	0	0
Fair Value of Financial Collateral in the Balance Sheet	697.6	712.4
Net Amount	0	0
Offsetting Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned [Abstract]
Gross Amounts Recognized	719.8	721.8
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	719.8	721.8
Financial Instruments	0.1	2.7
Fair Value of Financial Collateral in the Balance Sheet	697.6	716.5
Net Amount	22.1	2.6
Interest rate swap contracts
Financial Assets
Gross Amounts Recognized	21.9	8.8
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	21.9	8.8
Financial Instruments	0.1	2.7
Fair Value of Financial Collateral in the Balance Sheet	18	2.4
Net Amount	3.8	3.7
Offsetting Derivative Liabilities [Abstract]
Gross Amounts Recognized	21.9	8.8
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	21.9	8.8
Financial Instruments	0.1	2.7
Fair Value of Financial Collateral in the Balance Sheet	0	4.1
Net Amount	21.8	2
Mortgage related derivatives
Financial Assets
Gross Amounts Recognized	1.3	1.3
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	1.3	1.3
Financial Instruments	0	0
Fair Value of Financial Collateral in the Balance Sheet	0	0
Net Amount	1.3	1.3
Offsetting Derivative Liabilities [Abstract]
Gross Amounts Recognized	0.3	0.6
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	0.3	0.6
Financial Instruments	0	0
Fair Value of Financial Collateral in the Balance Sheet	0	0
Net Amount	$ 0.3	$ 0.6

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, 2019	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Amount of loss recognized in other comprehensive income (effective portion)			$ 1.1
Designated as Hedging Instrument
Derivative [Line Items]
Amount of loss recognized in other comprehensive income (effective portion)	$ 0	$ 0	1.1
Reclassification adjustment for derivative net (gains) losses included in income	0	0	1.1
Not Designated as Hedging Instrument
Derivative [Line Items]
Amount of gain recognized in other non-interest income	0	0.3
Amount of net fee income recognized in other non-interest income	2.5	1.3
Gain (Loss) on Fair Value Hedges Recognized in Earnings	$ 0.3	$ (0.5)
Not Designated as Hedging Instrument | Interest Rate Swap
Derivative [Line Items]
Amount of gain recognized in other non-interest income			0
Amount of net fee income recognized in other non-interest income			0.8
Amount of net gains (losses) recognized in mortgage banking revenues			$ (1.7)

Mortgage Servicing Rights - Sc

Mortgage Servicing Rights - Schedule of Mortgage Servicing Rights (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)
Servicing Asset at Amortized Value, Balance [Roll Forward]
Balance at end of year, net of valuation reserve	$ 30.2	$ 27.7
Mortgage loans
Servicing Asset at Amortized Value, Balance [Roll Forward]
Servicing asset at amortized cost, before valuation reserve, beginning	27.7	24.8	$ 18.7
Acquisitions of mortgage servicing rights	0	0	3.5
Originations of mortgage servicing rights	7.3	6	5.6
Amortization expense	(4.4)	(3.1)	(3)
Servicing asset at amortized cost, before valuation reserve, ending	30.6	27.7	24.8
Less valuation reserve	(0.4)	0	0
Balance at end of year, net of valuation reserve	30.2	27.7	24.8
Principal balance of serviced loans underlying mortgage servicing rights	$ 3,710.1	$ 3,698.2	$ 3,636.7
Mortgage servicing rights as a percentage of serviced loans	0.0081	0.0075	0.0068

Mortgage Servicing Rights - Na

Mortgage Servicing Rights - Narrative (Details) - Mortgage loans - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2016	Dec. 31, 2015
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]
Mortgage servicing rights, fair value	$ 34,800,000	$ 42,400,000
Servicing asset at fair value, assumptions used to estimate fair value, weighted average life	6 years	8 years
Impairment reversals	$ 0
Permanent impairment charged against carrying value	$ 0		$ 0	$ 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	9.30%	10.40%
Servicing assets at fair value, assumptions used to estimate fair value, prepayment speed	0.60%	0.40%
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	11.00%	12.10%
Servicing assets at fair value, assumptions used to estimate fair value, prepayment speed	1.50%	1.50%

Deposits (Details)

Deposits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deposits, by Type [Abstract]
Non-interest bearing demand	$ 3,426,500,000	$ 3,158,300,000
Interest bearing:
Demand	3,195,400,000	2,957,500,000
Savings	3,591,600,000	3,247,900,000
Total	651,100,000	547,600,000
Time, other	798,900,000	769,400,000
Total interest bearing	8,237,000,000	7,522,400,000	$ 7,522,400,000	$ 7,034,900,000
Total deposits	11,663,500,000	10,680,700,000
Brokered time deposits	2,900,000	24,100,000	0
Time deposits obtained through Certificate of Deposit Account Registry Service (CDARS)	117,700,000	87,100,000
Time deposits of $250,000 or more	278,400,000	221,000,000
FDIC deposit insurance limit	250,000,000
Time, $100 and Over
Due within 3 months or less	101,600,000
Due after 3 months and within 6 months	140,800,000
Due after 6 months and within 12 months	273,700,000
Due within 2021	104,000,000
Total	651,100,000	547,600,000
Total Time
Due within 3 months or less	323,600,000
Due after 3 months and within 6 months	301,400,000
Due after 6 months and within 12 months	515,900,000
Due within 2021	220,400,000
Total	1,450,000,000
Interest expense, time deposits, $100,000 or more	11,500,000	$ 5,000,000	$ 3,500,000
Contractual Maturities, Time Deposits, $100,000 or More, after 2 years	23,600,000
Time Deposits Maturities, after Next Two Years	64,000,000
Contractual Maturities, Time Deposits, $100,000 or More, after 3 years	3,500,000
Time Deposits Maturities, after Next Three Years	13,900,000
Contractual Maturities, Time Deposits, $100,000 or More, after 5 years and thereafter	3,900,000
Time Deposits Maturities, after Next Five Years and Thereafter	$ 10,800,000

Long-Term Debt and Other Borr_3

Long-Term Debt and Other Borrowed Funds (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument
Long-term debt and capital lease obligations	$ 13,900,000	$ 15,800,000
Fiscal Year Maturities of Long-term Debt
2018	100,000
2019	100,000
2020	5,100,000
2021	100,000
2022	100,000
Thereafter	8,400,000
Long-term debt and capital lease obligations	13,900,000	$ 15,800,000
Advances from FHLB	0		$ 0
Other borrowings	0
Subordinated term loan | Subordinated Term Loan, 6.81% | Parent Company
Debt Instrument
Stated interest rate			6.81%
Capital lease obligations | Capital Lease Obligation, 8.00% | Subsidiaries
Debt Instrument
Stated interest rate		8.00%
Capital Lease Obligations	1,200,000	$ 1,300,000
Notes payable | Subsidiaries
Fiscal Year Maturities of Long-term Debt
Long Term Debt, Acquired or Assumed	12,600,000	$ 14,400,000
Notes payable | Note Payable, 6.24% | Subsidiaries
Debt Instrument
Stated interest rate		6.24%
Long-term debt	0	$ 1,800,000
Notes payable | Note Payable, 2.28% | Subsidiaries
Debt Instrument
Stated interest rate		2.28%
Long-term debt	5,000,000	$ 5,000,000
Notes payable | Note Payable, 1.00% | Subsidiaries
Debt Instrument
Stated interest rate		1.00%
Long-term debt	5,100,000	$ 5,100,000
Notes payable | Note Payable, 1.30%, maturing March 31, 2038 [Member] | Subsidiaries
Debt Instrument
Long-term debt	2,000,000	2,000,000
Notes payable | Note Payable, 1.30%, Maturing June 1, 2034 [Member] | Subsidiaries
Debt Instrument
Long-term debt	600,000	$ 600,000
Federal Home Loan Bank | Notes Payable to FHLB | Subsidiaries
Fiscal Year Maturities of Long-term Debt
FHLB, advances, maximum amount available	$ 1,616,000,000
Secured Debt
Fiscal Year Maturities of Long-term Debt
Ownership percentage in subsidiary	99.90%
Line of credit facility, maximum borrowing capacity	$ 455,600,000
Federal Funds Purchased
Fiscal Year Maturities of Long-term Debt
Line of credit facility, maximum borrowing capacity	205,000,000
Unsecured Debt [Member]
Fiscal Year Maturities of Long-term Debt
Line of credit facility, maximum borrowing capacity	$ 50,000,000
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	6.24%

Subordinated Debentures Held _3

Subordinated Debentures Held by Subsidiary Trusts (Details) $ in Millions	Aug. 16, 2018USD ($)	Dec. 31, 2007USD ($)	Dec. 31, 2019USD ($)trusts	Dec. 31, 2008	Dec. 31, 2018USD ($)	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			$ 86.9		$ 86.9
Debt instrument, interest rate at period end			4.85%
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			4.35%
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			4.64%
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			4.29%
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			4.80%
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
Debt instrument, basis spread on variable rate			4.85%	2.75%
Debt instrument, interest rate at period end						1030000000.00%
Northwest Bancorporation Capital Trust 1 [Member] | Junior Subordinated Deferrable Interest Debentures Issued by FIST VI
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts			$ 4.4		$ 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			3.66%
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			$ 84.3
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	Aug. 16, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Sep. 25, 2017
Class of Stock
Common stock, shares authorized			200,000,000
Stock compensation expense			$ 8	$ 5.6	$ 3.8
Aggregate purchase price of shares repurchased and retired			$ 2.5	$ 1	$ 1.3
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			43,129,085	38,169,575
Common shares purchased and retired			0
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			22,117,254	22,453,672
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
Inland Northwest Bank [Member]
Class of Stock
Total consideration paid		$ 176.3
Class A common stock		$ 173.3
Inland Northwest Bank [Member] | Class A Common Stock
Class of Stock
Common stock issued (in shares)		3,840,000
Director [Member] | Class A Common Stock
Class of Stock
Shares issued to directors during period			22,417
Stock compensation expense			$ 0.8
Board of Directors
Class of Stock
Aggregate value of shares issued to directors				$ 0.5
Board of Directors | Class A Common Stock
Class of Stock
Shares issued to directors during period				11,389
Secondary Public Offering | Class A Common Stock
Class of Stock
Common stock, value authorized						$ 250

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, 2018	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net Income (Loss) Available to Common Stockholders, Basic [Abstract]
Net income, basic and diluted	$ 160.2		$ 181	$ 160.2	$ 106.5
Weighted Average Number of Shares Outstanding, Basic and Diluted [Abstract]
Weighted average common shares outstanding for basic earnings per share computation			63,645,029	57,778,857	51,429,366
Dilutive effect of stock-based compensation (in shares)			239,839	438,266	473,843
Weighted average common shares outstanding for diluted earnings per common share computation			63,884,868	58,217,123	51,903,209
Earnings Per Share, Basic and Diluted [Abstract]
Basic earnings per common share (in dollars per share)			$ 2.84	$ 2.77	$ 2.07
Diluted earnings per common share (in dollars per share)			$ 2.83	$ 2.75	$ 2.05
Restricted Stock [Member]
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)		448	150		113,874
Performance Shares
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)			138,298	83,475	83,635

Regulatory Capital - Schedule

Regulatory Capital - Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Total risk-based capital:
Actual amount	$ 1,495.3	$ 1,285
Capital to Risk Weighted Assets	14.10%	12.99%
Adequately capitalized Basel III phase-in schedule, amount	$ 848.5	$ 791.2
Adequately capitalized Basel III phase-in schedule, ratio (percent)	8.00%	8.00%
Adequately capitalized Basel III fully phased-in, amount	$ 1,113.6	$ 976.6
Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	10.50%	9.88%
Well capitalized, amount	$ 1,060.6	$ 989
Capital Required to be Well Capitalized to Risk Weighted Assets	10.00%	10.00%
Tier 1 risk-based capital:
Actual amount	$ 1,422.3	$ 1,212
Tier One Risk Based Capital to Risk Weighted Assets	13.41%	12.26%
Adequately capitalized Basel III phase-in schedule, amount	$ 636.3	$ 593.4
Adequately capitalized Basel III phase-in schedule, ratio (percent)	6.00%	6.00%
Adequately capitalized Basel III fully phased-in, amount	$ 901.5	$ 778.8
Tier One Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	8.50%	7.88%
Well capitalized, amount	$ 848.5	$ 791.2
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	$ 1,338.2	$ 1,127.8
Common Equity Tier One Risk Based Capital To Risk Weighted Assets	12.62%	11.40%
Adequately capitalized Basel III phase-in schedule, amount	$ 477.3	$ 445
Adequately capitalized Basel III phase-in schedule, ratio (percent)	4.50%	4.50%
Adequately capitalized Basel III fully phased-in, amount	$ 742.4	$ 630.5
Tier One Common Equity Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	7.00%	6.38%
Well capitalized, amount	$ 689.4	$ 642.8
Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets	6.50%	6.50%
Leverage capital ratio:
Actual amount	$ 1,422.3	$ 1,212
Tier One Leverage Capital to Average Assets	10.13%	9.47%
Adequately capitalized, amount	$ 561.6	$ 511.9
Adequately capitalized, ratio (percent)	4.00%	4.00%
Well capitalized, amount	$ 702	$ 639.9
Well capitalized, ratio (percent)	5.00%	5.00%
FIB
Total risk-based capital:
Actual amount	$ 1,321.4	$ 1,184.5
Capital to Risk Weighted Assets	12.50%	12.01%
Adequately capitalized Basel III phase-in schedule, amount	$ 845.8	$ 788.8
Adequately capitalized Basel III phase-in schedule, ratio (percent)	8.00%	8.00%
Adequately capitalized Basel III fully phased-in, amount	$ 1,110.1	$ 973.7
Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	10.50%	9.88%
Well capitalized, amount	$ 1,057.2	$ 986
Capital Required to be Well Capitalized to Risk Weighted Assets	10.00%	10.00%
Tier 1 risk-based capital:
Actual amount	$ 1,248.4	$ 1,111.6
Tier One Risk Based Capital to Risk Weighted Assets	11.81%	11.27%
Adequately capitalized Basel III phase-in schedule, amount	$ 634.3	$ 591.6
Adequately capitalized Basel III phase-in schedule, ratio (percent)	6.00%	6.00%
Adequately capitalized Basel III fully phased-in, amount	$ 898.6	$ 776.5
Tier One Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	8.50%	7.88%
Well capitalized, amount	$ 845.8	$ 788.8
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	$ 1,248.4	$ 1,111.6
Common Equity Tier One Risk Based Capital To Risk Weighted Assets	11.81%	11.27%
Adequately capitalized Basel III phase-in schedule, amount	$ 475.7	$ 443.7
Adequately capitalized Basel III phase-in schedule, ratio (percent)	4.50%	4.50%
Adequately capitalized Basel III fully phased-in, amount	$ 740	$ 628.6
Tier One Common Equity Capital Required For Capital Adequacy Under Basel III Fully Phased-In To Risk Weighted Assets	7.00%	6.38%
Well capitalized, amount	$ 687.2	$ 640.9
Common Equity Tier One Capital To Be Well Capitalized To Risk Weighted Assets	6.50%	6.50%
Leverage capital ratio:
Actual amount	$ 1,248.4	$ 1,111.6
Tier One Leverage Capital to Average Assets	8.91%	8.97%
Adequately capitalized, amount	$ 560.4	$ 495.9
Adequately capitalized, ratio (percent)	4.00%	4.00%
Well capitalized, amount	$ 700.4	$ 619.8
Well capitalized, ratio (percent)	5.00%	5.00%

Commitments and Contingencies

Commitments and Contingencies - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Apr. 08, 2019office
Unrecorded Unconditional Purchase Obligation
Operating leases, rent expense	$ 4.8	$ 3.2	$ 2.5
Construction Contracts
Unrecorded Unconditional Purchase Obligation
Commitments under construction contracts	$ 2.1
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	$ 0.9	$ 1.5

Commitments and Contingencies_2

Commitments and Contingencies - Operating Lease Commitments (Details) $ in Millions	Dec. 31, 2019USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018	$ 6.4
2019	6.1
2020	5.6
2021	5.2
2022	5.1
Thereafter	22.1
Operating leases, future minimum payments due	50.5
Third Parties
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018	5
2019	4.7
2020	4.4
2021	4.2
2022	4.1
Thereafter	20.5
Operating leases, future minimum payments due	42.9
Related Entity
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2018	1.4
2019	1.4
2020	1.2
2021	1
2022	1
Thereafter	1.6
Operating leases, future minimum payments due	$ 7.6

Financial Instruments with Of_2

Financial Instruments with Off-Balance Sheet Risk (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Entity Information
Credit extension commitments	$ 2,807.8	$ 2,620.4
Unused credit card lines		1,058.5
Unused Credit Card Lines
Entity Information
Credit extension commitments	764.1	726.7
Unused credit card lines	1,212.7
Standby Letters of Credit
Entity Information
Credit extension commitments	$ 42.7	$ 46.7

Income Taxes - Schedule of Inc

Income Taxes - Schedule of Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Deferred Tax Liabilities, Unrealized Gains on Trading Securities	$ (3.9)
Current:
Federal	40.5	$ 23.1	$ 24.3
State	8.2	7.2	5
Total current	48.7	30.3	29.3
Deferred:
Federal	3.7	12.3	18.4
State	1.7	3.5	2.5
Total deferred	5.4	15.8	20.9
Total income tax expense	$ 54.1	$ 46.1	$ 50.2

Income Taxes - Reconciliation

Income Taxes - Reconciliation of Tax Expense at Effective Tax Rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Expense, Continuing Operations Income Tax Reconciliation [Abstract]
Tax expense at the statutory tax rate	$ 49.4	$ 43.3	$ 54.9
Increase (decrease) in tax resulting from:
Tax-exempt income	(2.8)	(2.8)	(4.5)
State income tax, net of federal income tax benefit	9.9	8.5	4.9
Benefit of stock-based compensation plans			(2.6)
Federal tax credits	(2)	(2.6)	(2.5)
Benefit due to enactment of federal tax reform	0	0	2.2
Other, net	0.8	0.8	2.2
Total income tax expense	$ 54.1	$ 46.1	$ 50.2

Income Taxes - Deferred Tax As

Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Loans, principally due to allowance for loan losses	$ 18.1	$ 18.4
Loan discount	7.2	8.3
Investment securities, unrealized losses	0	8.9
Deferred compensation	18.1	17
Non-performing loan interest	1	1.2
Other real estate owned write-downs and carrying costs	0.1	0.3
Tax credit carryforwards	0.2	0.1
Net operating loss carryforwards	2.6	3.9
Lease liabilities	9.9	0
Other	0.9	0.1
Deferred tax assets	58.1	58.2
Deferred tax liabilities:
Fixed assets, principally differences in bases and depreciation	(8.8)	(6.9)
Deferred loan costs	(2.8)	(2.6)
Deferred Tax Liabilities, Unrealized Gains on Trading Securities	3.9
Deferred Tax Liabilities, Unrealized Gains on Available For Sale Securities		0
Investment in joint venture partnership, principally due to differences in depreciation of partnership assets	(0.7)	(0.8)
Right of use assets	(9.7)	0
Prepaid amounts	(0.5)	(0.6)
Government agency stock dividends	(1.2)	(1.5)
Goodwill and core deposit intangibles	(49.2)	(44.2)
Mortgage servicing rights	(7.2)	(6.4)
Other	(0.8)	(3.8)
Deferred tax liabilities	(84.8)	(66.8)
Deferred Tax Liabilities, Net	26.7	8.6
Operating Loss Carryforwards [Line Items]
Deferred Income Tax Expense (Benefit)	$ 5.4	$ 15.8	$ 20.9

Income Taxes - Narrative (Deta

Income Taxes - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Effective income tax rate reconciliation, at federal statutory income tax rate	21.00%	35.00%
Current net income tax receivable	$ 6.1	$ 2
Benefit due to enactment of federal tax reform	$ 0	$ 0	$ 2.2

Stock-Based Compensation - Nar

Stock-Based Compensation - Narrative (Details)	12 Months Ended
	Dec. 31, 2019USD ($)planshares	Dec. 31, 2018USD ($)shares	Dec. 31, 2017USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of stock-based compensation plans | plan	2
Options expiration period	10 years
Stock option awards granted | shares	0	0
Stock compensation expense	$ 8,000,000	$ 5,600,000	$ 3,800,000
Intrinsic value of fully-vested stock options outstanding	5,800,000
Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense		0
Tax benefits from stock-based compensation		0
Total intrinsic value of options exercised	4,900,000	4,900,000	6,300,000
Proceeds from Stock Options Exercised	1,000,000	1,800,000	2,400,000
Common Stock Redeemed For Payment of Stock Option Exercises, Aggregate Value	2,000,000	1,600,000	2,800,000
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense		5,600,000	3,300,000
Tax benefits from stock-based compensation	$ 400,000	$ 200,000	$ 600,000
Stock issued during period, shares, restricted stock award | shares	230,922
Unrecognized compensation cost related to nonvested restricted stock awards	$ 8,100,000
Unrecognized compensation cost related to nonvested restricted stock to be recognized over weighted average period (in years)	1 year 1 month 6 days
Restricted Stock [Member] | Upon Achievement of Performance Goals
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares, restricted stock award | shares	72,926
Restricted Stock [Member] | Upon Achievement of Defined Return on Equity Performance Goals
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares, restricted stock award | shares	36,463
Restricted Stock [Member] | Upon Achievement of Defined Total Return to Shareholder Goals
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares, restricted stock award | shares	36,463
Restricted Stock [Member] | Upon One-Third Annual Anniversary
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares, restricted stock award | shares	139,661
Restricted Stock [Member] | First Anniversary
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage		33.33%
Restricted Stock [Member] | Second Anniversary
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage		33.33%
Restricted Stock [Member] | Third Anniversary
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage		33.33%
2006 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for grant | shares	1,492,762

Stock-Based Compensation - Sch

Stock-Based Compensation - Schedule of Stock Options Activity (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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	1 year 10 days
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term	1 year 10 days
Stock option awards granted	0	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
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	428,176
Exercised, number of shares	(191,193)
Forfeited, number of shares	(15,786)
Outstanding options, end of year, number of shares	221,197	428,176
Outstanding options exercisable, end of year, number of shares	221,197
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)	$ 15.61
Exercised, weighted average exercise price (in dollars per share)	15.76
Forfeited, weighted average exercise price (in dollars per share)	17.05
Outstanding options, end of year, weighted average exercise price (in dollars per share)	15.33	$ 15.61
Outstanding options exercisable, end of year, weighted average exercise price (in dollars per share)	$ 15.33
Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total intrinsic value of options exercised	$ 4.9	$ 4.9	$ 6.3
Proceeds from Stock Options Exercised	1	1.8	2.4
Common Stock Redeemed For Payment of Stock Option Exercises, Aggregate Value	$ 2	$ 1.6	$ 2.8

Stock-Based Compensation - S_2

Stock-Based Compensation - Schedule of Restricted Stock Activity (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	$ 8	$ 5.6	$ 3.8
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	$ 2,013.9	1,693.9
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, shares, restricted stock award	230,922
Stock compensation expense		$ 5.6	3.3
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	380,584
Granted, number of shares	230,922
Vested, number of shares	(202,286)
Forfeited, number of shares	(46,198)
Canceled, number of shares	0
Restricted stock, end of year, number of shares	363,022	380,584
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)	$ 37.46
Granted, weighted average grant date fair value (in dollars per share)		$ 41.29
Vested, weighted average grant date fair value (in dollars per share)		33.24
Forfeited, weighted average grant date fair value (in dollars per share)		41.18
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)	$ 41.47	$ 37.46
Tax benefits from stock-based compensation	$ 0.4	$ 0.2	0.6
Unrecognized compensation cost related to nonvested restricted stock awards	$ 8.1
Share-based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition	1 year 1 month 6 days
Common Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	$ 8	$ 5.6	$ 3.8
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	$ 1,049.3
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	36,463

Employee Benefit Plans - Profi

Employee Benefit Plans - Profit Sharing and Savings Plans (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits
Other Comprehensive Income (Loss), Tax	$ 12,400,000	$ 12,400,000	$ (3,300,000)	$ (2,800,000)
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	1,600,000
Savings Plan, Employee Elected
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits
Compensation expense		7,000,000	$ 6,300,000	$ 5,500,000
Maximum annual contribution per employee, amount per dollar of employee contribution		$ 1
Maximum annual contribution per employee, percent		5.00%

Employee Benefit Plans - Postr

Employee Benefit Plans - Postretirement Healthcare Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure
Amortization of gain, weighted average remaining service period (in years)	4 years
Total stockholders’ equity	$ 2,013,900	$ 1,693,900		$ 982,600
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
Defined Benefit Plan, Amortization of Transition Asset (Obligation)	$ 700
Postretirement Healthcare Plan
Defined Benefit Plan Disclosure
Minimum age requirement	55 years
Requisite service period	15 years
Accumulated other comprehensive income (loss), before tax, related to the Plan	$ 500
Reduction in accumulated post-retirement benefit obligation due to curtailment				2,800
Reduction in net periodic benefit cost due to curtailment			$ (300)	$ (2,800)
Unfunded benefit obligation	(400)	(500)
Net periodic benefit cost	700	$ 800	$ 400
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
2019	90
2020	60
2021	50
2022	40
2023 through 2027	80
Estimated amount amortized from accumulated other comprehensive loss into net periodic benefit costs	$ 120
Minimum
Defined Benefit Plan Disclosure
Assumptions used calculating net periodic benefit cost, discount rate	3.60%	3.60%
Maximum [Member]
Defined Benefit Plan Disclosure
Assumptions used calculating net periodic benefit cost, discount rate	5.50%	6.00%

Other Comprehensive Income - S

Other Comprehensive Income - Schedule of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Equity [Abstract]
Other Comprehensive Income (Loss), Tax	$ 12.4	$ 12.4	$ (3.3)	$ (2.8)
Other Comprehensive Income (Loss), Transfers From Available-For-Sale To Held-To-Maturity Securities, Net Of Tax			1.2	1.2
Other Comprehensive Income (Loss), Net of Tax	35.6		(9.5)
Before Tax Amount
Change in net unrealized gain during period		54.9	(13.9)	(6.5)
Reclassification adjustment for net gain included in net income		(0.1)	0.1	(0.7)
Change in net unrealized loss on derivatives				(1.1)
Reclassification adjustment for derivative net (gains) losses included in net income		0	0	1.1
Change in net actuarial gains		(0.8)	(0.6)	(1.3)
Total other comprehensive income				(6.6)
Tax Expense (Benefit)
Change in net unrealized loss during period		14.1	(3.6)	(2.7)
Reclassification adjustment for net loss included in net income		0	0	(0.3)
Change in net unrealized loss on derivatives				(0.4)
Change in net actuarial gains		(0.1)	(0.1)	(0.5)
Total other comprehensive income (loss)				(2.8)
Net of Tax Amount
Change in net unrealized loss during period		40.8	(10.3)	(3.8)
Reclassification adjustment for net gain included in net income		(0.1)	0.1	(0.4)
Change in net unrealized loss on derivatives				(0.7)
Change in net actuarial loss		(0.7)	(0.5)	(0.8)
Other comprehensive income (loss), net of tax		35.6	(9.5)	(3.8)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax				0.4
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax				0.7
Other comprehensive income (loss), before tax	$ 48	48	(12.8)	(6.6)
Other Comprehensive Income (Loss), Transfers From Available-For-Sale To Held-To-Maturity Securities, Before Tax		281.1	1.6	1.9
Other Comprehensive Income (Loss), Transfers From Available-For-Sale To Held-To-Maturity Securities, Tax			0.4	0.7
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity		(6)	$ 0	$ 0
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, Tax		(1.6)
Other Comprehensive Income (Loss), Transfers from Held-to-maturity to Available-for-Sale Securities, Net of Tax		$ (4.4)

Other Comprehensive Income - A

Other Comprehensive Income - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in net unrealized gain during period	$ 54.9	$ (13.9)	$ (6.5)
Reclassification adjustment for net gain included in net income	(0.1)	0.1	$ (0.7)
Accumulated other comprehensive income (loss)	2,013.9	1,693.9		$ 982.6
Net unrealized gain (loss) on investment securities available-for-sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	10.6	(25.5)
Total other comprehensive income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	11	(24.6)		$ (8.2)
Postretirement Healthcare Plan
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss), before tax, related to the Plan	0.5
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	$ 0.4	$ 0.9

Condensed Financial Informati_3

Condensed Financial Information (Parent Company Only) - Condensed Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2016
Condensed balance sheets:
Cash and cash equivalents	$ 1,076.8	$ 822
Investment in subsidiaries, at equity:
Other assets	187.6	159.1
Total assets	14,644.2	13,300.2
Subordinated debentures held by subsidiary trusts	86.9	86.9
Total liabilities	12,630.3	11,606.3
Stockholders’ equity	2,013.9	1,693.9	$ 982.6
Total liabilities and stockholders’ equity	14,644.2	13,300.2
Parent Company
Condensed balance sheets:
Cash and cash equivalents	147.1	72.9
Investment in subsidiaries, at equity:
Equity method investments	1,899.3	1,648.9
Advances from subsidiaries, net	48.9	46
Other assets	61.7	59
Total assets	2,157	1,826.8
Other liabilities	56.2	46
Subordinated debentures held by subsidiary trusts	86.9	86.9
Total liabilities	143.1	132.9
Stockholders’ equity	2,013.9	1,693.9
Total liabilities and stockholders’ equity	$ 2,157	$ 1,826.8

Condensed Financial Informati_4

Condensed Financial Information (Parent Company Only) - Condensed Statements of Income (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2018	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions
Salaries and benefits			$ 155.3	$ 146.4	$ 122.7
Interest expense			59	40.9	28
Acquisition related expenses		$ 12.4	20.3	12.4	27.2
Earnings before income tax benefit			235.1	206.3	156.7
Income tax benefit			54.1	46.1	50.2
Net income	$ 160.2		181	160.2	106.5
Parent Company
Condensed Financial Statements, Captions
Dividends from subsidiaries			178	148.5	150
Other interest income			0.3	0.1	0.1
Other income, primarily management fees from subsidiaries			25.9	17	18
Total income			204.2	165.6	168.1
Salaries and benefits			34.2	25.3	21.8
Interest expense			4.7	4.5	4.7
Acquisition related expenses			17	8.1	25.3
Other operating expenses, net			14.8	14.5	13
Total expenses			70.7	52.4	64.8
Earnings before income tax benefit			133.5	113.2	103.3
Income tax benefit			(11.9)	(9.3)	(14.2)
Income before undistributed earnings of subsidiaries			145.4	122.5	117.5
Undistributed earnings of subsidiaries			35.6	37.7	(11)
Net income			$ 181	$ 160.2	$ 106.5

Condensed Financial Informati_5

Condensed Financial Information (Parent Company Only) - Condensed Statements of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net income	$ 160.2	$ 181	$ 160.2	$ 106.5
Adjustments to reconcile net income to cash provided by operating activities:
Stock-based compensation expense		8	5.6	3.9
Net cash provided by operating activities		127.3	219	154.6
Cash flows from investing activities:
Acquisition of intangible assets		0	0	(28)
Acquisition of banks and bank holding companies, net of cash and cash equivalents acquired		298.4	(28.1)	91.8
Net cash used in investing activities		(16.8)	181.1	(130.6)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of stock issuance costs		1	1.8	2.4
Purchase and retirement of common stock		(2.5)	(1)	(1.3)
Dividends paid to common stockholders		(79.2)	(64.1)	(48.6)
Net cash used in financing activities		144.3	25.2	(47.1)
Net change in cash and cash equivalents				(23.1)
Cash and cash equivalents at beginning of period	758.9		758.9	782
Cash and cash equivalents at end of period				758.9
Parent Company
Cash flows from operating activities:
Net income		181	160.2	106.5
Adjustments to reconcile net income to cash provided by operating activities:
Undistributed earnings of subsidiaries		(35.6)	(37.7)	11
Stock-based compensation expense		8	5.6	3.9
Other, net		8.1	16.4	14.7
Net cash provided by operating activities		161.5	144.5	136.1
Cash flows from investing activities:
Capital distributions from nonbank subsidiaries		0	0	18
Acquisition of intangible assets		0	0	(28)
Acquisition of banks and bank holding companies, net of cash and cash equivalents acquired		0	(14.7)	(128.3)
Investment in subsidiary		0	0	(18)
Net cash used in investing activities		0	(14.7)	(156.3)
Cash flows from financing activities:
Net (decrease) increase in advances from subsidiaries		(6.6)	(9.9)	(28.4)
Repayment of long-term debt		0	(26)	0
Proceeds from issuance of common stock, net of stock issuance costs		1	1.8	2.4
Purchase and retirement of common stock		(2.5)	(1)	(1.3)
Dividends paid to common stockholders		(79.2)	(64.1)	(48.6)
Net cash used in financing activities		(87.3)	(99.2)	(75.9)
Net change in cash and cash equivalents		74.2	30.6	(96.1)
Cash and cash equivalents at beginning of period	$ 42.3	72.9	42.3	138.4
Cash and cash equivalents at end of period		147.1	72.9	42.3
Community 1st Bank [Member]
Cash flows from financing activities:
Stock Issued		$ 176.1
Inland Northwest Bank [Member]
Cash flows from financing activities:
Stock Issued			$ 173.3
BOTC Acquisition
Cash flows from financing activities:
Stock Issued				$ 386

Fair Value Measurements - Sche

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, 2019USD ($)	Dec. 31, 2018USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	$ 2,960	$ 2,270.7
Derivative assets	23.2	10.1
Derivative liabilities	22.2	9.4
Deferred compensation plan assets	18.2	12.1
Deferred compensation plan liabilities	18.2	12.1
Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other real estate owned	1.2	0.6
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	100.9	33.3
Deferred compensation plan assets	18.2	12.1
Deferred compensation plan liabilities	18.2	12.1
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	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	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	0.6
US Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	9	2.6
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	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	2.6
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	0
State, county and municipal securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Fair Value	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	80.9
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	366.8	559.2
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	366.8	559.2
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	2,317.2	1,544.8
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	2,317.2	1,544.8
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	47.2	70.2
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	47.2	70.2
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	135.7	91.9
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	135.7	91.9
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	2,960	2,270.7
Derivative assets	23.2	10.1
Derivative liabilities	22.2	9.4
Deferred compensation plan assets	18.2	12.1
Deferred compensation plan liabilities	18.2	12.1
Estimated Fair Value | Fair Value, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for sale	100.9	33.3
Deferred compensation plan assets	18.2	12.1
Deferred compensation plan liabilities	18.2	12.1
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	2,960	2,270.7
Derivative assets	23.2	10.1
Derivative liabilities	22.2	9.4
Deferred compensation plan assets	18.2	12.1
Deferred compensation plan liabilities	18.2	12.1
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	0.6
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	2.6
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	80.9
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	366.8	559.2
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	2,317.2	1,544.8
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	47.2	70.2
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	135.7	91.9
Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	21.9	8.8
Derivative liabilities	21.9	8.8
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	21.9	8.8
Derivative liabilities	21.9	8.8
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	21.9	8.8
Derivative liabilities	21.9	8.8
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	1.3	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	1.3	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	0.3	0.6
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	$ 0.3	$ 0.6
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	0.56
Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Maximum [Member] | 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]
Servicing Asset, Measurement Input	0.65
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	0.22
Valuation, Market Approach [Member] | Measurement Input, Discount Rate [Member] | Weighted Average [Member] | 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]
Servicing Asset, Measurement Input	0.27

Fair Value Measurements - Fair

Fair Value Measurements - Fair Value Inputs, Quantitative Information (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)	Dec. 31, 2019USD ($)
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	$ 2,270.7	$ 2,960
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	(96.00%)
Discount Rate for Assets HFS Level 3 Fair Value	(43.00%)
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	(13.00%)
Discount Rate for OREO Level 3 Fair Value	(39.00%)
Discount Rate for Assets HFS Level 3 Fair Value	(10.00%)
Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Impaired loans	$ (12.2)	(13.7)
Other real estate owned	(0.6)	(1.2)
Long-lived assets to be disposed of by sale	(0.5)	(0.2)
Estimated Fair Value
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	2,270.7	2,960
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	2,270.7	2,960
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)
Impaired loans	$ (24.1)	(27.6)
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	(26.00%)
Other Debt Obligations [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Debt Securities, Available-for-sale	$ 2	3.2
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	2	3.2
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	2	3.2
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)
Impaired 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)
Impaired 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)
Impaired loans	(24.1)
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)
Impaired loans	(24.1)	$ (27.6)
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		0.56
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		0.22
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	$ 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	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	(0.6)	(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	(0.6)	$ (2.2)
Loans And Leases Receivable, Other Real Estate Owned [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		0.08
Loans And Leases Receivable, Other Real Estate Owned [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		0.65
Loans And Leases Receivable, Other Real Estate Owned [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		0.27
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	(4.9)	(6.2)
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	$ (4.9)	$ (6.2)
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		0.37
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		0.03

Fair Value Measurements - Narr

Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Related allowance	$ 3.6	$ 6.8	$ 10.8
Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	13.7	12.2
Fair Value, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Related allowance	3.6	6.8
Partial loan charge-off	10.1	5.4
Carrying Amount | Fair Value, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	41.3	36.3
Assets Held-for-sale, Long Lived, Fair Value Disclosure	6.4	5.4
Changes Measurement [Member] | Fair Value, Nonrecurring [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	0.5
Estimated Fair Value | Fair Value, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets Held-for-sale, Long Lived, Fair Value Disclosure	6.2	4.9
Estimated Fair Value | Fair Value, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	$ 27.6	$ 24.1

Fair Value Measurements - Sc_2

Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Millions	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Financial assets:
Estimated Fair Value	$ 2,960		$ 2,270.7
Held-to-maturity investment securities, estimated fair value	94.5	$ 275.1	400.7
Derivative assets	23.2		10.1
Deferred compensation plan assets	18.2		12.1
Financial liabilities:
Derivative liabilities	22.2		9.4
Deferred compensation plan liabilities	18.2		12.1
Carrying Amount
Financial assets:
Cash and cash equivalents	1,076.8		822
Estimated Fair Value	2,960		2,270.7
Held-to-maturity investment securities, estimated fair value	92.3		406.8
Accrued interest receivable	46.7		44.9
Mortgage servicing rights, net	30.2		27.7
Loans Held-for-sale, Fair Value Disclosure	100.9		33.3
Net loans held for investment	8,857.7		8,397.4
Derivative assets	23.2		10.1
Deferred compensation plan assets	18.2		12.1
Total financial assets	13,206		12,025
Financial liabilities:
Total deposits, excluding time deposits	10,213.5		9,363.7
Time deposits	1,450		1,317
Securities sold under repurchase agreements	697.6		712.4
Other borrowed funds			0
Accrued interest payable	12.1		7.8
Long-term debt	13.9		15.8
Subordinated debentures held by subsidiary trusts	86.9		86.9
Derivative liabilities	22.2		9.4
Deferred compensation plan liabilities	18.2		12.1
Total financial liabilities	12,514.4		11,525.1
Estimated Fair Value
Financial assets:
Cash and cash equivalents	1,076.8		822
Estimated Fair Value	2,960		2,270.7
Held-to-maturity investment securities, estimated fair value	94.5		400.7
Accrued interest receivable	46.7		44.9
Mortgage servicing rights, net	34.8		42.4
Loans Held-for-sale, Fair Value Disclosure	100.9		33.3
Net loans held for investment	8,930.7		8,439.7
Derivative assets	23.2		10.1
Deferred compensation plan assets	18.2		12.1
Total financial assets	13,285.8		12,075.9
Financial liabilities:
Total deposits, excluding time deposits	10,213.5		9,363.7
Time deposits	1,446.6		1,299
Securities sold under repurchase agreements	697.6		712.4
Other borrowed funds			0
Accrued interest payable	12.1		7.8
Long-term debt	10.4		13
Subordinated debentures held by subsidiary trusts	81.3		84.9
Derivative liabilities	22.2		9.4
Deferred compensation plan liabilities	18.2		12.1
Total financial liabilities	12,501.9		11,502.3
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	1,076.8		822
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	1,076.8		822
Financial liabilities:
Total deposits, excluding time deposits	10,213.5		9,363.7
Time deposits	0		0
Securities sold under repurchase agreements	0		0
Other borrowed funds			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	10,213.5		9,363.7
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0		0
Estimated Fair Value	2,960		2,270.7
Held-to-maturity investment securities, estimated fair value	94.5		400.7
Accrued interest receivable	46.7		44.9
Mortgage servicing rights, net	34.8		42.4
Loans Held-for-sale, Fair Value Disclosure	100.9		33.3
Net loans held for investment	8,906.7		8,415.6
Derivative assets	23.2		10.1
Deferred compensation plan assets	18.2		12.1
Total financial assets	12,185		11,229.8
Financial liabilities:
Total deposits, excluding time deposits	0		0
Time deposits	1,446.6		1,299
Securities sold under repurchase agreements	697.6		712.4
Other borrowed funds			0
Accrued interest payable	12.1		7.8
Long-term debt	10.4		13
Subordinated debentures held by subsidiary trusts	81.3		84.9
Derivative liabilities	22.2		9.4
Deferred compensation plan liabilities	18.2		12.1
Total financial liabilities	2,288.4		2,138.6
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	24		24.1
Derivative assets	0		0
Deferred compensation plan assets	0		0
Total financial assets	24		24.1
Financial liabilities:
Total deposits, excluding time deposits	0		0
Time deposits	0		0
Securities sold under repurchase agreements	0		0
Other borrowed funds			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, 2019	Dec. 31, 2018	Dec. 31, 2017
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Loan repayments	$ 18,300
Amount of loans removed due to changes in related parties	1,100
Airplane Usage by Company
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Related party transaction, expenses from transactions with related party	22	$ 53	$ 45
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	40,300	43,200
New loans and advances on existing loans	16,500
Board of Directors Chairman | Airplane Usage by Company
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Related party transactions, other revenues	30	25	17
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	$ 80	$ 73

Recent Authoritative Accounting

Recent Authoritative Accounting Guidance - New Accounting Pronouncements and Changes in Accounting Principles (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Benefit of stock-based compensation plans	$ (2.6)

Subsequent Events Subsequent Ev

Subsequent Events Subsequent Events (Details) - $ / shares	Feb. 19, 2020	Jan. 31, 2020
Class A Common Stock | Subsequent Event
Subsequent Event [Line Items]
Dividends (in dollars per share)	$ 0.60	$ 0.34

Uncategorized Items - fibk-2019

Label	Element	Value
Retained Earnings [Member]
Stock Issued, Value, Stock Options Exercised, Net of Tax Benefit (Expense)	us-gaap_StockIssuedDuringPeriodValueStockOptionsExercisedNetOfTaxBenefitExpense	$ 0
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
AOCI Attributable to Parent [Member]
Stock Issued, Value, Stock Options Exercised, Net of Tax Benefit (Expense)	us-gaap_StockIssuedDuringPeriodValueStockOptionsExercisedNetOfTaxBenefitExpense	$ 0