First Interstate BancSystem (FIBK) 10-K 28 Feb 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Jan. 31, 2018	Jun. 30, 2017
Entity Registrant Name	FIRST INTERSTATE BANCSYSTEM INC
Entity Central Index Key	860,413
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-K
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 1,317,809,107
Class A Common Stock
Entity Common Stock, Shares Outstanding		33,580,995
Class B Common Stock
Entity Common Stock, Shares Outstanding		22,907,947

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets
Cash and due from banks	$ 196,495	$ 146,779
Federal funds sold	146	95
Interest bearing deposits in banks	562,345	635,149
Total cash and cash equivalents	758,986	782,023
Investment securities:
Available-for-sale	2,208,712	1,611,698
Held-to-maturity (estimated fair values of $483,349 and $513,273 at December 31, 2017 and 2016, respectively)	484,494	512,770
Total investment securities	2,693,206	2,124,468
Loans held for investment	7,567,720	5,416,750
Mortgage loans held for sale	46,635	61,794
Total loans	7,614,355	5,478,544
Less allowance for loan losses	72,147	76,214
Net loans	7,542,208	5,402,330
Premises and equipment, net of accumulated depreciation	241,862	194,457
Goodwill	444,704	212,820
Company-owned life insurance	260,551	198,116
Other real estate owned (“OREO”)	10,053	10,019
Accrued interest receivable	38,031	29,852
Mortgage servicing rights, net of accumulated amortization and impairment reserve	24,770	18,457
Deferred tax asset, net	4,028	0
Core deposit intangibles, net of accumulated amortization	49,109	9,648
Other assets	145,747	81,705
Total assets	12,213,255	9,063,895
Deposits:
Non-interest bearing	2,900,042	1,906,257
Interest bearing	7,034,829	5,469,853
Total deposits	9,934,871	7,376,110
Securities sold under repurchase agreements	642,961	537,556
Accounts payable and accrued expenses	86,596	44,923
Accrued interest payable	5,599	5,421
Deferred tax liability, net	0	6,839
Long-term debt	13,126	27,970
Other borrowed funds	20,009	6
Subordinated debentures held by subsidiary trusts	82,477	82,477
Total liabilities	10,785,639	8,081,302
Stockholders’ equity:
Nonvoting noncumulative preferred stock without par value; authorized 100,000 shares; no shares issued or outstanding as of December 31, 2017 and 2016	0	0
Common stock	686,991	296,071
Retained earnings	752,588	694,650
Accumulated other comprehensive income (loss), net	(11,963)	(8,128)
Total stockholders’ equity	1,427,616	982,593
Total liabilities and stockholders’ equity	$ 12,213,255	$ 9,063,895

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets:
Held-to-maturity investment securities, estimated fair value	$ 483,349	$ 513,273
Stockholders' Equity:
Nonvoting, noncumulative preferred stock, shares authorized	100,000	100,000
Nonvoting, noncumulative preferred stock, shares issued	0	0
Nonvoting, noncumulative preferred stock, shares outstanding	0	0

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest income:
Interest and fees on loans	$ 324,769	$ 259,219	$ 246,015
Interest and dividends on investment securities:
Taxable	42,697	32,160	30,861
Exempt from federal taxes	3,156	3,447	3,998
Interest on deposits in banks	7,146	2,589	1,537
Interest on federal funds sold	9	11	12
Total interest income	377,777	297,426	282,423
Interest expense:
Interest on deposits	21,367	12,662	13,107
Interest on securities sold under repurchase agreements	1,253	429	231
Interest on other borrowed funds	1,537	0	0
Interest on long-term debt	599	1,815	2,300
Interest on subordinated debentures held by subsidiary trusts	3,178	2,755	2,422
Total interest expense	27,934	17,661	18,060
Net interest income	349,843	279,765	264,363
Provision for loan losses	11,053	9,991	6,822
Net interest income after provision for loan losses	338,790	269,774	257,541
Non-interest income:
Payment services revenues	43,308	34,374	32,750
Mortgage banking revenues	28,896	37,222	29,973
Wealth management revenues	21,067	20,460	19,907
Service charges on deposit accounts	21,276	18,426	17,031
Other service charges, commissions and fees	13,358	11,506	10,404
Loss on termination of interest rate swap	(1,132)	0	0
Investment securities gains, net	718	330	137
Other income	14,262	10,028	11,313
Non-recurring litigation recovery	0	4,150	0
Total non-interest income	141,753	136,496	121,515
Non-interest expense:
Salaries and wages	122,751	108,684	101,451
Employee benefits	37,561	35,193	31,324
Occupancy, net	22,410	17,714	17,879
Furniture and equipment	11,467	9,584	15,524
Outsourced technology services	25,087	20,547	10,124
FDIC insurance premiums	4,605	4,548	4,858
Professional fees	6,847	4,990	6,458
OREO expense, net of income	467	(44)	(1,475)
Mortgage servicing rights amortization	3,038	2,957	2,434
Mortgage servicing rights impairment recovery	(93)	(35)	(97)
Core deposit intangibles amortization	5,438	3,427	3,388
Other expenses	57,087	50,625	50,936
Loss contingency expense	0	0	5,000
Acquisition related expenses	27,156	2,821	795
Total non-interest expense	323,821	261,011	248,599
Income before income tax expense	156,722	145,259	130,457
Income tax expense	50,201	49,623	43,662
Net income	$ 106,521	$ 95,636	$ 86,795
Basic earnings per common share (in dollars per share)	$ 2.07	$ 2.15	$ 1.92
Diluted earnings per common share (in dollars per share)	$ 2.05	$ 2.13	$ 1.90

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 106,521	$ 95,636	$ 86,795
Investment securities available-for-sale:
Change in net unrealized gains (losses) during the period	(6,545)	(19,379)	3,151
Reclassification adjustment for net gains included in income	(718)	(330)	(137)
Change in unamortized loss on available-for-sale investment securities transferred into held-to-maturity	1,858	1,858	1,611
Change in net unrealized gain (loss) on derivatives	(1,095)	(203)	165
Reclassification adjustment for derivative net (gains) losses included in net income	1,132	0	0
Defined benefit post-retirement benefit plans:
Change in net actuarial loss	(1,264)	3,983	58
Other comprehensive income (loss), before tax	(6,632)	(14,071)	4,848
Deferred tax benefit (expense) related to other comprehensive income (loss)	2,797	5,537	(1,908)
Other comprehensive income (loss), net of tax	(3,835)	(8,534)	2,940
Comprehensive income	$ 102,686	$ 87,102	$ 89,735

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
Beginning Balance at Dec. 31, 2014	$ 908,924	$ 323,596	$ 587,862	$ (2,534)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	86,795		86,795
Other comprehensive loss, net of tax	2,940			2,940
Common stock transactions:
Common shares purchased and retired	(20,647)	(20,647)
Common shares issued	0
Non-vested common shares issued	0
Non-vested common shares forfeited or canceled	0
Stock options exercised, net of shares tendered in payment of option price and income tax withholding amounts	3,369	3,369
Tax benefit of stock-based compensation	1,443	1,443
Stock-based compensation expense	3,959	3,959
Cash dividends declared:
Common (per share)	(36,290)		(36,290)
Ending Balance at Dec. 31, 2015	950,493	311,720	638,367	406
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	95,636		95,636
Other comprehensive loss, net of tax	(8,534)			(8,534)
Common stock transactions:
Common shares purchased and retired	(26,854)	(26,854)
Common shares issued	0
Non-vested common shares issued	0
Non-vested common shares forfeited or canceled	0
Stock options exercised, net of shares tendered in payment of option price and income tax withholding amounts	4,683	4,683
Tax benefit of stock-based compensation	2,146	2,146
Stock-based compensation expense	4,376	4,376
Cash dividends declared:
Common (per share)	(39,353)		(39,353)
Ending Balance at Dec. 31, 2016	982,593	296,071	694,650	(8,128)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	106,521		106,521
Other comprehensive loss, net of tax	(3,835)			(3,835)
Common stock transactions:
Common shares purchased and retired	(1,348)	(1,348)
Non-vested common shares issued	0
Non-vested common shares forfeited or canceled	0
Stock options exercised, net of shares tendered in payment of option price and income tax withholding amounts	2,431	2,431
Stock-based compensation expense	3,868	3,868
Cash dividends declared:
Common (per share)	(48,583)		(48,583)
Ending Balance at Dec. 31, 2017	1,427,616	686,991	$ 752,588	$ (11,963)
Common stock transactions:
Common Stock, Value, Issued	$ 385,969	$ 385,969

Consolidated Statements of Sto7

Consolidated Statements of Stockholders' Equity (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common stock transactions:
Stock options exercised (in shares)	285,887
Cash dividends declared:
Common (in dollars per share)	$ 0.96	$ 0.88	$ 0.8
Common stock
Common stock transactions:
Common shares purchased and retired	33,063	1,015,389	793,077
Common shares issued	11,267,676	16,347	21,414
Non-vested common shares issued	140,246	190,239	169,577
Non-vested common shares forfeited	53,571	29,844	19,184
Stock options exercised (in shares)	218,095	336,598	261,080
Shares tendered in payment of option price and income tax withholding amounts	67,792	104,643	89,358

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 106,521	$ 95,636	$ 86,795
Adjustments to reconcile net income from operations to net cash provided by operating activities:
Provision for loan losses	11,053	9,991	6,822
Net (gain) loss on disposal of property and equipment	137	621	(731)
Depreciation and amortization	18,432	19,480	18,313
Net premium amortization on investment securities	11,616	12,520	14,880
Net (gain) loss on investment securities transactions	(718)	(330)	(137)
Realized and unrealized net gains on mortgage banking activities	(25,174)	(26,806)	(21,748)
Net loss (gain) on sale of OREO	59	(925)	(2,991)
Write-down of OREO and other assets pending disposal	406	792	1,013
Net (gain) on sale of Health Savings Accounts	(3,068)	0	0
Mortgage servicing rights recovery	(93)	(35)	(97)
Deferred income tax expense (benefit)	20,917	3,369	12,449
Net increase in cash surrender value of company-owned life insurance policies	(5,397)	(4,477)	(3,432)
Stock-based compensation expense	3,868	4,376	3,959
Tax benefits from stock-based compensation	0	2,146	1,443
Excess tax benefits from stock-based compensation	0	(1,566)	(1,184)
Originations of mortgage loans held for sale	(945,200)	(1,113,292)	(1,148,098)
Proceeds from sales of mortgage loans held for sale	990,266	1,125,365	1,164,804
Changes in operating assets and liabilities:
(Increase) decrease in interest receivable	(574)	(1,053)	(101)
Decrease (increase) in other assets	(6,063)	(2,214)	(3,622)
Increase (decrease) in interest payable	178	445	(1,006)
Increase (decrease) in accounts payable and accrued expenses	(22,483)	(5,973)	(11,931)
Net cash provided by operating activities	154,683	118,070	115,400
Purchases of investment securities:
Held-to-maturity	(12,793)	(18,173)	(45,179)
Available-for-sale	(614,267)	(905,940)	(474,846)
Proceeds from maturities, pay-downs, calls and sales of investment securities:
Held-to-maturity	97,407	112,563	118,406
Available-for-sale	426,221	814,598	648,683
Purchase of company-owned life insurance	0	0	(30,000)
Extensions of credit to customers, net of repayments	(99,722)	(168,321)	(327,878)
Recoveries of loans charged-off	7,749	9,249	7,432
Proceeds from sales of OREO	5,913	5,325	15,644
Acquisition of intangible assets	(28,013)	0	0
Proceeds from the sale of Health Savings Accounts	6,138	0	0
Proceeds from sale of loan production office	0	932	0
Acquisition of bank and bank holding company, net of cash and cash equivalents, received	91,779	18,554	(1,636)
Capital expenditures, net of proceeds from sales	(10,977)	(11,879)	(5,965)
Net cash provided by (used in) investing activities	(130,565)	(143,092)	(95,339)
Cash flows from financing activities:
Net increase (decrease) in deposits	(110,216)	77,586	19,160
Net increase (decrease) in securities sold under repurchase agreements	105,405	8,871	6,385
Net increase (decrease) in other borrowed funds	64	4	(7)
Repayments of long-term debt	92	(65)	(16,538)
Advances on long-term debt	5,000	150	5,103
Write-off of debt issuance costs	0	0	7
Proceeds from issuance of common stock	2,431	4,683	3,369
Excess tax benefits from stock-based compensation	0	1,566	1,184
Purchase and retirement of common stock	(1,348)	(26,854)	(20,647)
Dividends paid to common stockholders	(48,583)	(39,353)	(36,290)
Net cash used in financing activities	(47,155)	26,588	(38,274)
Net change in cash and cash equivalents	(23,037)	1,566	(18,213)
Cash and cash equivalents at beginning of period	782,023	780,457	798,670
Cash and cash equivalents at end of period	758,986	782,023	780,457
Supplemental disclosures of cash flow information:
Cash paid during the period for income taxes	28,759	54,419	27,335
Cash paid during the period for interest expense	27,757	17,200	18,933
Supplemental disclosures of noncash investing and financing activities:
Transfer of loans to loans held for sale	3,009	0	0
Transfer from long-term debt to other borrowed funds	20,000	0	0
Transfer of loans to other real estate owned	5,219	7,648	5,560
Capitalization of internally originated mortgage servicing rights	5,563	5,758	3,614
Supplemental schedule of noncash investing activities from acquisitions:
Investment securities available for sale	424,302	0	0
Investment securities held to maturity	57,307	0	0
Loans held for sale	10,253	0	0
Loans	2,079,336	0	0
Premises and equipment	46,732	0	0
Goodwill	231,886	0	0
Core deposit intangible	47,968	0	0
Mortgage servicing rights	3,491	0	0
Interest receivable	7,605	0	0
Company-owned life insurance	57,038	0	0
Deferred tax assets	28,640	0	0
Other real estate owned	1,192	0	0
Other assets	31,572	0	0
Total noncash assets acquired	3,027,322	0	0
Deposits	(2,668,976)	0	0
Accounts payable and accrued expenses	(64,156)	0	0
Total liabilities assumed	$ (2,733,132)	$ 0	$ 0

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
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, 2017 , 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 2016 and 2015 to conform to the 2017 presentation. These reclassifications did not change previously reported net income or stockholders’ equity. 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”) and FI Statutory Trust VI (“Trust VI”) 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 and Trust VI 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, 2017 and 2016 , the Company had cash of $552.0 million and $625.3 million , respectively, on deposit with the Federal Reserve Bank. In addition, the Company maintained compensating balances with the Federal Reserve Bank of approximately $16.1 million and $19.3 million as of December 31, 2017 and 2016 , 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, and marketable equity securities are classified as available-for-sale and carried at fair value. The unrealized gains and losses on these securities are reported, net of applicable income taxes, as a separate component of stockholders’ equity and comprehensive income. Management determines the appropriate classification of securities at the time of purchase and at each reporting date management reassesses the appropriateness of the classification. The amortized cost of debt securities classified as held-to-maturity or available-for-sale is adjusted for accretion of discounts to maturity and amortization of premiums over the estimated average life of the security, 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. Beginning January 1, 2016, the Company 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. Prior to 2016, the Company carried loans held for sale at the lower of aggregate cost or estimated market value. 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 loan, on current delivery prices. Gains and losses on loan 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 $46.6 million and $61.8 million as of December 31, 2017 and 2016 , respectively. As of December 31, 2017 , the Company had no 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 two-step 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. Accumulated core deposit intangibles amortization was $36.9 million as of December 31, 2017 and $31.5 million as of December 31, 2016 . 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 capital lease are amortized over the shorter of their estimated useful lives or the terms of the related leases. Land is recorded at cost. Company-Owned Life Insurance . 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, 2017 and 2016 , deferred compensation plan assets were $ 12.2 million and $ 10.6 million , respectively. Corresponding deferred compensation plan liabilities were $ 12.2 million and $ 10.6 million as of December 31, 2017 and 2016 , 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 2017 and impairment losses of $0.2 million and $0.8 million were recognized in other non-interest expense in 2016 and 2015 , respectively. 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 $0.4 million , $0.6 million and $0.2 million were recorded in 2017 , 2016 and 2015 , respectively. The carrying value of foreclosed residential real estate properties included in other real estate owned was $2.7 million as of December 31, 2017 , and $2.3 million as of December 31, 2016 . 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, 2017 , restricted equity securities of the Federal Reserve Bank and the FHLB of $ 31.8 million and $ 10.2 million , respectively, were included in other assets at cost. As of December 31, 2016 , restricted equity securities of the Federal Reserve Bank and the FHLB were $ 16.4 million and $ 10.1 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 2017 , 2016 or 2015 . 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, 2017 , 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 customer 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 customer, 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 2014 . The Company had no material penalties as of December 31, 2017 , 2016 or 2015 . 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. 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 Company has one operating segment, community banking, which encompasses commercial and consumer banking services offered to individuals, businesses, municipalities and other entities. Advertising Costs. Advertising costs are expensed as incurred. Advertising expense was $3.5 million , $2.8 million , and $3.5 million in 2017 , 2016 and 2015 , respectively. 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 . Compensation cost for all stock-based awards is measured at fair value on the date of grant and is recognized over the requisite service period for awards expected to vest. The impact of forfeitures of stock-based payment awards on compensation expense is recognized as forfeitures occur. Stock-based compensation expense of $3.9 million , $4.4 million and $4.0 million for the years ended December 31, 2017 , 2016 and 2015 , respectively, is included in benefits expense in the Company’s consolidated statements of income. Related income tax benefits recognized for the years ended December 31, 2017 , 2016 and 2015 were $2.6 million , $2.1 million and $1.4 million , respectively. 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 an

Acquisitions

Acquisitions	12 Months Ended
	Dec. 31, 2017
Business Combinations [Abstract]
Acquisitions	ACQUISITIONS Cascade Bancorp. On November 17, 2016 , the Company entered into an agreement and plan of merger (the "Agreement") to acquire all of the outstanding stock of Cascade Bancorp (“Cascade”), parent company of Bank of the Cascades, an Oregon-based community bank with 46 banking offices across Oregon, Idaho and Washington. This transaction solidifies the Company's ability to strategically expand its community banking footprint in the Northwest corridor of the United States. The merger was completed on May 30, 2017 . Holders of each share of Cascade common stock received 0.14864 shares of First Interstate Class A common stock and $1.91 in cash, without interest, for each share of Cascade common stock. In connection with the merger, the Company issued approximately 11.3 million shares of First Interstate Class A common stock, which was valued at $34.30 per share, which was the closing price of First Interstate Class A common stock on the acquisition date. Cash paid by First Interstate was approximately $155.0 million , which included the cash portion of the merger consideration and the cash in lieu of fractional shares that Cascade Bancorp shareholders would have otherwise been entitled to receive. Total consideration exchanged in connection with the merger amounted to $541.0 million . All “in-the-money” Cascade options and all Cascade restricted stock units outstanding immediately prior to the transaction close were canceled in exchange for the right to receive a cash payment as provided in the Agreement. The Company paid approximately $9.3 million in cash related to Cascade options and restricted stock units, which was included in the consideration paid. Unvested Cascade restricted stock awards outstanding immediately prior to the transaction close were canceled in exchange for the right to receive a cash payment and Company shares as provided in the Agreement. The Company paid a total of approximately $2.2 million in cash and issued approximately 168 thousand Company shares, valued at $34.30 per share, related to Cascade unvested restricted stock awards. Of the cash paid and shares issued related to Cascade unvested restricted stock awards, approximately $2.4 million was allocated to expense and excluded from consideration paid due to the acceleration of award vesting at the Company’s discretion. The remaining balance of approximately $5.5 million related to unvested Cascade restricted stock awards is included in the consideration paid. The assets and liabilities of Cascade 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 provisional goodwill of $231.9 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 Cascade and the Company. The following table summarizes the consideration paid, fair values of the Cascade assets acquired and liabilities assumed and the resulting goodwill. The amounts reported below for net deferred tax assets and goodwill are provisional pending completion of the Company's review of tax items. As Recorded Fair Value As Recorded As of May 30, 2017 by Cascade Adjustments by the Company Assets acquired: Cash and cash equivalents $ 246.8 $ — $ 246.8 Investment securities 476.7 4.9 (1) 481.6 Loans held for investment 2,111.0 (31.7 ) (2) 2,079.3 Mortgage loans held for sale 10.3 — 10.3 Allowance for loan losses (24.0 ) 24.0 (3) — Premises and equipment 46.6 0.1 (4) 46.7 Other real estate owned ("OREO") 1.2 — 1.2 Core deposit intangible assets — 48.0 (5) 48.0 Deferred tax assets, net 47.6 (19.0 ) (6) 28.6 Other assets 98.6 1.1 (7) 99.7 Total assets acquired 3,014.8 27.4 3,042.2 Liabilities assumed: Deposits 2,669.9 (0.9 ) (8) 2,669.0 Accounts payable and accrued expense 62.2 1.9 (9) 64.1 Total liabilities assumed 2,732.1 1.0 2,733.1 Net assets acquired $ 282.7 $ 26.4 $ 309.1 Consideration paid: Cash $ 155.0 Class A common stock 386.0 Total consideration paid $ 541.0 Goodwill $ 231.9 Explanation of fair value adjustments. Note marks for the deferred tax assets, loans held for investment, and accounts payable and accrued expenses were adjusted due to loan and deferred compensation valuation adjustments since prior quarter reporting. Certain other balances have been reclassified, none of which are material. The adjustments had no impact on 2017 earnings and a net reduction to goodwill of $0.3 million from 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. Shared National Credits (SNC) were recorded at quoted sales prices where available. The fair value of the remaining 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 Cascade 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 appraisals obtained from an independent third party appraiser or broker's opinion of value. (5) Adjustment represents the value of the core deposit base assumed in the acquisition based upon valuation from an independent accounting and advisory firm. (6) Adjustment consists of the write-off of pre-existing deferred tax assets and purchase accounting adjustments as a result of the acquisition. (7) Adjustment consists of various other assets recorded as a result of the acquisition, including mortgage servicing rights, SBA servicing rights, and favorable leases offset by reductions to the fair value of other items. (8) Decrease 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. (9) Increase in fair value due to credit card incentive program, unfavorable leases, write-off of balance sheet reserve, and swap liability offset. Core deposit intangible assets of $48.0 million are being amortized using an accelerated method over the estimated useful lives of the related deposits of 10 years. The Company recorded $27.2 million in pre-tax acquisition related expenses for the year ended December 31, 2017, including legal and professional fees of $9.6 million , of which $6.5 million were incurred to directly consummate the merger. These costs are incorporated in non-interest expense in the Company’s consolidated statements of income and are summarized below. December 31, 2017 Legal and professional fees $ 9.6 Employee expenses 5.1 Technology conversion and contract termination 10.2 Other 2.3 Total acquisition related expenses $ 27.2 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 underlying credit-impaired loans, (ii) assumptions regarding future principal and interest amounts collected, and (iii) indices used to fair value variable rate loans. Information regarding loans acquired credit-impaired as of the May 30, 2017 acquisition date is as follows: Contractually required principal and interest payments $ 49.7 Contractual cash flows not expected to be collected ("non-accretable discount") 24.7 Cash flows expected to be collected 25.0 Interest component of cash flows expected to be collected ("accretable discount") 1.9 Fair value of acquired credit-impaired loans $ 23.1 Information regarding acquired loans not deemed credit-impaired at the acquisition date is as follows: Contractually required principal and interest payments $ 2,098.1 Contractual cash flows not expected to be collected 23.3 Fair value at acquisition $ 2,066.5 The accompanying consolidated statements of income include the results of operations of the acquired entity from the May 30, 2017 acquisition date. For the period from May 30, 2017 to June 30, 2017, Cascade reported revenues of $12.9 million and net income of $3.0 million . The acquired entity continued to operate as Bank of the Cascades until August 11, 2017 at which point the operations were integrated with the Company's operations, and Bank of the Cascades merged with First Interstate Bank. Standalone amounts for the Bank of the Cascades were no longer available after that date. The following table presents unaudited pro forma consolidated revenues and net income as if the acquisition had occurred as of January 1, 2016. Year ended December 31, (unaudited) 2017 2016 Interest income $ 420.8 $ 392.7 Non-interest income 153.3 165.9 Total revenues $ 574.1 $ 558.6 Net income $ 126.4 $ 72.3 EPS - basic $ 2.03 $ 1.30 EPS - diluted 2.01 1.29 The unaudited supplemental pro forma net income presented in the table above for 2017 was adjusted to exclude acquisition related costs, including change in control expenses related to employee benefit plans and legal and professional expenses, of $21.8 million , net of tax. Pro forma net income presented in the table above for 2016 was adjusted to include the aforementioned acquisition related costs. The unaudited pro forma net income presented in the table above for 2017 and 2016 includes adjustments for scheduled amortization of core deposit intangible assets acquired in the acquisition. The unaudited supplemental pro forma net income presented in the table above for 2017 and 2016 does not capture operating costs savings and other business synergies expected as a result of the acquisition. Flathead Bank of Bigfork. On April 6, 2016 , the Company's bank subsidiary entered into a stock purchase agreement to acquire all of the outstanding stock of Flathead Bank of Bigfork ("Flathead"), a Montana-based bank wholly owned by Flathead Holding Company. The acquisition was completed as of August 12, 2016 for cash consideration of $34.1 million . The acquisition allowed the Company to gain market share in several of its current market areas and expand its market presence in Montana. The assets and liabilities of Flathead 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. Goodwill arising from the acquisition consists largely of the synergies and economies of scale expected from combining the operations of Flathead and the Company. The Flathead acquisition was accounted for using the acquisition method with cash consideration funded from cash on hand. The assets and liabilities of Flathead were recorded in the Company's consolidated financial statements at their estimated fair values as of the acquisition dates. The excess value of the consideration paid over the fair value of assets acquired and liabilities assumed was recorded as goodwill. Core deposit intangible assets related to the Flathead acquisition of $2.5 million are being amortized using an accelerated method over the estimated useful lives of the related deposits of 9 years. The accompanying consolidated statements of income include the results of operations of Flathead from its acquisition date. The operations of Flathead were immediately integrated with the Company's operations and the acquired bank was merged with FIB. As such, the Company has determined it is not practical to report post-acquisition date revenues and net income of the acquired entity that were included in the Company's consolidated statements of income for the years ended December 31, 2017 and 2016 and unaudited pro forma consolidated revenues and net income as if the Flathead acquisition had occurred as of January 1, 2015, are not presented. Goodwill arising from the Flathead acquisition consists largely of the synergies and economies of scale expected from combining the operations of the acquired entity and the Company. The Flathead acquisition was accounted for as a deemed asset purchase; therefore, goodwill recorded in conjunction with the Flathead acquisition is deductible for income tax purposes. Fair values of assets acquired and liabilities assumed as part of the Flathead acquisition were estimated using relevant market information and significant other inputs and generally fall within Levels 2 and 3 of the fair value hierarchy. Acquisition related expenses The Company recorded third party acquisition related costs of $27.2 million , $2.8 million and $0.8 million in 2017 , 2016 and 2015 , respectively. These costs are included in acquisition related expenses in the Company's consolidated statements of income.

Goodwill and Core Deposit Intan

Goodwill and Core Deposit Intangibles	12 Months Ended
	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Core Deposit Intangibles	GOODWILL AND CORE DEPOSIT INTANGIBLES Goodwill Year Ended December 31, 2017 2016 2015 Net carrying value at beginning of period $ 212.8 $ 204.5 $ 205.6 Acquisitions and measurement period adjustments 231.9 8.3 (1.1 ) Net carrying value at end of period $ 444.7 $ 212.8 $ 204.5 Goodwill increased $231.9 million , or 109.0% , to $444.7 million as of December 31, 2017 , from $212.8 million as of December 31, 2016, attributable to the provisional goodwill recorded in conjunction with the acquisition of BOTC. Goodwill increased $8.3 million to $212.8 million as of December 31, 2016 , from $204.5 million as of December 31, 2015 , attributable to the goodwill recorded in conjunction with the Flathead acquisition. In 2015, goodwill decreased $1.1 million primarily due to the decrease in recorded goodwill resulting from the finalization of the fair valuation of deferred tax assets acquired in the Mountain West Financial Corp, or MWFC, acquisition. The Company performed an impairment assessment as of July 1, 2017, 2016 , and 2015 and concluded that there was no impairment to goodwill. The following table sets forth activity for identifiable core deposit intangibles subject to amortization: Core deposit intangibles ("CDI") Year Ended December 31, 2017 2016 2015 CDI, net, beginning of period $ 9.7 $ 10.6 $ 13.3 Established through acquisitions 48.0 2.5 0.7 Reductions due to sale of accounts (3.2 ) — — Current period amortization 5.4 3.4 3.4 Total CDI, net, at end of period $ 49.1 $ 9.7 $ 10.6 Core deposit intangibles 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 Company sold the custodial rights to our Health Savings Account ("HSA") portfolio to HealthEquity, Inc. for $6.2 million , of which $3.2 million was attributable to BOTC acquired deposits which were sold at fair market value. The following table provides estimated future CDI amortization expense: Years ending December 31, 2018 $ 6.9 2019 6.6 2020 6.2 2021 5.7 2022 5.3 Thereafter 18.4 Total $ 49.1

Investment Securities

Investment Securities	12 Months Ended
	Dec. 31, 2017
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, 2017 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Available-for-Sale U.S. Treasury notes $ 3.2 $ — $ — $ 3.2 Obligations of U.S. government agencies 569.5 — (8.0 ) 561.5 U.S. agency residential mortgage-backed securities & collateralized mortgage obligations 1,474.1 3.8 (15.4 ) 1,462.5 Private mortgage-backed securities 91.5 — (0.8 ) 90.7 Corporate Securities 88.0 0.1 (0.3 ) 87.8 Other investments 3.0 — — 3.0 Total $ 2,229.3 $ 3.9 $ (24.5 ) $ 2,208.7 December 31, 2017 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Held-to Maturity State, county and municipal securities $ 172.4 $ 2.6 $ (0.6 ) $ 174.4 Corporate securities 61.6 0.1 (0.3 ) 61.4 Obligations of U.S. government agencies 19.8 — (0.2 ) 19.6 U.S agency residential mortgage-backed securities & 230.5 8.8 (11.6 ) 227.7 Other investments 0.2 — — 0.2 Total $ 484.5 $ 11.5 $ (12.7 ) $ 483.3 December 31, 2016 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Available-for-Sale U.S. Treasury notes $ 3.6 $ — $ — $ 3.6 Obligations of U.S. government agencies 397.4 0.3 (6.4 ) 391.3 U.S. agency residential mortgage-backed securities & collateralized mortgage obligations 1,220.9 6.4 (13.6 ) 1,213.7 Private mortgage-backed securities 0.1 — — 0.1 Other investments 3.0 — — 3.0 Total $ 1,625.0 $ 6.7 $ (20.0 ) $ 1,611.7 December 31, 2016 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Held-to Maturity State, county and municipal securities $ 160.2 $ 2.7 $ (0.5 ) $ 162.4 Corporate securities 53.0 0.2 (0.2 ) 53.0 Obligations of U.S. government agencies 19.8 — (0.2 ) 19.6 U.S. agency residential mortgage-backed securities & collateralized mortgage obligations 279.6 7.8 (9.2 ) 278.2 Other investments 0.2 — — 0.2 Total $ 512.8 $ 10.7 $ (10.1 ) $ 513.4 Gross gains of $1.1 million and gross losses of $0.4 million were realized on the disposition of available-for-sale securities in 2017 . Gross gains of $0.4 million and $0.2 million were realized on the disposition of available-for-sale securities in 2016 and 2015 , respectively. Gross losses of $0.1 million and $0.02 million were realized on the disposition of available-for-sale securities in 2016 and 2015 , respectively. As of December 31, 2017 , the Company had general obligation securities with amortized costs of $133.5 million included in state, county and municipal securities, of which $108.2 million were issued by political subdivisions or agencies within the states of Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming. On October 30, 2015 , the Company transferred available-for-sale U.S. agency residential mortgage-backed securities and collateralized mortgage obligations with amortized costs and fair values of $100.3 million and $100.1 million , respectively, into the held-to-maturity category. Unrealized net losses of $0.2 million included in accumulated other comprehensive income at the time of the transfer are being amortized to yield over the remaining expected lives of the transferred securities of 4.0 years . 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, 2017 and 2016 . Less than 12 Months 12 Months or More Total December 31, 2017 Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Available-for-Sale Obligations of U.S. government agencies $ 284.9 $ (3.4 ) $ 266.1 $ (4.6 ) $ 551.0 $ (8.0 ) U.S. agency residential mortgage-backed securities & collateralized mortgage obligations 670.1 (6.2 ) 439.2 (9.2 ) 1,109.3 (15.4 ) Private mortgage-backed securities 74.0 (0.8 ) — — 74.0 (0.8 ) Corporate securities 51.3 (0.3 ) — — 51.3 (0.3 ) Total $ 1,080.3 $ (10.7 ) $ 705.3 $ (13.8 ) $ 1,785.6 $ (24.5 ) Less than 12 Months 12 Months or More Total December 31, 2017 Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Held-to-Maturity State, county and municipal securities $ 53.3 $ (0.4 ) $ 12.3 $ (0.2 ) $ 65.6 $ (0.6 ) Corporate securities 41.2 (0.2 ) 5.0 (0.1 ) 46.2 (0.3 ) U.S. agency residential mortgage-backed 76.4 (9.1 ) 60.5 (2.5 ) 136.9 (11.6 ) Obligations of U.S. government agencies 9.7 — 9.9 (0.2 ) 19.6 (0.2 ) Total $ 180.6 $ (9.7 ) $ 87.7 $ (3.0 ) $ 268.3 $ (12.7 ) Less than 12 Months 12 Months or More Total December 31, 2016 Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Available-for-Sale U.S. Treasury notes $ 0.6 $ — $ — $ — $ 0.6 $ — Obligations of U.S. government agencies 316.5 (6.4 ) — — 316.5 (6.4 ) U.S. agency residential mortgage-backed securities & collateralized mortgage obligations 746.3 (13.1 ) 15.8 (0.5 ) 762.1 (13.6 ) Total $ 1,063.4 $ (19.5 ) $ 15.8 $ (0.5 ) $ 1,079.2 $ (20.0 ) Less than 12 Months 12 Months or More Total December 31, 2016 Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Held-to-Maturity State, county and municipal securities $ 42.5 $ (0.5 ) $ 2.8 $ — $ 45.3 $ (0.5 ) Corporate securities 32.5 (0.2 ) — — $ 32.5 $ (0.2 ) U.S. agency residential mortgage-backed $ 108.8 $ (7.9 ) $ 20.0 $ (1.3 ) $ 128.8 $ (9.2 ) Obligations of U.S. government agencies $ 19.6 $ (0.2 ) $ — $ — $ 19.6 $ (0.2 ) Total $ 203.4 $ (8.8 ) $ 22.8 $ (1.3 ) $ 226.2 $ (10.1 ) 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 581 and 396 individual investment securities that were in an unrealized loss position as of December 31, 2017 and 2016 , respectively. Unrealized losses as of December 31, 2017 and 2016 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, 2017 , 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 2017 , 2016 or 2015 . Maturities of investment securities at December 31, 2017 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, 2017 Amortized Cost Estimated Fair Value Amortized Cost Estimated Fair Value Within one year $ 450.7 $ 447.5 $ 76.1 $ 76.7 After one year but within five years 1,422.7 1,409.3 266.2 263.1 After five years but within ten years 277.6 274.2 111.7 113.3 After ten years 78.3 77.7 30.5 30.2 Total $ 2,229.3 $ 2,208.7 $ 484.5 $ 483.3 At December 31, 2017 , the Company had investment securities callable within one year with amortized costs and estimated fair values of $133.2 million and $132.9 million , respectively. These investment securities are primarily classified as available-for-sale and included in the after one year but within five years category in the table above. At December 31, 2017 , 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, 2017 and 2016 , the Company had variable rate mortgage-backed securities with amortized costs of $247.9 million and $66.2 million , respectively, classified as available-for-sale in the table above. There are no significant concentrations of investments at December 31, 2017 , (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 $2,087.7 million and $1,400.1 million at December 31, 2017 and 2016 , respectively, were pledged to secure public deposits and securities sold under repurchase agreements. The approximate fair value of securities pledged at December 31, 2017 and 2016 was $2,062.6 million and $1,388.2 million , respectively. All securities sold under repurchase agreements are with customers 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, 2017
Loans Receivable, Net [Abstract]
Loans	LOANS The following table presents loans by class as of the dates indicated: December 31, 2017 2016 Real estate loans: Commercial $ 2,822.9 $ 1,834.4 Construction: Land acquisition & development 348.7 208.5 Residential 240.2 147.9 Commercial 119.4 125.6 Total construction loans 708.3 482.0 Residential 1,487.4 1,027.4 Agricultural 158.2 170.2 Total real estate loans 5,176.8 3,514.0 Consumer: Indirect consumer 784.7 752.4 Other consumer 175.1 148.1 Credit card 74.6 69.8 Total consumer loans 1,034.4 970.3 Commercial 1,215.4 797.9 Agricultural 136.2 132.9 Other, including overdrafts 4.9 1.6 Loans held for investment 7,567.7 5,416.7 Mortgage loans held for sale 46.6 61.8 Total loans $ 7,614.3 $ 5,478.5 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 $397.0 million and $321.5 million as of December 31, 2017 and 2016 , respectively. 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, 2017 and 2016 . December 31, 2017 2016 Outstanding principal $ 38.2 $ 35.8 Carrying value: Loans on accrual status 24.9 21.9 Total carrying value $ 24.9 $ 21.9 The following table summarizes changes in the accretable yield for loans acquired credit impaired for the years ended December 31, 2017 and 2016 : Year Ended December 31, 2017 2016 2015 Beginning balance $ 6.8 $ 6.7 $ 5.8 Acquisitions 1.9 1.1 0.4 Accretion income (2.9 ) (2.5 ) (2.9 ) Additions 0.1 — 0.6 Reductions due to exit events (1.5 ) (1.1 ) (0.5 ) Reclassifications from nonaccretable differences 2.9 2.6 3.3 Ending balance $ 7.3 $ 6.8 $ 6.7 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, 2017 Past Due Past Due Past Due Past Due Loans Loans Loans Real estate Commercial $ 2.9 $ 0.5 $ 0.3 $ 3.7 $ 2,792.4 $ 26.8 $ 2,822.9 Construction: Land acquisition & development 7.3 0.3 0.3 7.9 337.8 3.0 348.7 Residential 2.1 — — 2.1 236.4 1.7 240.2 Commercial — — — — 115.6 3.8 119.4 Total construction loans 9.4 0.3 0.3 10.0 689.8 8.5 708.3 Residential 13.3 1.4 0.4 15.1 1,464.1 8.2 1,487.4 Agricultural 0.3 — 0.2 0.5 154.3 3.4 158.2 Total real estate loans 25.9 2.2 1.2 29.3 5,100.6 46.9 5,176.8 Consumer: Indirect consumer 7.8 2.1 0.4 10.3 772.6 1.8 784.7 Other consumer 1.6 0.5 0.1 2.2 172.6 0.3 175.1 Credit card 0.9 0.6 0.7 2.2 72.4 — 74.6 Total consumer loans 10.3 3.2 1.2 14.7 1,017.6 2.1 1,034.4 Commercial 3.9 1.7 0.7 6.3 1,189.5 19.6 1,215.4 Agricultural 1.8 0.1 — 1.9 133.5 0.8 136.2 Other, including overdrafts — — — — 4.9 — 4.9 Loans held for investment 41.9 7.2 3.1 52.2 7,446.1 69.4 7,567.7 Mortgage loans originated for sale — — — — 46.6 — 46.6 Total loans $ 41.9 $ 7.2 $ 3.1 $ 52.2 $ 7,492.7 $ 69.4 $ 7,614.3 Total Loans 30 - 59 60 - 89 > 90 30 or More Days Days Days Days Current Non-accrual Total As of December 31, 2016 Past Due Past Due Past Due Past Due Loans Loans Loans Real estate Commercial $ 7.3 $ 1.1 $ 0.3 $ 8.7 $ 1,799.5 $ 26.2 $ 1,834.4 Construction: Land acquisition & development 0.6 0.4 0.3 1.3 202.2 5.0 208.5 Residential 0.9 0.3 — 1.2 146.2 0.5 147.9 Commercial — — — — 124.8 0.8 125.6 Total construction loans 1.5 0.7 0.3 2.5 473.2 6.3 482.0 Residential 4.0 1.3 0.7 6.0 1,015.0 6.4 1,027.4 Agricultural 0.3 0.3 — 0.6 165.3 4.3 170.2 Total real estate loans 13.1 3.4 1.3 17.8 3,453.0 43.2 3,514.0 Consumer: Indirect consumer 8.4 2.3 0.7 11.4 740.2 0.8 752.4 Other consumer 1.3 0.2 0.2 1.7 146.1 0.3 148.1 Credit card 0.5 0.3 0.6 1.4 68.4 — 69.8 Total consumer loans 10.2 2.8 1.5 14.5 954.7 1.1 970.3 Commercial 3.2 0.7 0.7 4.6 767.9 25.4 797.9 Agricultural 1.5 0.4 — 1.9 128.0 3.0 132.9 Other, including overdrafts — — 0.3 0.3 1.3 — 1.6 Loans held for investment 28.0 7.3 3.8 39.1 5,304.9 72.7 5,416.7 Mortgage loans originated for sale — — — — 61.8 — 61.8 Total loans $ 28.0 $ 7.3 $ 3.8 $ 39.1 $ 5,366.7 $ 72.7 $ 5,478.5 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 $3.5 million , $3.4 million and $3.2 million during the years ended December 31, 2017 , 2016 and 2015 , 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, 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 December 31, 2016 Unpaid Total Principal Balance Recorded Investment With No Allowance Recorded Investment With Allowance Total Recorded Investment Related Allowance Real estate: Commercial $ 57.0 $ 24.4 $ 21.4 $ 45.8 $ 2.8 Construction: Land acquisition & development 12.1 4.3 1.8 6.1 0.8 Residential 1.6 0.2 0.6 0.8 — Commercial 4.8 3.9 0.7 4.6 0.7 Total construction loans 18.5 8.4 3.1 11.5 1.5 Residential 8.2 4.1 2.5 6.6 0.3 Agricultural 5.1 4.5 0.2 4.7 — Total real estate loans 88.8 41.4 27.2 68.6 4.6 Commercial 40.3 13.2 19.2 32.4 9.3 Agricultural 3.7 3.3 0.4 3.7 0.1 Total $ 132.8 $ 57.9 $ 46.8 $ 104.7 $ 14.0 December 31, 2015 Unpaid Total Principal Balance Recorded Investment With No Allowance Recorded Investment With Allowance Total Recorded Investment Related Allowance Real estate: Commercial $ 58.2 $ 27.9 $ 17.6 $ 45.5 $ 3.4 Construction: Land acquisition & development 15.5 7.2 0.8 8.0 0.3 Residential 1.0 0.3 — 0.3 — Commercial 1.3 0.3 0.7 1.0 0.7 Total construction loans 17.8 7.8 1.5 9.3 1.0 Residential 7.1 3.6 2.3 5.9 0.4 Agricultural 6.4 5.6 0.2 5.8 — Total real estate loans 89.5 44.9 21.6 66.5 4.8 Commercial 29.6 10.8 13.7 24.5 6.5 Agricultural 1.3 0.6 0.4 1.0 0.3 Total $ 120.4 $ 56.3 $ 35.7 $ 92.0 $ 11.6 The following tables present the average recorded investment in and income recognized on impaired loans for the periods indicated: Year Ended December 31, 2017 2016 2015 Average Recorded Investment Income Recognized Average Recorded Investment Income Recognized Average Recorded Investment Income Recognized Real estate: Commercial $ 40.4 $ 0.3 $ 46.9 $ 0.2 $ 39.2 $ 0.8 Construction: Land acquisition & development 5.0 — 8.0 0.1 8.3 0.1 Residential 1.3 — 0.8 — 0.3 — Commercial 4.2 — 2.9 — 1.9 — Total construction loans 10.5 — 11.7 0.1 10.5 0.1 Residential 8.4 — 6.2 — 4.1 — Agricultural 4.2 — 4.4 — 7.2 — Total real estate loans 63.5 0.3 69.2 0.3 61.0 0.9 Commercial 28.1 0.2 30.1 0.3 18.5 0.1 Agricultural 2.4 — 1.8 — 0.9 — Total $ 94.0 $ 0.5 $ 101.1 $ 0.6 $ 80.4 $ 1.0 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 2017 , 2016 and 2015 would have been approximately $3.5 million , $4.2 million and $3.9 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 $44.5 million as of December 31, 2017 , of which $31.9 million were included in non-accrual loans and $12.6 million were on accrual status. The Company had loans renegotiated in troubled debt restructurings of $49.6 million as of December 31, 2016 , of which $27.3 million were included in non-accrual loans and $22.3 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, 2017 Interest only period Extension of terms or maturity Interest rate adjustment Other Real estate: Commercial 5 $ 1.5 $ 0.4 $ — $ 0.9 $ 2.8 Agriculture 1 — 0.8 — — 0.8 Total real estate loans 6 1.5 1.2 — 0.9 3.6 Commercial 17 1.2 2.0 — 6.0 9.2 Agriculture 1 — 0.1 — — 0.1 Total 24 $ 2.7 $ 3.3 $ — $ 6.9 $ 12.9 Number of Notes Type of Concession Principal Balance at Restructure Date Year ended December 31, 2016 Interest only period Extension of terms or maturity Interest rate adjustment Other Real estate: Commercial 18 $ 0.4 $ 5.5 $ 0.2 $ 1.8 $ 7.9 Commercial construction 1 — 3.7 — — 3.7 Residential 1 — 0.1 — — 0.1 Total real estate loans 20 0.4 9.3 0.2 1.8 11.7 Commercial 13 4.4 0.4 — 3.3 8.1 Agriculture 2 — 0.3 — — 0.3 Total 35 $ 4.8 $ 10.0 $ 0.2 $ 5.1 $ 20.1 Number of Notes Type of Concession Principal Balance at Restructure Date Year ended December 31, 2015 Interest only period Extension of terms or maturity Interest rate adjustment Other Commercial real estate 2 $ — $ 0.6 $ — $ 0.1 $ 0.7 Consumer 1 — — — — — Commercial 12 — 8.7 3.3 0.5 12.5 Total 15 $ — $ 9.3 $ 3.3 $ 0.6 $ 13.2 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 2017 , 2016 or 2015 . 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 following table presents information on the Company's troubled debt restructurings during the previous 12 months for which there was a payment default. Year ended December 31, 2017 Number of Notes Balance Commercial 1 $ 1.3 Total 1 $ 1.3 As of December 31, 2016 and 2015 , loans modified in troubled debt restructurings within the previous 12 months for which there was a payment default during the period were not significant. As of December 31, 2017 and 2016 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, 2017 , 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, 2017 Other Assets Especially Mentioned Substandard Doubtful Total Criticized Loans Real estate: Commercial $ 78.0 $ 96.4 $ 10.3 $ 184.7 Construction: Land acquisition & development 3.2 16.4 — 19.6 Residential 2.3 1.7 0.5 4.5 Commercial 2.4 3.6 3.5 9.5 Total construction loans 7.9 21.7 4.0 33.6 Residential 3.9 12.5 1.9 18.3 Agricultural 4.3 19.1 — 23.4 Total real estate loans 94.1 149.7 16.2 260.0 Consumer: Indirect consumer 0.8 2.2 0.3 3.3 Other consumer 0.4 0.7 0.2 1.3 Total consumer loans 1.2 2.9 0.5 4.6 Commercial 54.7 56.3 11.1 122.1 Agricultural 5.1 8.3 0.4 13.8 Total $ 155.1 $ 217.2 $ 28.2 $ 400.5 As of December 31, 2016 Other Assets Especially Mentioned Substandard Doubtful Total Criticized Loans Real estate: Commercial $ 85.3 $ 85.3 $ 10.8 $ 181.4 Construction: Land acquisition & development 13.4 6.2 1.4 21.0 Residential 0.4 1.6 0.7 2.7 Commercial 1.6 6.3 0.7 8.6 Total construction loans 15.4 14.1 2.8 32.3 Residential 5.0 12.5 0.8 18.3 Agricultural 3.8 17.8 — 21.6 Total real estate loans 109.5 129.7 14.4 253.6 Consumer: Indirect consumer 0.8 1.5 0.1 2.4 Other consumer 0.7 1.0 0.3 2.0 Total consumer loans 1.5 2.5 0.4 4.4 Commercial 46.4 29.3 21.2 96.9 Agricultural 6.2 10.7 0.4 17.3 Total $ 163.6 $ 172.2 $ 36.4 $ 372.2 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, 2017
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, 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 (2.8 ) (0.2 ) — 11.1 Less loans charged-off (4.4 ) (11.3 ) (6.8 ) (0.4 ) — (22.9 ) Add back recoveries of loans previously charged-off 1.4 4.2 2.1 — — 7.7 Ending balance $ 31.6 $ 8.7 $ 30.6 $ 1.2 $ — $ 72.1 Individually evaluated for impairment $ 6.2 $ — $ 4.4 $ 0.2 $ — $ 10.8 Collectively evaluated for impairment 25.4 8.7 26.2 1.0 — 61.3 Ending balance $ 31.6 $ 8.7 $ 30.6 $ 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 Year ended December 31, 2016 Real Estate Consumer Commercial Agriculture Other Total Allowance for loan losses: Beginning balance $ 52.3 $ 5.1 $ 18.8 $ 0.6 $ — $ 76.8 Provision charged (credited) to operating expense (21.7 ) 8.4 21.9 1.4 — 10.0 Less loans charged-off (5.2 ) (8.6 ) (5.8 ) (0.2 ) — (19.8 ) Add back recoveries of loans previously charged-off 3.2 2.8 3.2 — — 9.2 Ending balance $ 28.6 $ 7.7 $ 38.1 $ 1.8 $ — $ 76.2 Individually evaluated for impairment $ 4.6 $ — $ 9.3 $ 0.1 $ — $ 14.0 Collectively evaluated for impairment 24.0 7.7 28.8 1.7 — 62.2 Ending balance $ 28.6 $ 7.7 $ 38.1 $ 1.8 $ — $ 76.2 Total loans: Individually evaluated for impairment $ 68.6 $ — $ 32.4 $ 3.7 $ — $ 104.7 Collectively evaluated for impairment 3,445.4 970.3 765.5 129.2 1.6 5,312.0 Total loans held for investment $ 3,514.0 $ 970.3 $ 797.9 $ 132.9 $ 1.6 $ 5,416.7 Year ended December 31, 2015 Real Estate Consumer Commercial Agriculture Other Total Allowance for loan losses: Beginning balance $ 53.9 $ 5.0 $ 14.3 $ 1.0 $ — $ 74.2 Provision charged (credited) to operating expense (0.6 ) 3.2 4.4 (0.2 ) — 6.8 Less loans charged-off (4.1 ) (5.7 ) (1.7 ) (0.2 ) — (11.7 ) Add back recoveries of loans previously charged-off 3.1 2.6 1.8 — — 7.5 Ending balance $ 52.3 $ 5.1 $ 18.8 $ 0.6 $ — $ 76.8 Individually evaluated for impairment $ 4.8 $ — $ 6.5 $ 0.3 $ — $ 11.6 Collectively evaluated for impairment 47.5 5.1 12.3 0.3 — 65.2 Ending balance $ 52.3 $ 5.1 $ 18.8 $ 0.6 $ — $ 76.8 Total loans: Individually evaluated for impairment $ 66.5 $ — $ 24.5 $ 1.0 $ — $ 92.0 Collectively evaluated for impairment 3,346.6 844.4 767.9 141.2 1.3 5,101.4 Total loans held for investment $ 3,413.1 $ 844.4 $ 792.4 $ 142.2 $ 1.3 $ 5,193.4 The Company performs a quarterly assessment of the adequacy of its allowance for loan losses in accordance with generally accepted accounting principles. 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, loss factor percentages are based on a one-year loss history. 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 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, 2017 and 2016 , the Company's allowance for loans losses included $1.0 million and $0.4 million , respectively, related to loans acquired credit impaired.

Premises and Equipment

Premises and Equipment	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Premises and Equipment	PREMISES AND EQUIPMENT Premises and equipment and related accumulated depreciation are as follows: December 31, 2017 2016 Land $ 49.4 $ 39.8 Buildings and improvements 257.1 218.3 Furniture and equipment 97.7 88.6 404.2 346.7 Less accumulated depreciation (162.3 ) (152.2 ) Premises and equipment, net $ 241.9 $ 194.5 The Parent Company and a FIB branch office lease premises from an affiliated entity. See Note 17—Commitments and Contingencies.

Company-Owned Life Insurance

Company-Owned Life Insurance	12 Months Ended
	Dec. 31, 2017
Investments, All Other Investments [Abstract]
Company-Owned Life Insurance	COMPANY-OWNED LIFE INSURANCE Company-owned life insurance consists of the following: December 31, 2017 2016 Key executive, principal shareholder $ 4.2 $ 4.0 Key executive split dollar 4.9 4.8 Group life 251.5 189.3 Total $ 260.6 $ 198.1 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 $4.2 million and $4.0 million at December 31, 2017 and 2016 , respectively. The Company also has life insurance policies covering selected other key officers. The net cash surrender value of these policies was $4.9 million and $4.8 million at December 31, 2017 and 2016 , 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 $251.5 million and $189.3 million at December 31, 2017 and 2016 , respectively. Under these policies, the Company receives benefits payable upon death of the insured. The Company has entered into either an endorsement split dollar agreement or a survivor income benefit agreement with each insured officer. Under the endorsement split dollar agreements, a portion of the policy death benefit is payable to the insured's designated beneficiary if the insured is employed by the Company at the time of death. Under the survivor income benefit agreements, the Company makes a lump-sum payment to the insured's designated beneficiary if the insured is employed by the Company at the time of death.

Other Real Estate Owned

Other Real Estate Owned	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 2015 Balance at beginning of year $ 10.0 $ 6.3 $ 13.6 Acquisitions 1.2 1.1 — Additions 5.4 7.6 5.6 Valuation adjustments (0.4 ) (0.6 ) (0.2 ) Dispositions (6.1 ) (4.4 ) (12.7 ) Balance at end of year $ 10.1 $ 10.0 $ 6.3 Write-downs of $0.4 million during 2017 included adjustments of $0.3 million directly related to receipt of updated appraisals and adjustments of $0.1 million based on other sources, including management estimates of the current fair value of properties. Write-downs of $0.6 million during 2016 included adjustments of $0.55 million directly related to receipt of updated appraisals and adjustments of $0.05 million based on other sources, including management estimates of the current fair value of properties. Write-downs of $0.2 million during 2015 included adjustments of $0.1 million directly related to receipt of updated appraisals and adjustments of $0.1 million based on other sources, including management estimates of the current fair value of properties.

Derivatives and Hedging Activit

Derivatives and Hedging Activities	12 Months Ended
	Dec. 31, 2017
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, 2017 December 31, 2016 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 $ 344.2 $ 7.5 $ 53.6 $ 1.3 Interest rate lock commitments 60.7 1.3 73.4 1.1 Forward loan sales contracts — — 126.8 0.3 Total derivative assets $ 404.9 $ 8.8 $ 253.8 $ 2.7 Derivative Liabilities (included in accounts payable and accrued expenses on the consolidated balance sheets) Derivatives designated as hedges: Interest rate swap contracts $ — $ — $ 100.0 $ — Non-hedging interest rate derivatives: Interest rate swap contracts 344.2 7.8 53.6 1.3 Forward loan sales contracts 88.8 0.1 — — Total derivative liabilities $ 433.0 $ 7.9 $ 153.6 $ 1.3 On September 6, 2017, the Company paid $1.1 million to terminate an existing interest rate swap contract designated as a cash flow hedge, originally entered into on September 22, 2015, with a notional amount of $100 million . Under the terms of the interest rate swap contract, the Company would have paid a fixed interest rate of 1.94% and the counterparty would have paid to the Company a variable interest rate equal to the three-month LIBOR. As the contract was terminated prior to the effective date of September 15, 2017, no cash was exchanged outside of the termination payment. The effective portion of changes in the fair value of derivatives designated and that qualify as cash flow hedges is recorded in Accumulated Other Comprehensive Income and is subsequently reclassified into earnings in the period that the hedged forecasted transaction affects earnings. The ineffective portion of the change in fair value of the derivatives is recognized directly in earnings. The Company did not record any hedge ineffectiveness at December 31, 2017 and 2016 . 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, 2017 and December 31, 2016 : December 31, 2017 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 $ 7.5 $ — $ 7.5 $ 2.4 $ — $ 5.1 Mortgage related derivatives 1.3 — 1.3 — — 1.3 Total derivatives 8.8 — 8.8 2.4 — 6.4 Total assets $ 8.8 $ — $ 8.8 $ 2.4 $ — $ 6.4 Financial Liabilities Interest rate swap contracts $ 7.8 $ — $ 7.8 $ 2.4 $ 3.3 $ 2.1 Mortgage related derivatives 0.1 — 0.1 — — 0.1 Total derivatives 7.9 — 7.9 2.4 3.3 2.2 Repurchase agreements 643.0 — 643.0 — 643.0 — Total liabilities $ 650.9 $ — $ 650.9 $ 2.4 $ 646.3 $ 2.2 December 31, 2016 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 $ 1.3 $ — $ 1.3 $ 0.5 $ — $ 0.8 Mortgage related derivatives 1.4 — 1.4 — — 1.4 Total derivatives 2.7 — 2.7 0.5 — 2.2 Total assets $ 2.7 $ — $ 2.7 $ 0.5 $ — $ 2.2 Financial Liabilities Interest rate swap contracts $ 1.3 $ — $ 1.3 $ 0.5 $ — $ 0.8 Total derivatives 1.3 — 1.3 0.5 — 0.8 Repurchase agreements 537.6 — 537.6 — 537.6 — Total liabilities $ 538.9 $ — $ 538.9 $ 0.5 $ 537.6 $ 0.8 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, 2017 2016 2015 Derivatives designated as hedges: Amount of loss recognized in other comprehensive income (effective portion) $ (1.1 ) $ (0.2 ) $ 0.2 Reclassification adjustment for derivative net (gains) losses included in income 1.1 — — Non-hedging interest rate derivatives: Amount of gain (loss) recognized in other non-interest income — 0.1 — Amount of net fee income recognized in other non-interest income 0.8 0.9 0.1 Amount of net gains (losses) recognized in mortgage banking revenues (1.7 ) 1.4 —

Mortgage Servicing Rights

Mortgage Servicing Rights	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 2015 Balance at beginning of year $ 18.7 $ 15.9 $ 14.4 Acquisitions of mortgage servicing rights 3.5 — 0.3 Originations of mortgage servicing rights 5.6 5.8 3.6 Amortization expense (3.0 ) (3.0 ) (2.4 ) Balance at end of year 24.8 18.7 15.9 Less valuation reserve — (0.2 ) (0.2 ) Balance at end of year, net of valuation reserve $ 24.8 $ 18.5 $ 15.7 Principal balance of serviced loans underlying mortgage servicing rights $ 3,636.7 $ 3,127.5 $ 2,906.2 Mortgage servicing rights as a percentage of serviced loans 0.68 % 0.59 % 0.54 % At December 31, 2017 , the estimated fair value and weighted average remaining life of the Company’s mortgage servicing rights were $40.1 million and 7.3 years, respectively. The fair value of mortgage servicing rights was determined using discount rates ranging from 9.5% to 11.3% and monthly prepayment speeds ranging from 0.5% to 1.8% depending upon the risk characteristics of the underlying loans. At December 31, 2016 , the estimated fair value and weighted average remaining life of the Company’s mortgage servicing rights were $35.7 million and 8.0 years, respectively. The fair value of mortgage servicing rights was determined using discount rates ranging from 9.6% to 11.4% and monthly prepayment speeds ranging from 0.5% to 1.5% depending upon the risk characteristics of the underlying loans. The Company reversed impairment of $0.09 million , $0.04 million and $0.10 million in 2017 , 2016 and 2015 , respectively. No permanent impairment was recorded in 2017 , 2016 or 2015 .

Deposits

Deposits	12 Months Ended
	Dec. 31, 2017
Deposits [Abstract]
Deposits	DEPOSITS Deposits are summarized as follows: December 31, 2017 2016 Non-interest bearing demand $ 2,900.0 $ 1,906.3 Interest bearing: Demand 2,787.5 2,276.5 Savings 3,095.4 2,141.8 Time, $100 and over 432.0 461.4 Time, other 720.0 590.1 Total interest bearing 7,034.9 5,469.8 Total deposits $ 9,934.9 $ 7,376.1 The Company had no brokered time deposits as of December 31, 2017 and 2016 . Other time deposits include deposits obtained through the Company’s participation in the Certificate of Deposit Account Registry Service (“CDARS”). CDARS deposits totaled $94.2 million and $25.7 million as of December 31, 2017 and 2016 , respectively. As of December 31, 2017 and 2016 , the Company had time deposits of $182.0 million and $208.5 million , respectively, that met or exceeded the FDIC insurance limit of $0.25 million . Maturities of time deposits at December 31, 2017 are as follows: Time, $100 and Over Total Time Due within 3 months or less $ 88.6 $ 313.8 Due after 3 months and within 6 months 65.3 167.3 Due after 6 months and within 12 months 110.6 259.2 Due after 12 months 167.5 411.6 Total $ 432.0 $ 1,151.9 Interest expense on time deposits of $100 or more was $4.3 million , $3.7 million and $3.7 million for the years ended December 31, 2017 , 2016 and 2015 , respectively.

Long-Term Debt and Other Borrow

Long-Term Debt and Other Borrowed Funds	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 Parent Company: 6.81% subordinated term loan maturing January 9, 2018, principal due at maturity, interest payable quarterly $ — $ 20.0 Subsidiaries: 8.00% capital lease obligation with term ending October 25, 2029 1.4 1.5 6.24% note payable maturing September 6, 2032, principal due at maturity, interest payable monthly 1.6 1.5 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 — Total long-term debt $ 13.1 $ 28.0 Maturities of long-term debt at December 31, 2017 are as follows: 2018 $ 0.1 2019 0.1 2020 0.1 2021 0.1 2022 5.1 Thereafter 7.6 Total $ 13.1 On January 10, 2008 , the Company borrowed $20.0 million on a 6.81% unsecured subordinated term loan maturing January 9, 2018 , with interest payable quarterly and principal due at maturity. As the maturity of the debt is under one year, the balance was reclassified to other borrowed funds as of December 31, 2017 . On January 8, 2018, the Company paid $20.0 million to redeem in full the principal and interest related to the 6.81% unsecured subordinated term loan. The Company has available lines of credit with the FHLB of approximately $1,313.3 million , subject to collateral availability. As of December 31, 2017 and 2016 , there were no long or short-term advances outstanding with the FHLB. The Company has a capital lease obligation on a banking office. The balance of the obligation was $1.4 million and $1.5 million as of December 31, 2017 and 2016 , respectively. Assets acquired under capital lease, consisting solely of a building and leasehold improvements, are included in premises and equipment and are subject to depreciation. In conjunction with acquisitions in 2014, the Company assumed a 6.24% fixed rate unsecured note payable related to a new markets tax credit. The note payable matures on September 6, 2032 , with interest payable monthly and principal due at maturity. The balance of the obligation was $1.6 million and $1.5 million as of December 31, 2017 and 2016 , respectively. The note is collateralized by the Company's equity interest in ONE Sub-CDE, LLC, a CDE owned 99.9% by the Company. On January 29, 2015 , the Company borrowed $5.0 million on a 2.28% note payable maturing July 29, 2022 , with interest payable monthly and principal due at maturity. The note is collateralized by the Company's equity interest in Universal Sub CDE, LLC, a CDE owned 99.9% by the Company. On November 16, 2017 , the Company borrowed $5.1 million on a 1.00% fixed rate note payable maturing on December 31, 2041 , with interest only, payable quarterly, until December 31, 2025 and then principal and interest payable until maturity. The note is collateralized by the Company's equity interest in BFCC Sub CDE, LLC, a CDE owned 99.9% by the Company. The Company had other borrowed funds of $20.01 million and $0.01 million as of December 31, 2017 and 2016 , respectively, consisting of demand notes issued to the United States Treasury, secured by investment securities and bearing no interest, and unsecured subordinated term loans in 2017. The Company has federal funds lines of credit with third parties amounting to $185.0 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 $ 464.8 million secured by a blanket pledge of indirect consumer loans.

Subordinated Debentures Held by

Subordinated Debentures Held by Subsidiary Trusts	12 Months Ended
	Dec. 31, 2017
Subordinated Borrowings [Abstract]
Subordinated Debentures Held by Subsidiary Trusts	SUBORDINATED DEBENTURES HELD BY SUBSIDIARY TRUSTS The Company sponsors six wholly-owned business trusts, Trust I, Trust II, Trust III, Trust IV, Trust V and Trust VI (collectively, the “Trusts”). The Trusts were formed for the exclusive purpose of issuing an aggregate of $80.0 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, $2.5 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 2017 2016 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 Total subordinated debentures held by subsidiary trusts $ 82.5 $ 82.5 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, 2017 the interest rate on the Subordinated Debentures was 3.94% . 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, 2017 , the interest rate on the Subordinated Debentures was 4.34% . 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, 2017 , the interest rate on the Subordinated Debentures was 3.99% . 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, 2017 the interest rate on the Subordinated Debentures was 4.39% . 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, 2017 the interest rate on the Subordinated Debentures was 4.44% . 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, 2017 , the interest rate on the Subordinated Debentures was 4.44% . 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. Subject to certain limitations, the Trust Preferred Securities qualify as tier 1 capital of the Parent Company under the Federal Reserve Board’s capital adequacy guidelines. Proceeds from the issuance of the Trust Preferred Securities were used to fund acquisitions.

Capital Stock and Dividend Rest

Capital Stock and Dividend Restrictions	12 Months Ended
	Dec. 31, 2017
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 33,560,202 shares of Class A common stock and 22,905,357 shares of Class B common stock outstanding as of December 31, 2017 . The Company had 21,613,885 shares of Class A common stock and 23,312,291 shares of Class B common stock outstanding as of December 31, 2016 . During 2017 , the Company issued 14,926 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 2017 . During 2016 , the Company issued 16,347 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 2016 . 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 2017 , there were no stock repurchases made pursuant to stock repurchase programs. During 2016 , the Company repurchased and retired 975,877 shares of its Class A common stock in a combination of open market and privately negotiated transactions at an aggregate purchase price of $25.5 million , or a weighted average price of $26.16 per share. The repurchases were made pursuant to stock repurchase programs approved by the Company's board of directors. All other stock repurchases during 2017 and 2016 were redemptions of vested restricted shares tendered in lieu of cash for payment of income tax withholding amounts by participants of the Company's equity compensation plans. On January 26, 2017, we filed a registration statement on Form S-4, as amended on March 20, 2017 and April 6, 2017 with registration statements on Form S-4/A, to register 11,839,179 shares of Class A common stock to be issued as partial consideration for our acquisition of Cascade Bancorp. On May 30, 2017, the Company issued 11,252,750 shares of its Class A common stock with an aggregate value of $386.0 million as partial consideration for the acquisition of Cascade Bancorp. 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. The payment of dividends by subsidiary banks is subject to various federal and state regulatory limitations. In general, a bank is limited, without the prior consent of its regulators, to paying dividends that do not exceed current year net profits together with retained earnings from the two preceding calendar years. The Company’s debt instruments also include limitations on the payment of dividends.

Earnings per Common Share

Earnings per Common Share	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 2015 Net income, basic and diluted $ 106.5 $ 95.6 $ 86.8 Weighted average common shares outstanding for basic earnings per share computation 51,429,366 44,511,774 45,184,091 Dilutive effects of stock-based compensation 473,843 398,622 462,327 Weighted average common shares outstanding for diluted earnings per common share computation 51,903,209 44,910,396 45,646,418 Basic earnings per common share $ 2.07 $ 2.15 $ 1.92 Diluted earnings per common share 2.05 2.13 1.90 The Company had 83,635 , 7,215 and 12,890 unvested time restricted stock outstanding as of December 31, 2017 , 2016 , and 2015 respectively, that were not included in the computation of diluted earnings per common share because their effect would be anti-dilutive. The Company had 113,874 , 155,637 and 142,512 shares of unvested restricted stock as of December 31, 2017 , 2016 and 2015 , 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, 2017
Banking and Thrift [Abstract]
Regulatory Capital	REGULATORY CAPITAL The Company is subject to the regulatory capital requirements administered by federal banking regulators and the Federal Reserve. Failure to meet minimum capital requirements can initiate certain mandatory and possible additional discretionary actions by regulators that, if undertaken, could have a direct material effect on the Company’s financial statements. Under capital adequacy guidelines and the regulatory framework for prompt corrective action, the Company must meet specific capital guidelines that involve quantitative measures of the Company’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. 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, 2017 , the Company exceeded all capital adequacy requirements to which it is subject. The Company’s actual capital amounts and ratios and selected minimum regulatory thresholds and prompt corrective action provisions as of December 31, 2017 and 2016 are presented in the following tables: Actual Adequately Capitalized Basel III Phase-In Schedule Adequately Capitalized Basel III Fully Phased-In Well Capitalized (1) Amount Ratio Amount Ratio Amount Ratio Amount Ratio December 31, 2017 Total risk-based capital: Consolidated $ 1,112.5 12.76 % $ 806.5 9.25 % $ 915.5 10.50 % $ 871.9 10.00 % FIB 1,066.6 12.29 802.7 9.25 911.2 10.50 867.8 10.00 Tier 1 risk-based capital: Consolidated 1,040.3 11.93 632.1 7.25 741.1 8.50 697.5 8.00 FIB 994.4 11.46 629.1 7.25 737.6 8.50 694.2 8.00 Common equity tier 1 risk-based capital: Consolidated 962.4 11.04 501.3 5.75 610.3 7.00 566.7 6.50 FIB 994.4 11.46 499.0 5.75 607.4 7.00 564.1 6.50 Leverage capital ratio: Consolidated 1,040.3 8.86 469.9 4.00 469.9 4.00 587.4 5.00 FIB 994.4 8.48 469.1 4.00 469.1 4.00 586.3 5.00 Actual Adequately Capitalized Basel III Phase-In Schedule Adequately Capitalized Basel III Fully Phased-In Well Capitalized (1) Amount Ratio Amount Ratio Amount Ratio Amount Ratio December 31, 2016 Total risk-based capital: Consolidated $ 974.5 15.13 % $ 555.7 8.63 % $ 676.1 10.50 % $ 643.9 10.00 % FIB 842.0 13.13 553.5 8.63 673.4 10.50 641.3 10.00 Tier 1 risk-based capital: Consolidated 894.3 13.89 426.9 6.63 547.3 8.50 515.1 8.00 FIB 765.8 11.94 425.2 6.63 545.1 8.50 513.1 8.00 Common equity tier 1 risk-based capital: Consolidated 814.3 12.65 330.3 5.13 450.7 7.00 418.5 6.50 FIB 765.8 11.94 329.0 5.13 448.9 7.00 416.9 6.50 Leverage capital ratio: Consolidated 894.3 10.11 353.8 4.00 353.8 4.00 442.3 5.00 FIB 765.8 8.69 352.3 4.00 352.3 4.00 440.4 5.00 (1) The ratios for the well capitalized requirement are only applicable to FIB. However, the Company manages its capital position as if the requirement applies to the consolidated entity and has presented the ratios as if they also applied on a consolidated basis. 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. When fully phased in on January 1, 2019, the Basel III Capital Rules will require the Company to maintain (i) a minimum ratio of common equity tier 1 capital to risk-weighted assets of at least 4.5% , plus a 2.5% “capital conservation buffer” (which is added to the 4.5% tier 1 capital to risk-weighted assets of at least 7.0% upon full implementation), (ii) a minimum ratio of tier 1 capital to risk-weighted assets of at least 6.0% , plus the capital conservation buffer (which is added to the 6.0% tier 1 capital ratio as that buffer is phased in, effectively resulting in a minimum tier 1 capital ratio of 8.5% upon full implementation), (iii) a minimum ratio of total capital to risk-weighted assets of at least 8.0% , plus the capital conservation buffer (which is added to the 8.0% total capital ratio as that buffer is phased in, effectively resulting in a minimum total capital ratio of 10.5% upon full implementation) and (iv) a minimum leverage ratio of 4.0% , calculated as the ratio of tier 1 capital to average quarterly assets. The implementation of the capital conservation buffer began on January 1, 2016 at the 0.625% level and will be phased in over a four-year period (increasing by that amount on each subsequent January 1, until it reaches 2.5% on January 1, 2019). The Basel III Capital Rules also provide for a “countercyclical capital buffer” that is applicable to only certain covered institutions and does not have any current applicability to us. The capital conservation buffer is designed to absorb losses during periods of economic stress and, as detailed above, effectively increases the minimum required risk-weighted capital ratios. Banking institutions with a ratio of common equity tier 1 capital to risk-weighted assets below the effective minimum ( 4.5% plus the capital conservation buffer and, if applicable, the countercyclical capital buffer) will face constraints on dividends, equity repurchases and compensation based on the amount of the shortfall. As of December 31, 2017 , the Company's capital conservation buffer was 4.76% for the consolidated company and 4.29% for FIB.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	COMMITMENTS AND CONTINGENCIES The Company had commitments under construction contracts of $1.1 million as of December 31, 2017 . 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% ownership interest. The Company leases certain premises and equipment from third parties under operating leases. Total rental expense to third parties was $2.5 million , $1.9 million , and $2.7 million , in 2017 , 2016 and 2015 , 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, 2017 , are as follows: Third Parties Related Entity Total For the year ending December 31: 2018 $ 3.5 $ 2.5 $ 6.0 2019 3.3 2.5 5.8 2020 2.6 2.5 5.1 2021 2.1 2.5 4.6 2022 1.6 2.5 4.1 Thereafter 9.4 9.1 18.5 Total $ 22.5 $ 21.6 $ 44.1 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 $1.9 million and $2.7 million of sold residential mortgage loans with recourse provisions still in effect as of December 31, 2017 and 2016 , 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, 2017 , 2016 and 2015 . 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. 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, 2017
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 customers. 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 customer’s creditworthiness on a case-by-case basis. The amount of collateral obtained is based on management’s credit evaluation of the customer. 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 customer 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 $2,179.5 million at December 31, 2017 , which included $635.3 million on unused credit card lines and $775.0 million with commitment maturities beyond one year. Commitments to extend credit to borrowers approximated $1,600.0 million at December 31, 2016 , which included $544.9 million on unused credit card lines and $424.5 million with commitment maturities beyond one year. Standby letters of credit are conditional commitments issued by the Company to guarantee the performance of a customer 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 customers. At December 31, 2017 and 2016 , the Company had outstanding stand-by letters of credit of $50.5 million and $50.6 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, 2017
Income Tax Disclosure [Abstract]
Income Taxes	INCOME TAXES Income tax expense consists of the following: Year ended December 31, 2017 2016 2015 Current: Federal $ 24.3 $ 40.3 $ 26.7 State 5.0 5.9 4.5 Total current 29.3 46.2 31.2 Deferred: Federal 18.4 2.9 11.1 State 2.5 0.5 1.4 Total deferred 20.9 3.4 12.5 Total income tax expense $ 50.2 $ 49.6 $ 43.7 Total income tax expense differs from the amount computed by applying the statutory federal income tax rate of 35% in 2017 , 2016 and 2015 , respectively, to income before income taxes as a result of the following: Year ended December 31, 2017 2016 2015 Tax expense at the statutory tax rate $ 54.9 $ 50.8 $ 45.7 Increase (decrease) in tax resulting from: Tax-exempt income (4.5 ) (4.4 ) (4.1 ) State income tax, net of federal income tax benefit 4.9 4.3 3.8 Benefit of stock-based compensation plans (2.6 ) — — Federal tax credits (2.4 ) (2.1 ) (2.3 ) Benefit due to enactment of federal tax reform (2.2 ) — — Other, net 2.2 1.0 0.6 Tax expense at effective tax rate $ 50.2 $ 49.6 $ 43.7 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) with a tax rate of 21.0% and 35.0% as of December 31, 2017 and December 31, 2016 , respectively, relate to the following: December 31, 2017 2016 Deferred tax assets: Loans, principally due to allowance for loan losses $ 18.0 $ 30.1 Loan discount 6.7 3.1 Investment securities, unrealized losses 5.6 6.6 Employee benefits 11.5 9.3 Non-performing loan interest 1.1 1.2 Other real estate owned write-downs and carrying costs 0.6 1.3 Tax credit carryforwards (1) 3.7 — Net operating loss carryforwards (2) 8.7 — Other 4.7 2.5 Deferred tax assets 60.6 54.1 Deferred tax liabilities: Fixed assets, principally differences in bases and depreciation (6.7 ) (4.7 ) Deferred loan costs (1.7 ) (3.0 ) Investment in joint venture partnership, principally due to differences in depreciation of partnership assets (0.8 ) (1.2 ) Prepaid amounts (0.6 ) (1.5 ) Government agency stock dividends (1.6 ) (2.3 ) Goodwill and core deposit intangibles (38.1 ) (42.4 ) Mortgage servicing rights (5.7 ) (5.5 ) Other (1.4 ) (0.3 ) Deferred tax liabilities (56.6 ) (60.9 ) Net deferred tax assets (liabilities) $ 4.0 $ (6.8 ) (1) Based on filed tax returns and amounts expected to be reported in current year tax returns (December 31, 2017), we had remaining federal tax credit carryforwards of $3.7 million from acquired companies. The federal tax credits were primarily generated from low income housing and AMT tax credit carryforwards. These tax credits expire beginning in 2027 and ending in 2037 and their use is subject to annual limitations. (2) As of December 31, 2017, we had remaining federal net operating loss carryforwards of $20.1 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 2018 and ending in 2031. The use of these carryforwards is subject to annual limitations. The Company had a current net income tax receivable of $5.7 million and $5.8 million at December 31, 2017 and December 31, 2016 , respectively. On December 22, 2017, H.R.1, commonly known as the Tax Cuts and Jobs Act (the “Act”), was signed into law. The Act includes many provisions that will affect our income tax expense, including reducing our federal tax rate from 35% to 21%, effective January 1, 2018. As a result of this rate reduction, we are required to re-measure, through income tax expense in the period of enactment, our deferred tax assets and liabilities using the enacted rate at which we expect them to be recovered or settled. This re-measurement resulted in a 2017 income tax benefit of $2.2 million . Also on December 22, 2017, the U.S. Securities and Exchange Commission (“SEC”) released Staff Accounting Bulletin No. 118 (“SAB 118”) to address any uncertainty or diversity of views in practice in accounting for the income tax effects of the Act in situations where a registrant does not have the necessary information available, prepared, or analyzed in reasonable detail to complete this accounting in the reporting period that includes the enactment date. SAB 118 allows for a measurement period, not to extend beyond one year from the Act’s enactment date, to complete the necessary accounting. We recorded provisional amounts of deferred income taxes using reasonable estimates in three areas where the information necessary to determine the final deferred tax asset or liability was either not available, not prepared, or not sufficiently analyzed as of the report filing date: 1) Our deferred tax liability for temporary differences between the tax and financial reporting bases of fixed assets is awaiting completion and implementation of software updates to process the calculations associated with the Act's provisions allowing for 100% bonus depreciation on fixed assets placed in service after September 27, 2017. 2) Our deferred tax assets and liabilities for temporary differences acquired from Cascade Bancorp are awaiting final determinations of those amounts from the Cascade Bancorp 2017 income tax returns. 3) Our deferred tax liability for temporary differences associated with equity investments in partnerships is awaiting the receipt of Schedules K-1 from outside preparers, which is necessary to determine our 2017 tax impact from these investments. In a fourth area, we made no adjustments to deferred tax assets representing future deductions for accrued compensation that may be subject to new limitations under Internal Revenue Code Section 162(m) which, generally, limits the annual deduction for certain compensation paid to certain employees to $1.0 million. As of the report filing date, there is uncertainty regarding how the newly-enacted rules in this area apply to existing contracts. Consequently, we are seeking further clarification of these matters before completing our analysis. We will complete and record the income tax effects of these provisional items during the period the necessary information becomes available. This measurement period will not extend beyond December 22, 2018.

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [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, 2017 , there were 1,954,046 common shares available for future grant under the 2015 Plan. As of December 31, 2017 , all outstanding stock based compensation awards are for shares of Class A common stock. 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 2017 or 2016 . 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 2017 . Compensation expense related to stock option awards of $0.04 million and $0.48 million was included in benefits on the Company’s consolidated statements of income for the years ended December 31, 2016 and 2015 , respectively and the related income tax benefits recognized for the years ended December 31, 2016 and 2015 were $0.02 million and $0.18 million , respectively. The following table summarizes stock option activity under the Company’s active stock option plans for the year ended December 31, 2017 : Number of Shares Weighted-Average Exercise Price Weighted-Average Remaining Contract Life Outstanding options, beginning of year 940,843 $ 16.77 Exercised (285,887 ) 18.15 Forfeited (12,701 ) 17.86 Outstanding options, end of year 642,255 $ 16.13 2.34 years Outstanding options exercisable, end of year 642,255 $ 16.13 2.34 years The total intrinsic value of fully-vested stock options outstanding as of December 31, 2017 was $15.4 million . The total intrinsic value of options exercised was $6.3 million , $5.6 million and $3.9 million during the years ended December 31, 2017 , 2016 and 2015 , respectively. The actual tax benefit realized for the tax deduction from option exercises totaled $2.0 million , $2.1 million and $1.5 million for the years ended December 31, 2017 , 2016 and 2015 , respectively. The Company received cash of $2.4 million , $4.7 million and $3.4 million from stock option exercises during the years ended December 31, 2017 , 2016 and 2015 , respectively. The Company redeemed common stock with aggregate values of $2.8 million , $3.1 million and $2.5 million tendered in payment for stock option exercises during the years ended December 31, 2017 , 2016 and 2015 , 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 $3.3 million , $3.9 million and $2.9 million was included in benefits on the Company’s consolidated statements of income for the years ended December 31, 2017 , 2016 and 2015 , respectively. Related income tax benefits recognized for the years ended December 31, 2017 , 2016 and 2015 were $0.6 million , $1.5 million and $1.1 million , respectively. The following table presents information regarding the Company’s restricted stock as of December 31, 2017 : Number of Shares Weighted-Average Measurement Date Fair Value Restricted stock, beginning of year 317,055 $ 26.22 Granted 140,246 41.46 Vested (111,224 ) 26.14 Forfeited (53,571 ) 27.96 Restricted stock, end of year 292,506 $ 33.24 During 2017 , the Company issued 140,246 restricted common shares. The 2017 restricted share awards included 35,634 performance restricted shares, of which 17,817 vest in varying percentages upon achievement of defined return on equity performance goals, and 17,817 vest in varying percentages upon achievement of defined total return to shareholder goals. Vesting of the performance restricted shares is also contingent on employment as of December 31, 2019 . Additionally, 104,612 time-restricted shares were issued during 2017 that vest one-third on each annual anniversary of the grant date through February 15, 2020 , contingent on continued employment through the vesting date. As of December 31, 2017 , there was $5.8 million of unrecognized compensation cost related to non-vested, restricted stock awards expected to be recognized over a period of 1.85 years.

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2017
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 three years of vesting service. Accrued contribution expense for this plan of $1.6 million , $2.7 million and $0.7 million in 2017 , 2016 and 2015 , 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. Eligibility requirements for this plan are the same as those for the profit sharing plan discussed in the preceding paragraph. Employee participation in the plan is at the option of the employee. The Company contributes $1.25 for each $1.00 of employee contributions up to 4% of the participating employee’s compensation. Contribution expense for this plan of $5.5 million , $4.8 million and $4.7 million in 2017 , 2016 and 2015 , 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 $0.7 million and $0.7 million as of December 31, 2017 and 2016 , respectively, is included in accounts payable and accrued expenses in the Company’s consolidated balance sheets. Net periodic benefit costs of $0.4 million , $0.2 million and $0.4 million for the years ended December 31, 2017 , 2016 and 2015 , 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, 2017 , and the net periodic benefit costs for the year then ended, included a discount rate of 2.4% and a 6.0% 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, 2016 , and the net periodic benefit costs for the year then ended, included a discount rate of 2.4% 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 $0.13 million , $0.13 million , $0.10 million , $0.07 million , $0.05 million and $0.10 million for 2018 , 2019 , 2020 , 2021 , 2022 , and 2023 through 2027 , respectively. At December 31, 2017 , the Company had accumulated other comprehensive gain related to the plan of $2.2 million , or $1.3 million net of related income tax benefit, comprised of net actuarial gains of $ 2.1 million and an unamortized transition asset of $0.1 million . The Company estimates $0.8 million will be amortized from accumulated other comprehensive gain into net period benefit costs in 2018 .

Other Comprehensive Income

Other Comprehensive Income	12 Months Ended
	Dec. 31, 2017
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, 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 gains included in net income (0.7 ) (0.3 ) (0.4 ) Change in unamortized loss on available-for-sale securities transferred into held-to-maturity 1.8 0.7 1.1 Change in net unrealized loss on derivatives (1.1 ) (0.4 ) (0.7 ) Reclassification adjustment for derivative net (gains) losses included in net income 1.1 0.4 0.7 Defined benefits post-retirement benefit plan: Change in net actuarial loss (gain) (1.2 ) (0.5 ) (0.7 ) Total other comprehensive loss $ (6.6 ) $ (2.8 ) $ (3.8 ) Year ended December 31, 2016 Before Tax Amount Tax Expense (Benefit) Net of Tax Amount Investment securities available-for sale: Change in net unrealized loss during period $ (19.4 ) $ (7.6 ) $ (11.8 ) Reclassification adjustment for net gains included in net income (0.3 ) (0.1 ) (0.2 ) Unamortized premium on available-to-sale securities transferred into held-for-maturity 1.9 0.7 1.2 Change in net unrealized gain on derivatives (0.2 ) (0.1 ) (0.1 ) Defined benefits post-retirement benefit plan: Change in net actuarial loss 4.0 1.6 2.4 Total other comprehensive loss $ (14.0 ) $ (5.5 ) $ (8.5 ) Year ended December 31, 2015 Before Tax Amount Tax Expense (Benefit) Net of Tax Amount Investment securities available-for sale: Change in net unrealized loss during period $ 3.1 $ 1.2 $ 1.9 Reclassification adjustment for net gains included in net income (0.2 ) (0.1 ) (0.1 ) Change in unamortized gain on available-for-sale securities transferred into held-to-maturity 1.6 0.6 1.0 Change in net unrealized gain on derivatives 0.2 0.1 0.1 Defined benefits post-retirement benefit plan: Change in net actuarial loss 0.1 0.1 — Total other comprehensive income $ 4.8 $ 1.9 $ 2.9 The components of accumulated other comprehensive income (loss), net of income taxes, are as follows: Year ended December 31, 2017 2016 Net unrealized gain (loss) on investment securities available-for-sale $ (13.2 ) $ (10.1 ) Net actuarial gain (loss) on defined benefit post-retirement benefit plans 1.3 2.0 Net unrealized gain (loss) on derivatives — — Net accumulated other comprehensive income (loss) $ (11.9 ) $ (8.1 )

Condensed Financial Information

Condensed Financial Information (Parent Company Only)	12 Months Ended
	Dec. 31, 2017
Condensed Financial Information of Parent Company Only 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, 2017 2016 Condensed balance sheets: Cash and cash equivalents $ 42.3 $ 138.4 Investment in subsidiaries, at equity: Bank subsidiary 1,432.3 932.0 Total investment in subsidiaries 1,432.3 932.0 Advances to subsidiaries, net 25.7 — Other assets 61.7 35.4 Total assets $ 1,562.0 $ 1,105.8 Other liabilities $ 51.9 $ 18.0 Advances from subsidiaries, net — 2.7 Long-term debt — 20.0 Subordinated debentures held by subsidiary trusts 82.5 82.5 Total liabilities 134.4 123.2 Stockholders’ equity 1,427.6 982.6 Total liabilities and stockholders’ equity $ 1,562.0 $ 1,105.8 Years Ended December 31, 2017 2016 2015 Condensed statements of income: Dividends from subsidiaries $ 150.0 $ 140.0 $ 70.0 Other interest income 0.1 — — Other income, primarily management fees from subsidiaries 18.0 15.1 13.2 Total income 168.1 155.1 83.2 Salaries and benefits 21.8 18.8 16.3 Interest expense 4.7 4.1 3.8 Acquisition expenses 25.3 1.5 0.8 Other operating expenses, net 13.0 10.4 9.9 Total expenses 64.8 34.8 30.8 Earnings before income tax benefit 103.3 120.3 52.4 Income tax benefit (14.2 ) (7.7 ) (7.0 ) Income before undistributed earnings of subsidiaries 117.5 128.0 59.4 Undistributed earnings of subsidiaries (11.0 ) (32.4 ) 27.4 Net income $ 106.5 $ 95.6 $ 86.8 Years Ended December 31, 2017 2016 2015 Condensed statements of cash flows: Cash flows from operating activities: Net income $ 106.5 $ 95.6 $ 86.8 Adjustments to reconcile net income to cash provided by operating activities: Undistributed earnings of subsidiaries 11.0 32.4 (27.4 ) Stock-based compensation expense 3.9 4.4 3.9 Tax benefits from stock-based compensation — 2.1 1.4 Excess tax benefits from stock-based compensation — (1.6 ) (1.2 ) Other, net 14.7 (0.8 ) (11.3 ) Net cash provided by operating activities 136.1 132.1 52.2 Cash flows from investing activities: Capital distributions from nonbank subsidiaries 18.0 2.0 — Acquisition of intangible assets (28.0 ) — — Acquisition of bank holding company, net of cash and cash equivalents received (128.3 ) — (7.2 ) Investment in subsidiary (18.0 ) — — Net cash used in investing activities $ (156.3 ) $ 2.0 $ (7.2 ) Years Ended December 31, 2017 2016 2015 Cash flows from financing activities: Net (decrease) increase in advances from nonbank subsidiaries $ (28.4 ) $ 9.1 $ (2.0 ) Repayment of long-term debt — — (1.0 ) Proceeds from issuance of common stock, net of stock issuance costs 2.4 4.7 3.4 Excess tax benefits from stock-based compensation — 1.6 1.2 Purchase and retirement of common stock (1.3 ) (26.9 ) (20.6 ) Dividends paid to common stockholders (48.6 ) (39.4 ) (36.3 ) Net cash used in financing activities (75.9 ) (50.9 ) (55.3 ) Net change in cash and cash equivalents (96.1 ) 83.2 (10.3 ) Cash and cash equivalents, beginning of year 138.4 55.2 65.5 Cash and cash equivalents, end of year $ 42.3 $ 138.4 $ 55.2 During 2017 , there was $386.0 million of noncash financing activities for the issuance of common stock for the BOTC acquisition. There were no noncash investing or financing activities during 2016 or 2015 .

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2017
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 of assets or liabilities 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, 2017 and 2016 . 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. There have been no significant changes in the valuation techniques during the periods ended December 31, 2017 and 2016 . Further details on the methods used to estimate the fair value of each class of financial instruments above are discussed below: Investment Securities Available-for-Sale . 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 as classified within Level 2 of the valuation hierarchy. 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. 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. For investment securities, if needed, a broker may be utilized to determine the reported fair value. Loans Held for Sale. Beginning January 1, 2016, the Company elected to account for loans held for sale using the fair value option. Prior to 2016, the Company carried loans held for sale at the lower of aggregate cost or estimated market value. 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. As of December 31, 2015, all loans held for sale were recorded at cost. Interest Rate Swap Contracts. Fair values for 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 and, therefore, are classified within Level 2 of the valuation hierarchy. The inputs used to determine fair value include the 3 month LIBOR forward curve to estimate variable rate cash inflows and the fed 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. The fair values of deferred compensation plan assets 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. 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, 2017 Balance Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Investment securities available-for-sale: U.S. Treasury Notes $ 3.2 $ — $ 3.2 $ — Obligations of U.S. government agencies 561.5 — 561.5 — U.S. agency mortgage-backed securities & collateralized mortgage obligations 1,462.5 — 1,462.5 — Private mortgage-backed securities 90.7 — 90.7 — Corporate securities 87.9 — 87.9 — Other investments 3.0 — 3.0 — Loans held for sale 46.6 — 46.6 — Derivative assets: Interest rate swap contracts 7.5 — 7.5 — Interest rate lock commitments 1.3 — 1.3 — Derivative liabilities: Interest rate swap contracts 7.8 — 7.8 — Forward loan sales contracts 0.1 — 0.1 — Deferred compensation plan assets 12.2 — 12.2 — Deferred compensation plan liabilities 12.2 — 12.2 — Fair Value Measurements at Reporting Date Using As of December 31, 2016 Balance Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Investment securities available-for-sale: U.S. Treasury Notes $ 3.6 $ — $ 3.6 $ — Obligations of U.S. government agencies 391.3 — 391.3 — U.S. agency mortgage-backed securities & collateralized mortgage obligations 1,213.7 — 1,213.7 — Private mortgage-backed securities 0.1 — 0.1 — Other investments 3.0 — 3.0 — Loans held for sale 61.8 — 61.8 — Derivative assets: Interest rate swap contracts 1.3 — 1.3 — Interest rate lock commitments 1.1 — 1.1 — Forward loan sales contracts 0.3 — 0.3 — Derivative liabilities: Interest rate swap contracts 1.3 — 1.3 — Deferred compensation plan assets 10.6 — 10.6 — Deferred compensation plan liabilities 10.6 — 10.6 — 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, 2017 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 $ 32.6 $ — $ — $ 32.6 $ 22.2 Other real estate owned 1.3 — — 1.3 (1.6 ) Long-lived assets to be disposed of by sale 0.8 — — 0.8 — Fair Value Measurements at Reporting Date Using As of December 31, 2016 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 $ 39.3 $ — $ — $ 39.3 $ (25.8 ) Other real estate owned 2.1 — — 2.1 (1.7 ) Long-lived assets to be disposed of by sale 1.3 — — 1.3 (1.0 ) 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, 2017 , certain impaired loans with a carrying value of $54.7 million were reduced by specific valuation allowance allocations of $10.7 million and partial loan charge-offs of $11.4 million resulting in a reported fair value of $32.6 million . As of December 31, 2016 , certain impaired loans with a carrying value of $65.2 million were reduced by specific valuation allowance allocations of $14.0 million and partial loan charge-offs of $11.9 million resulting in a reported fair value of $39.3 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, 2017 , long-lived assets to be disposed of by sale with carrying values of $0.8 million , had zero write-downs, resulting in a reported fair value of $0.8 million . As of December 31, 2016 , the Company had long-lived assets to be disposed of by sale of $2.4 million that were reduced by write-downs of $1.1 million charged to other expense resulting in a reported fair value of $1.3 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, 2017 Fair Value Valuation Technique Unobservable Inputs Range (Weighted Average) Impaired loans $ 32.6 Appraisal Appraisal adjustment 0% - 78.11% (26.1%) Other real estate owned 1.3 Appraisal Appraisal adjustment 8% - 96% (11.55%) Long-lived assets to be disposed of by sale 0.8 Appraisal Appraisal adjustment 0% - 0% 0% As of December 31, 2016 Fair Value Valuation Technique Unobservable Inputs Range (Weighted Average) Impaired loans $ 39.3 Appraisal Appraisal adjustment 0% - 66% (31%) Other real estate owned 2.1 Appraisal Appraisal adjustment 8% - 96% (18%) Long-lived assets to be disposed of by sale 1.3 Appraisal Appraisal adjustment 0% - 9% (6%) 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 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. A summary of the estimated fair values of financial instruments follows: ` Fair Value Measurements at Reporting Date Using As of December 31, 2017 Carrying Amount Estimated Fair Value Quoted Prices in Significant Other Significant Financial assets: Cash and cash equivalents $ 759.0 $ 759.0 $ 759.0 $ — $ — Investment securities available-for-sale 2,208.7 2,208.7 — 2,208.7 — Investment securities held-to-maturity 484.5 483.3 — 483.3 — Accrued interest receivable 38.0 38.0 — 38.0 — Mortgage servicing rights, net 24.8 40.1 — 40.1 — Net loans 7,542.2 7,298.8 — 7,266.2 32.6 Derivative assets 8.8 8.8 — 8.8 — Deferred compensation plan assets 12.2 12.2 — 12.2 — Total financial assets $ 11,078.2 $ 10,848.9 $ 759.0 $ 10,057.3 $ 32.6 Financial liabilities: Total deposits, excluding time deposits $ 8,783.0 $ 8,783.0 $ 8,783.0 $ — $ — Time deposits 1,151.9 1,137.9 — 1,137.9 — Securities sold under repurchase agreements 643.0 643.0 — 643.0 — Other borrowed funds 20.0 20.0 — 20.0 — Accrued interest payable 5.6 5.6 — 5.6 — Long-term debt 13.1 11.3 — 11.3 — Subordinated debentures held by subsidiary trusts 82.5 76.7 — 76.7 — Derivative liabilities 7.9 7.9 — 7.9 — Deferred compensation plan liabilities 12.2 12.2 — 12.2 — Total financial liabilities $ 10,719.2 $ 10,697.6 $ 8,783.0 $ 1,914.6 $ — Fair Value Measurements at Reporting Date Using As of December 31, 2016 Carrying Amount Estimated Fair Value Quoted Prices in Significant Other Significant Financial assets: Cash and cash equivalents $ 782.0 $ 782.0 $ 782.0 $ — $ — Investment securities available-for-sale 1,611.7 1,611.7 — 1,611.7 — Investment securities held-to-maturity 512.8 513.3 — 513.3 — Accrued interest receivable 29.9 29.9 — 29.9 — Mortgage servicing rights, net 18.5 35.7 — 35.7 — Net loans 5,402.3 5,309.9 — 5,270.6 39.3 Derivative assets 2.7 2.7 — 2.7 — Deferred compensation plan assets 10.6 10.6 — 10.6 — Total financial assets $ 8,370.5 $ 8,295.8 $ 782.0 $ 7,474.5 $ 39.3 Financial liabilities: Total deposits, excluding time deposits $ 6,324.5 $ 6,324.5 $ 6,324.5 $ — $ — Time deposits 1,051.6 1,044.7 — 1,044.7 — Securities sold under repurchase agreements 537.6 537.6 — 537.6 — Other borrowed funds — — — — — Accrued interest payable 5.4 5.4 — 5.4 — Long-term debt 28.0 27.5 — 27.5 — Subordinated debentures held by subsidiary trusts 82.5 73.6 — 73.6 — Derivative liabilities 1.3 1.3 — 1.3 — Deferred compensation plan liabilities 10.6 10.6 — 10.6 — Total financial liabilities $ 8,041.5 $ 8,025.2 $ 6,324.5 $ 1,700.7 $ —

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2017
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, incurred indebtedness in the form of loans, as customers, of $54.6 million and $ 53.3 million at December 31, 2017 and 2016 , respectively. During 2017 , new loans and advances on existing loans of $15.0 million were funded and loan repayments totaled $15.7 million . In addition, $2.0 million of loans were added due to changes in related parties during the year. These loans were made on substantially the same terms, including interest rates and collateral, as those prevailing at the time for comparable loans 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 previously leased an aircraft from an entity wholly-owned by a greater than 10% shareholder, who retired as chairman of the Company's Board of Directors in January 2016. Under the terms of the lease, the Company paid a fee for each flight hour plus certain third party operating expenses related to the aircraft. There were no fees or expenses incurred during 2017 . The Company paid total fees and operating expenses of $108 thousand and $332 thousand , during 2016 and 2015 , respectively, for its use of the aircraft. In addition, the Company previously leased a portion of its hanger and provided pilot services to the related entity. There were no payments received from the related entity during 2017 . The Company received payments from the related entity of $53 thousand and $64 thousand , in 2016 and 2015 , 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 leases an aircraft from an entity that is wholly-owned by the chairman of the Board of Directors of the Company, who was formerly the executive vice chairman of the Company's Board of Directors. This related entity and a director of the Company own a combined 66% of the aircraft leased. During 2017 , 2016 and 2015 , the Company paid total fees and operating expenses of $45 thousand , $121 thousand and $28 thousand respectively, for its use of the aircraft. In addition, during 2017 , 2016 and 2015 , the Company received payments from the related entity of $17 thousand , $19 thousand and $23 thousand , respectively, for reimbursement of certain third party operating expenses related to the chairman’s personal use of the aircraft. The Company leases an aircraft from an entity in which one director and one greater than 10% shareholder, who retired as chairman of the Company's Board of Directors in January 2016, have a combined 66% ownership interest. Under the terms of the lease, the Company pays a fee for each flight hour plus certain third party operating expenses related to the aircraft. During 2017 and 2016 , the Company paid total fees and operating expenses of $17 thousand and $175 thousand , respectively, for its use of the aircraft. As of December 31, 2017 and 2016 , the entity had received loans from FIB with an aggregate outstanding principal balances of $1.9 million and $2.0 million , respectively, which are 100% guaranteed by the greater than 10% shareholder and former chairman. These loans were made on substantially the same terms, including interest rates and collateral, as those prevailing at the time for comparable loans with persons not related to the Company. The Company purchases services from an entity in which one control group member not included as a director or greater than 5% shareholder of the Company, two greater than 5% shareholders and five directors, including the chairman of the Board of Directors, have an aggregate ownership interest of 15% , and in which one director is the chairman and three directors are members of the board of such entity. Services provided for the Company’s benefit include shareholder education and communication, strategic enterprise planning and corporate governance consultation. During 2017 , 2016 and 2015 , the Company paid $73 thousand , $100 thousand and $210 thousand , respectively, for these services.

Authoritative Accounting Guidan

Authoritative Accounting Guidance	12 Months Ended
	Dec. 31, 2017
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
Recent Authoritative Accounting Guidance	RECENT AUTHORITATIVE ACCOUNTING GUIDANCE ASU 2014-09, “Revenue from Contracts with Customers.” In May 2014, the FASB issued ASU 2014-09, “Revenue from Contracts with Customers (Topic 606).” ASU 2014-09 introduced a new five-step revenue recognition model in which an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. ASU 2014-09 also requires disclosures sufficient to enable users to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers, including qualitative and quantitative disclosures about contracts with customers, significant judgments and changes in judgments, and assets recognized from the costs to obtain or fulfill a contract. ASU 2015-14, “Revenue from Contracts with Customers (Topic 606) – Deferral of the Effective Date” was released in August of 2015 deferring the effective date of ASU 2014-09 for all entities by one year until January 1, 2018. In March 2016, the FASB issued ASU 2016-08, “Revenue from Contracts with Customers (Topic 606): Principal vs Agent Considerations (Reporting Revenue Gross versus Net).” The amendments in ASU 2016-08 were issued to clarify certain principal versus agent considerations within the implementation guidance of ASC Topic 606, “Revenue from Contracts with Customers.” The effective date and transition of ASU 2016-08 is the same as the effective date and transition of ASU 2014-09, Revenue from Contracts with Customers (Topic 606), as discussed above. In April 2016, the FASB issued ASU 2016-10, “Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing.” The amendments in ASU 2016-10 were issued to clarify ASC Topic 606, “Revenue from Contracts with Customers” related to (i) identifying performance obligations; and (ii) the licensing implementation guidance. The effective date and transition of ASU 2016-10 is the same as the effective date and transition of ASU 2014-09, “Revenue from Contracts with Customers (Topic 606),” as discussed above. The Company's revenue is comprised of net interest income on financial assets and financial liabilities, which is explicitly excluded from the scope of of ASU 2014-09, and non-interest income. The Company will adopt the new revenue recognition standards effective January 1, 2018 using a modified retrospective transition approach. The adoption and implementation of the new guidance will not significantly change how we recognize revenue for the various revenue streams, have a material impact to our net income on an ongoing basis, or have a significant impact on the Company's consolidated financial statements, results of operations or liquidity. ASU 2016-01 “Financial Instruments – Overall : Recognition and Measurement of Financial Assets and Financial Liabilities.” In January 2016, the FASB issued ASU 2016-01, “Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities.” The amendments in ASU 2016-01, among other things, (i) require equity investments, with certain exceptions, to be measured at fair value with changes in fair value recognized in net income, (ii) simplifies the impairment assessment of equity investments without readily determinable fair values by requiring a qualitative assessment to identify impairment, (iii) eliminates the requirement for public business entities to disclose the methods and significant assumptions used to estimate the fair value that is required to be disclosed for financial instruments measured at amortized cost on the balance sheet, (iv) requires public business entities to use the exit price notion when measuring the fair value of financial instruments for disclosure purposes, (v) requires an entity to present separately in other comprehensive income the portion of the total change in the fair value of a liability resulting from a change in the instrument-specific credit risk when the entity has elected to measure the liability at fair value in accordance with the fair value option for financial instruments, (vi) requires separate presentation of financial assets and financial liabilities by measurement category and form of financial asset on the balance sheet or the accompanying notes to the financial statements and (viii) clarifies that an entity should evaluate the need for a valuation allowance on a deferred tax asset related to available-for-sale investment securities. The Company will adopt the ASU 2016-01 amendments effective for the Company on January 1, 2018. The amendments will not have a significant impact on the Company’s consolidated financial statements, results of operations or liquidity. 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. ASU 2016-02 will be effective for the Company on January 1, 2019 and will be applied using a modified retrospective approach for leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements. Additionally, in January 2018, the Financial Accounting Standards Board issued a proposal to provide an additional transition method that would allow entities to not apply the guidance in ASU 2016-02 in the comparative periods presented in the financial statements and instead recognized a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. The Company is evaluating the new guidance to determine the impact ASU 2016-02 will have on its consolidated financial statements, results of operations or liquidity. ASU 2016-05 "Derivatives and Hedging (Topic 815): Effect of Derivative Contract Novations on Existing Hedge Accounting Relationships." The amendments in ASU 2016-05 clarify that a change in the counterparty to a derivative instrument that has been designated as the hedging instrument under ASC Topic 815 does not, in and of itself, require redesignation of that hedging relationship provided that all other hedge accounting criteria continue to be met. The amendments in ASU 2016-05 became effective for the Company on January 1, 2017, and did not have a material impact on the Company’s consolidated financial statements, results of operations or liquidity. ASU 2016-07 "Investments – Equity Method and Joint Ventures (Topic 323): Simplifying the Transition to the Equity Method of Accounting." The amendments in ASU 2016-07 eliminate the requirement that when an investment qualified for use of the equity method as a result of an increase in the level of ownership interest or degree of influence, an investor must adjust the investments, results of operations and retained earnings retroactively on a step-by-step basis as if the equity method had been in effect during all previous periods that the investment had been held. The amendments in ASU 2016-07 also simplify the transition to the equity method of accounting by eliminating retroactive adjustment of the investment when an investment qualifies for use of the equity method, among other things. The amendments in ASU 2016-07 became effective for the Company on January 1, 2017, and did not have a material impact on the Company’s consolidated financial statements, results of operations or liquidity. ASU 2016-09 "Compensation – Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting." Under the amendments in ASU 2016-09, all excess tax benefits and tax deficiencies related to share-based payment awards should be recognized as income tax expense or benefit in the income statement during the period in which they occur. Previously, such amounts were recorded in the pool of excess tax benefits included in additional paid-in capital, if such pool was available. Because excess tax benefits are no longer recognized in additional paid-in capital, the assumed proceeds from applying the treasury stock method when computing earnings per share excludes the amount of excess tax benefits that would have previously been recognized in additional paid-in capital. Additionally, excess tax benefits are classified along with other income tax cash flows as an operating activity rather than a financing activity, as was previously the case. The amendments in ASU 2016-09 also provide that an entity can make an entity-wide accounting policy election to either estimate the number of awards that are expected to vest (current GAAP) or account for forfeitures when they occur. The Company has elected to account for forfeitures when they occur. The amendments in ASU 2016-09 change the threshold to qualify for equity classification to permit withholding up to the maximum statutory tax rates (rather than the minimum as was previously the case) in the applicable jurisdictions. The amendments in ASU 2016-09 became effective for the Company on January 1, 2017, and resulted in a $2.6 million reduction in income tax expense for the period ending December 31, 2017, of which $2.0 million was due to the recognition of excess tax benefits related to outstanding stock option awards exercised during the period and $0.6 million was due to the recognition of excess tax benefits related to the vesting of restricted stock. ASU No. 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. The Company is currently evaluating the new guidance to determine the impact it will have on its consolidated financial statements, results of operations and liquidity. ASU No. 2016-15 "Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments." The amendments in ASU 2016-15 are intended to reduce diversity in practice in how eight particular transactions are classified in the statement of cash flows. ASU 2016-15 is effective for interim and annual reporting periods beginning after December 15, 2017. Early adoption is permitted, provided that all of the amendments are adopted in the same period. Entities will be required to apply the guidance retrospectively. If it is impracticable to apply the guidance retrospectively for an issue, the amendments related to that issue would be applied prospectively. The amendments in ASU 2016-15 will become effective for the Company on January 1, 2018. These amendments only affect the classification of certain items within the statement of cash flows, and will not have a significant impact on the Company’s consolidated financial statements, results of operations or liquidity. ASU No. 2017-01, “Business Combinations (Topic 805): Clarifying the Definition of a Business.” ASU 2017-01 clarifies the definition of a business with the objective of adding guidance to assist entities with evaluating whether transactions should be accounted for as acquisitions (or disposals) of assets or businesses. The amendments in this update provide a screen to determine when a set is not a business. The screen requires that when substantially all of the fair value of the gross assets acquired (or disposed of) is concentrated in a single identifiable asset or a group of similar identifiable assets, the set is not a business. This screen reduces the number of transactions that need to be further evaluated. If the screen is not met, the amendments in this update (1) require that to be considered a business, a set must include, at a minimum, an input and a substantive process that together significantly contribute to the ability to create output and (2) remove the evaluation of whether a market participant could replace missing elements. The amendments provide a framework to assist entities in evaluating whether both an input and a substantive process are present. The framework includes two sets of criteria to consider that depend on whether a set has outputs. ASU 2017-01 is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. The Company will adopt the ASU 2017-01 amendment effective for the Company on January 1, 2018. The amendment will not have a significant impact on the Company’s consolidated financial statements, results of operations or liquidity. ASU No. 2017-04, “Intangibles – Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment.” The amendments in ASU 2017-04 remove Step 2 of the goodwill impairment test, which requires a hypothetical purchase price allocation. Goodwill impairment will now be the amount by which a reporting unit’s carrying value exceeds its fair value, not to exceed the carrying amount of goodwill. All other goodwill impairment guidance will remain largely unchanged. ASU No. 2017-04 is effective for interim and annual reporting periods beginning after December 15, 2019, applied prospectively. Early adoption is permitted for any impairment tests performed after January 1, 2017. The amendments in ASU 2017-04 are not expected to have a significant impact on the Company’s consolidated financial statements, results of operations or liquidity. ASU No. 2017-07, “Compensation – Retirement Benefits (Topic 750): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost.” The amendments in ASU 2017-07 requires employers to present the service cost component of the net periodic benefit cost in the same income statement line item as other employee compensation costs arising from services rendered during the period. The other components of net benefit cost are required to be presented in the income separately from the service cost component and outside a subtotal of income from operations, if one is presented. The amendments in ASU 2017-07 also allow only the service cost component to be eligible for capitalization. ASU No. 2017-07 is effective for the Company for interim and annual reporting periods beginning after December 15, 2017, with early adoption permitted. The amendments in ASU 2017-07 are applied retrospectively for the presentation of the service cost and other components of net periodic benefit cost in the income statement and prospectively for the capitalization of the service cost in other assets. ASU 2017-07 allow the use of a practical expedient that permits an employer to use the amounts disclosed in its pension and other post-retirement benefits plan footnote for the prior comparative periods as the estimation basis for applying the retrospective presentation requirements. The Company will adopt the ASU 2017-07 amendment effective for the Company on January 1, 2018. The amendment will not have a significant impact on the Company’s consolidated financial statements, results of operations or liquidity. ASU No. 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, with early adoption permitted. The guidance calls for a modified retrospective transition approach under which a cumulative-effect adjustment will be made to retained earnings as of the beginning of the first reporting period in which the guidance is adopted. The Company currently amortizes premiums on callable debt securities to the earliest call date. As such, the amendments in ASU 2017-08 will not impact the Company's consolidated financial statements, results of operations and liquidity. ASU 2017-09, "Compensation - Stock Compensation (Topic 718): Scope of Modification Accounting." In May 2017, the FASB issued ASU 2017-09, which clarifies when a change to the terms or conditions of a share-based payment award must be accounted for as a modification. Under the new guidance, an entity will not apply modification accounting to a share-based payment award if there is no change to the award's fair value, vesting conditions and classification as an equity or liability instrument. The guidance is effective prospectively for all companies for annual periods beginning after December 15, 2017. Early adoption is permitted. The Company is currently evaluating the impact the standard may have on its consolidated financial statements, results of operations and liquidity. The Company will adopt the ASU 2017-09 amendment effective for the Company on January 1, 2018. The amendment will 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. ASU 2017-12 is effective for public business entities for fiscal years beginning after December 15, 2018, with early adoption, including adoption in an interim period, permitted. The Company plans to adopt ASU 2017-12 on January 1, 2019. ASU 2017-12 requires a modified retrospective transition method in which the Company will recognize the cumulative effect of the change on the beginning balance of each affected component of equity in the statement of financial position as of the date of adoption. 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. ASU 2018-02, "Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income." In February 2018, the FASB issued ASU 2018-02, Income Statement - Reporting Comprehensive Income: Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income (ASU 2018-02). The purpose of this updated guidance is to allow a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act. However, because the amendments only relate to the reclassification of the income tax effects of the Tax Cuts and Jobs Act, the underlying guidance that requires that the effect of a change in tax laws or rates be included in income from continuing operations is not affected. The updated amendments also require certain disclosures about stranded tax effects. The updated amendments are effective for all entities for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years. Early adoption is permitted, including adoption in any interim period, (1) for public business entities for reporting periods for which financial statements have not yet been issued and (2) for all other entities for reporting periods for which financial statements have not yet been made available for issuance and should be applied either in the period of adoption or retrospectively to each period (or periods) in which the effect of the change in the U.S. federal corporate income tax rate in the Tax Cuts and Jobs Act is recognized. Accumulated other comprehensive income at December 31, 2017 included $3.1 million related to stranded amounts resulting from the remeasurement of deferred tax assets and liabilities in connection with the enactment of the Tax Cuts and Jobs Act on December 22, 2017.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	SUBSEQUENT EVENTS Subsequent events have been evaluated for potential recognition and disclosure through the date financial statements were filed with the Securities and Exchange Commission. On January 8, 2018 , the Company paid $20 million to redeem in full the principal and interest related to the 6.81% unsecured subordinated term loan. On January 30, 2018 , the Company declared a quarterly dividend to common shareholders of $0.28 per share, to be paid on February 22, 2018 to shareholders of record as of February 12, 2018 . No other events requiring recognition or disclosure were identified.

Summary of Significant Accoun37

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
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, 2017 , 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 2016 and 2015 to conform to the 2017 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”) and FI Statutory Trust VI (“Trust VI”) 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 and Trust VI 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	Assets Held in Fiduciary or Agency Capacity. The Company holds certain trust assets in a fiduciary or agency capacity. The Company also purchases and sells federal funds as an agent. These and other assets held in an agency or fiduciary capacity are not assets of the Company and, accordingly, are not included in the accompanying consolidated financial statements.
Use of Estimates	Use of Estimates. The preparation of consolidated financial statements in conformity with accounting principles generally accepted in the United States of America requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the financial statements and income and expenses during the reporting period. Actual results could differ from those estimates. Material estimates that are particularly susceptible to change relate to the determination of the allowance for 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 . 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.
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, and marketable equity securities are classified as available-for-sale and carried at fair value. The unrealized gains and losses on these securities are reported, net of applicable income taxes, as a separate component of stockholders’ equity and comprehensive income. Management determines the appropriate classification of securities at the time of purchase and at each reporting date management reassesses the appropriateness of the classification. The amortized cost of debt securities classified as held-to-maturity or available-for-sale is adjusted for accretion of discounts to maturity and amortization of premiums over the estimated average life of the security, 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. Loans held for sale include residential mortgage loans originated for immediate sale. Beginning January 1, 2016, the Company 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. Prior to 2016, the Company carried loans held for sale at the lower of aggregate cost or estimated market value. 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 loan, on current delivery prices. Gains and losses on loan 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.
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 two-step 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.
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 years for furniture and equipment. Leasehold improvements and assets acquired under capital lease are amortized over the shorter of their estimated useful lives or the terms of the related leases. Land is recorded at cost.
Company-Owned Life Insurance	Company-Owned Life Insurance . Key executive and group life insurance policies are recorded at their cash surrender value. Separate account group life insurance policies are subject to a stable value contract that offsets the impact of interest rate fluctuations on the market value of the policies and are recorded at the stabilized investment value. Increases in the cash surrender or stabilized investment value of insurance policies, as well as insurance proceeds received, are recorded as other non-interest income, and are not subject to income taxes.
Deferred Compensation Plan	Deferred Compensation Plan. The Company has a deferred compensation plan for the benefit of certain highly compensated officers and directors of the Company. The plan allows for discretionary employer contributions in excess of tax limits applicable to the Company's 401(k) 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. 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.
Other Real Estate Owned	Other Real Estate Owned . Real estate acquired in satisfaction of loans is initially carried at current fair value less estimated selling costs. Any excess of loan carrying value over the fair value of the real estate acquired is recorded as a charge to the allowance for 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.
Restricted Equity Securities	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. 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.
Derivatives and Hedging Activities	Derivatives and Hedging Activities. For asset and liability management purposes, the Company enters into interest rate swap contracts to hedge against changes in forecasted cash flows due to interest rate exposures. Interest rate swaps are contracts in which a series of interest payments are exchanged over a prescribed period. The notional amount upon which the interest payments are based is not exchanged. The 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, 2017 , 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 customer 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 customer, 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	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	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 2014 .
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	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 Company has one operating segment, community banking, which encompasses commercial and consumer banking services offered to individuals, businesses, municipalities and other entities.
Advertising Costs	Advertising Costs. Advertising costs are expensed as incurred.
Transfers of Financial Assets	Transfers of Financial Assets. Transfers of financial assets are accounted for as sales when control over the assets has been surrendered. Control over transferred assets is deemed to be surrendered when the assets have been isolated from the Company; the transferee obtains the right, free of conditions that constrain it from taking advantage of that right, to pledge or exchange the transferred assets; and, the Company does not maintain effective control over the transferred assets through an agreement to repurchase them before their maturity.
Stock-Based Compensation	Stock-Based Compensation . Compensation cost for all stock-based awards is measured at fair value on the date of grant and is recognized over the requisite service period for awards expected to vest. The impact of forfeitures of stock-based payment awards on compensation expense is recognized as forfeitures occur.
Fair Value Measurements	Fair Value Measurements. In general, fair value measurements are based upon quoted market prices, where available. If quoted market prices are not available, fair value measurements are estimated using relevant market information and other assumptions. Fair value estimates involve uncertainties and require some degree of judgment regarding interest rates, credit risk, prepayments and other factors. The use of different assumptions or estimation techniques may have a significant effect on the fair value amounts reported.

Acquisitions (Tables)

Acquisitions (Tables)	12 Months Ended
	Dec. 31, 2017
Business Combinations [Abstract]
Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed	The following table summarizes the consideration paid, fair values of the Cascade assets acquired and liabilities assumed and the resulting goodwill. The amounts reported below for net deferred tax assets and goodwill are provisional pending completion of the Company's review of tax items. As Recorded Fair Value As Recorded As of May 30, 2017 by Cascade Adjustments by the Company Assets acquired: Cash and cash equivalents $ 246.8 $ — $ 246.8 Investment securities 476.7 4.9 (1) 481.6 Loans held for investment 2,111.0 (31.7 ) (2) 2,079.3 Mortgage loans held for sale 10.3 — 10.3 Allowance for loan losses (24.0 ) 24.0 (3) — Premises and equipment 46.6 0.1 (4) 46.7 Other real estate owned ("OREO") 1.2 — 1.2 Core deposit intangible assets — 48.0 (5) 48.0 Deferred tax assets, net 47.6 (19.0 ) (6) 28.6 Other assets 98.6 1.1 (7) 99.7 Total assets acquired 3,014.8 27.4 3,042.2 Liabilities assumed: Deposits 2,669.9 (0.9 ) (8) 2,669.0 Accounts payable and accrued expense 62.2 1.9 (9) 64.1 Total liabilities assumed 2,732.1 1.0 2,733.1 Net assets acquired $ 282.7 $ 26.4 $ 309.1 Consideration paid: Cash $ 155.0 Class A common stock 386.0 Total consideration paid $ 541.0 Goodwill $ 231.9 Explanation of fair value adjustments. Note marks for the deferred tax assets, loans held for investment, and accounts payable and accrued expenses were adjusted due to loan and deferred compensation valuation adjustments since prior quarter reporting. Certain other balances have been reclassified, none of which are material. The adjustments had no impact on 2017 earnings and a net reduction to goodwill of $0.3 million from 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. Shared National Credits (SNC) were recorded at quoted sales prices where available. The fair value of the remaining 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 Cascade 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 appraisals obtained from an independent third party appraiser or broker's opinion of value. (5) Adjustment represents the value of the core deposit base assumed in the acquisition based upon valuation from an independent accounting and advisory firm. (6) Adjustment consists of the write-off of pre-existing deferred tax assets and purchase accounting adjustments as a result of the acquisition. (7) Adjustment consists of various other assets recorded as a result of the acquisition, including mortgage servicing rights, SBA servicing rights, and favorable leases offset by reductions to the fair value of other items. (8) Decrease 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. (9) Increase in fair value due to credit card incentive program, unfavorable leases, write-off of balance sheet reserve, and swap liability offset.
Acquisition Related Expenses	These costs are incorporated in non-interest expense in the Company’s consolidated statements of income and are summarized below. December 31, 2017 Legal and professional fees $ 9.6 Employee expenses 5.1 Technology conversion and contract termination 10.2 Other 2.3 Total acquisition related expenses $ 27.2
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities Acquired During Period	Information regarding loans acquired credit-impaired as of the May 30, 2017 acquisition date is as follows: Contractually required principal and interest payments $ 49.7 Contractual cash flows not expected to be collected ("non-accretable discount") 24.7 Cash flows expected to be collected 25.0 Interest component of cash flows expected to be collected ("accretable discount") 1.9 Fair value of acquired credit-impaired loans $ 23.1
Schedule of Acquired Loans not Deemed to Have Credit Impairment	Information regarding acquired loans not deemed credit-impaired at the acquisition date is as follows: Contractually required principal and interest payments $ 2,098.1 Contractual cash flows not expected to be collected 23.3 Fair value at acquisition $ 2,066.5
Business Acquisition, Pro Forma Information	The following table presents unaudited pro forma consolidated revenues and net income as if the acquisition had occurred as of January 1, 2016. Year ended December 31, (unaudited) 2017 2016 Interest income $ 420.8 $ 392.7 Non-interest income 153.3 165.9 Total revenues $ 574.1 $ 558.6 Net income $ 126.4 $ 72.3 EPS - basic $ 2.03 $ 1.30 EPS - diluted 2.01 1.29

Goodwill and Core Deposit Int39

Goodwill and Core Deposit Intangibles (Tables)	12 Months Ended
	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Change in Carrying Amount of Goodwill	Goodwill Year Ended December 31, 2017 2016 2015 Net carrying value at beginning of period $ 212.8 $ 204.5 $ 205.6 Acquisitions and measurement period adjustments 231.9 8.3 (1.1 ) Net carrying value at end of period $ 444.7 $ 212.8 $ 204.5
Schedule of Activity of Identifiable Core Deposit Intangibles	The following table sets forth activity for identifiable core deposit intangibles subject to amortization: Core deposit intangibles ("CDI") Year Ended December 31, 2017 2016 2015 CDI, net, beginning of period $ 9.7 $ 10.6 $ 13.3 Established through acquisitions 48.0 2.5 0.7 Reductions due to sale of accounts (3.2 ) — — Current period amortization 5.4 3.4 3.4 Total CDI, net, at end of period $ 49.1 $ 9.7 $ 10.6
Schedule of Finite-Lived Intangible Assets, Future Amortization Expense	The following table provides estimated future CDI amortization expense: Years ending December 31, 2018 $ 6.9 2019 6.6 2020 6.2 2021 5.7 2022 5.3 Thereafter 18.4 Total $ 49.1

Investment Securities (Tables)

Investment Securities (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Available-for-Sale U.S. Treasury notes $ 3.2 $ — $ — $ 3.2 Obligations of U.S. government agencies 569.5 — (8.0 ) 561.5 U.S. agency residential mortgage-backed securities & collateralized mortgage obligations 1,474.1 3.8 (15.4 ) 1,462.5 Private mortgage-backed securities 91.5 — (0.8 ) 90.7 Corporate Securities 88.0 0.1 (0.3 ) 87.8 Other investments 3.0 — — 3.0 Total $ 2,229.3 $ 3.9 $ (24.5 ) $ 2,208.7 December 31, 2017 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Held-to Maturity State, county and municipal securities $ 172.4 $ 2.6 $ (0.6 ) $ 174.4 Corporate securities 61.6 0.1 (0.3 ) 61.4 Obligations of U.S. government agencies 19.8 — (0.2 ) 19.6 U.S agency residential mortgage-backed securities & 230.5 8.8 (11.6 ) 227.7 Other investments 0.2 — — 0.2 Total $ 484.5 $ 11.5 $ (12.7 ) $ 483.3 December 31, 2016 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Available-for-Sale U.S. Treasury notes $ 3.6 $ — $ — $ 3.6 Obligations of U.S. government agencies 397.4 0.3 (6.4 ) 391.3 U.S. agency residential mortgage-backed securities & collateralized mortgage obligations 1,220.9 6.4 (13.6 ) 1,213.7 Private mortgage-backed securities 0.1 — — 0.1 Other investments 3.0 — — 3.0 Total $ 1,625.0 $ 6.7 $ (20.0 ) $ 1,611.7 December 31, 2016 Amortized Cost Gross Unrealized Gains Gross Unrealized Losses Estimated Fair Value Held-to Maturity State, county and municipal securities $ 160.2 $ 2.7 $ (0.5 ) $ 162.4 Corporate securities 53.0 0.2 (0.2 ) 53.0 Obligations of U.S. government agencies 19.8 — (0.2 ) 19.6 U.S. agency residential mortgage-backed securities & collateralized mortgage obligations 279.6 7.8 (9.2 ) 278.2 Other investments 0.2 — — 0.2 Total $ 512.8 $ 10.7 $ (10.1 ) $ 513.4
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, 2017 and 2016 . Less than 12 Months 12 Months or More Total December 31, 2017 Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Available-for-Sale Obligations of U.S. government agencies $ 284.9 $ (3.4 ) $ 266.1 $ (4.6 ) $ 551.0 $ (8.0 ) U.S. agency residential mortgage-backed securities & collateralized mortgage obligations 670.1 (6.2 ) 439.2 (9.2 ) 1,109.3 (15.4 ) Private mortgage-backed securities 74.0 (0.8 ) — — 74.0 (0.8 ) Corporate securities 51.3 (0.3 ) — — 51.3 (0.3 ) Total $ 1,080.3 $ (10.7 ) $ 705.3 $ (13.8 ) $ 1,785.6 $ (24.5 ) Less than 12 Months 12 Months or More Total December 31, 2017 Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Held-to-Maturity State, county and municipal securities $ 53.3 $ (0.4 ) $ 12.3 $ (0.2 ) $ 65.6 $ (0.6 ) Corporate securities 41.2 (0.2 ) 5.0 (0.1 ) 46.2 (0.3 ) U.S. agency residential mortgage-backed 76.4 (9.1 ) 60.5 (2.5 ) 136.9 (11.6 ) Obligations of U.S. government agencies 9.7 — 9.9 (0.2 ) 19.6 (0.2 ) Total $ 180.6 $ (9.7 ) $ 87.7 $ (3.0 ) $ 268.3 $ (12.7 ) Less than 12 Months 12 Months or More Total December 31, 2016 Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Available-for-Sale U.S. Treasury notes $ 0.6 $ — $ — $ — $ 0.6 $ — Obligations of U.S. government agencies 316.5 (6.4 ) — — 316.5 (6.4 ) U.S. agency residential mortgage-backed securities & collateralized mortgage obligations 746.3 (13.1 ) 15.8 (0.5 ) 762.1 (13.6 ) Total $ 1,063.4 $ (19.5 ) $ 15.8 $ (0.5 ) $ 1,079.2 $ (20.0 ) Less than 12 Months 12 Months or More Total December 31, 2016 Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Fair Value Gross Unrealized Losses Held-to-Maturity State, county and municipal securities $ 42.5 $ (0.5 ) $ 2.8 $ — $ 45.3 $ (0.5 ) Corporate securities 32.5 (0.2 ) — — $ 32.5 $ (0.2 ) U.S. agency residential mortgage-backed $ 108.8 $ (7.9 ) $ 20.0 $ (1.3 ) $ 128.8 $ (9.2 ) Obligations of U.S. government agencies $ 19.6 $ (0.2 ) $ — $ — $ 19.6 $ (0.2 ) Total $ 203.4 $ (8.8 ) $ 22.8 $ (1.3 ) $ 226.2 $ (10.1 )
Maturities of Investment Securities	All other investment securities maturities are shown at contractual maturity dates. Available-for-Sale Held-to-Maturity December 31, 2017 Amortized Cost Estimated Fair Value Amortized Cost Estimated Fair Value Within one year $ 450.7 $ 447.5 $ 76.1 $ 76.7 After one year but within five years 1,422.7 1,409.3 266.2 263.1 After five years but within ten years 277.6 274.2 111.7 113.3 After ten years 78.3 77.7 30.5 30.2 Total $ 2,229.3 $ 2,208.7 $ 484.5 $ 483.3

Loans (Tables)

Loans (Tables)	12 Months Ended
	Dec. 31, 2017
Loans Receivable, Net [Abstract]
Schedule of Loans by Class	The following table presents loans by class as of the dates indicated: December 31, 2017 2016 Real estate loans: Commercial $ 2,822.9 $ 1,834.4 Construction: Land acquisition & development 348.7 208.5 Residential 240.2 147.9 Commercial 119.4 125.6 Total construction loans 708.3 482.0 Residential 1,487.4 1,027.4 Agricultural 158.2 170.2 Total real estate loans 5,176.8 3,514.0 Consumer: Indirect consumer 784.7 752.4 Other consumer 175.1 148.1 Credit card 74.6 69.8 Total consumer loans 1,034.4 970.3 Commercial 1,215.4 797.9 Agricultural 136.2 132.9 Other, including overdrafts 4.9 1.6 Loans held for investment 7,567.7 5,416.7 Mortgage loans held for sale 46.6 61.8 Total loans $ 7,614.3 $ 5,478.5
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, 2017 and 2016 . December 31, 2017 2016 Outstanding principal $ 38.2 $ 35.8 Carrying value: Loans on accrual status 24.9 21.9 Total carrying value $ 24.9 $ 21.9 The following table summarizes changes in the accretable yield for loans acquired credit impaired for the years ended December 31, 2017 and 2016 : Year Ended December 31, 2017 2016 2015 Beginning balance $ 6.8 $ 6.7 $ 5.8 Acquisitions 1.9 1.1 0.4 Accretion income (2.9 ) (2.5 ) (2.9 ) Additions 0.1 — 0.6 Reductions due to exit events (1.5 ) (1.1 ) (0.5 ) Reclassifications from nonaccretable differences 2.9 2.6 3.3 Ending balance $ 7.3 $ 6.8 $ 6.7
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, 2017 Past Due Past Due Past Due Past Due Loans Loans Loans Real estate Commercial $ 2.9 $ 0.5 $ 0.3 $ 3.7 $ 2,792.4 $ 26.8 $ 2,822.9 Construction: Land acquisition & development 7.3 0.3 0.3 7.9 337.8 3.0 348.7 Residential 2.1 — — 2.1 236.4 1.7 240.2 Commercial — — — — 115.6 3.8 119.4 Total construction loans 9.4 0.3 0.3 10.0 689.8 8.5 708.3 Residential 13.3 1.4 0.4 15.1 1,464.1 8.2 1,487.4 Agricultural 0.3 — 0.2 0.5 154.3 3.4 158.2 Total real estate loans 25.9 2.2 1.2 29.3 5,100.6 46.9 5,176.8 Consumer: Indirect consumer 7.8 2.1 0.4 10.3 772.6 1.8 784.7 Other consumer 1.6 0.5 0.1 2.2 172.6 0.3 175.1 Credit card 0.9 0.6 0.7 2.2 72.4 — 74.6 Total consumer loans 10.3 3.2 1.2 14.7 1,017.6 2.1 1,034.4 Commercial 3.9 1.7 0.7 6.3 1,189.5 19.6 1,215.4 Agricultural 1.8 0.1 — 1.9 133.5 0.8 136.2 Other, including overdrafts — — — — 4.9 — 4.9 Loans held for investment 41.9 7.2 3.1 52.2 7,446.1 69.4 7,567.7 Mortgage loans originated for sale — — — — 46.6 — 46.6 Total loans $ 41.9 $ 7.2 $ 3.1 $ 52.2 $ 7,492.7 $ 69.4 $ 7,614.3 Total Loans 30 - 59 60 - 89 > 90 30 or More Days Days Days Days Current Non-accrual Total As of December 31, 2016 Past Due Past Due Past Due Past Due Loans Loans Loans Real estate Commercial $ 7.3 $ 1.1 $ 0.3 $ 8.7 $ 1,799.5 $ 26.2 $ 1,834.4 Construction: Land acquisition & development 0.6 0.4 0.3 1.3 202.2 5.0 208.5 Residential 0.9 0.3 — 1.2 146.2 0.5 147.9 Commercial — — — — 124.8 0.8 125.6 Total construction loans 1.5 0.7 0.3 2.5 473.2 6.3 482.0 Residential 4.0 1.3 0.7 6.0 1,015.0 6.4 1,027.4 Agricultural 0.3 0.3 — 0.6 165.3 4.3 170.2 Total real estate loans 13.1 3.4 1.3 17.8 3,453.0 43.2 3,514.0 Consumer: Indirect consumer 8.4 2.3 0.7 11.4 740.2 0.8 752.4 Other consumer 1.3 0.2 0.2 1.7 146.1 0.3 148.1 Credit card 0.5 0.3 0.6 1.4 68.4 — 69.8 Total consumer loans 10.2 2.8 1.5 14.5 954.7 1.1 970.3 Commercial 3.2 0.7 0.7 4.6 767.9 25.4 797.9 Agricultural 1.5 0.4 — 1.9 128.0 3.0 132.9 Other, including overdrafts — — 0.3 0.3 1.3 — 1.6 Loans held for investment 28.0 7.3 3.8 39.1 5,304.9 72.7 5,416.7 Mortgage loans originated for sale — — — — 61.8 — 61.8 Total loans $ 28.0 $ 7.3 $ 3.8 $ 39.1 $ 5,366.7 $ 72.7 $ 5,478.5
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, 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 December 31, 2016 Unpaid Total Principal Balance Recorded Investment With No Allowance Recorded Investment With Allowance Total Recorded Investment Related Allowance Real estate: Commercial $ 57.0 $ 24.4 $ 21.4 $ 45.8 $ 2.8 Construction: Land acquisition & development 12.1 4.3 1.8 6.1 0.8 Residential 1.6 0.2 0.6 0.8 — Commercial 4.8 3.9 0.7 4.6 0.7 Total construction loans 18.5 8.4 3.1 11.5 1.5 Residential 8.2 4.1 2.5 6.6 0.3 Agricultural 5.1 4.5 0.2 4.7 — Total real estate loans 88.8 41.4 27.2 68.6 4.6 Commercial 40.3 13.2 19.2 32.4 9.3 Agricultural 3.7 3.3 0.4 3.7 0.1 Total $ 132.8 $ 57.9 $ 46.8 $ 104.7 $ 14.0 December 31, 2015 Unpaid Total Principal Balance Recorded Investment With No Allowance Recorded Investment With Allowance Total Recorded Investment Related Allowance Real estate: Commercial $ 58.2 $ 27.9 $ 17.6 $ 45.5 $ 3.4 Construction: Land acquisition & development 15.5 7.2 0.8 8.0 0.3 Residential 1.0 0.3 — 0.3 — Commercial 1.3 0.3 0.7 1.0 0.7 Total construction loans 17.8 7.8 1.5 9.3 1.0 Residential 7.1 3.6 2.3 5.9 0.4 Agricultural 6.4 5.6 0.2 5.8 — Total real estate loans 89.5 44.9 21.6 66.5 4.8 Commercial 29.6 10.8 13.7 24.5 6.5 Agricultural 1.3 0.6 0.4 1.0 0.3 Total $ 120.4 $ 56.3 $ 35.7 $ 92.0 $ 11.6 The following tables present the average recorded investment in and income recognized on impaired loans for the periods indicated: Year Ended December 31, 2017 2016 2015 Average Recorded Investment Income Recognized Average Recorded Investment Income Recognized Average Recorded Investment Income Recognized Real estate: Commercial $ 40.4 $ 0.3 $ 46.9 $ 0.2 $ 39.2 $ 0.8 Construction: Land acquisition & development 5.0 — 8.0 0.1 8.3 0.1 Residential 1.3 — 0.8 — 0.3 — Commercial 4.2 — 2.9 — 1.9 — Total construction loans 10.5 — 11.7 0.1 10.5 0.1 Residential 8.4 — 6.2 — 4.1 — Agricultural 4.2 — 4.4 — 7.2 — Total real estate loans 63.5 0.3 69.2 0.3 61.0 0.9 Commercial 28.1 0.2 30.1 0.3 18.5 0.1 Agricultural 2.4 — 1.8 — 0.9 — Total $ 94.0 $ 0.5 $ 101.1 $ 0.6 $ 80.4 $ 1.0
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, 2017 Interest only period Extension of terms or maturity Interest rate adjustment Other Real estate: Commercial 5 $ 1.5 $ 0.4 $ — $ 0.9 $ 2.8 Agriculture 1 — 0.8 — — 0.8 Total real estate loans 6 1.5 1.2 — 0.9 3.6 Commercial 17 1.2 2.0 — 6.0 9.2 Agriculture 1 — 0.1 — — 0.1 Total 24 $ 2.7 $ 3.3 $ — $ 6.9 $ 12.9 Number of Notes Type of Concession Principal Balance at Restructure Date Year ended December 31, 2016 Interest only period Extension of terms or maturity Interest rate adjustment Other Real estate: Commercial 18 $ 0.4 $ 5.5 $ 0.2 $ 1.8 $ 7.9 Commercial construction 1 — 3.7 — — 3.7 Residential 1 — 0.1 — — 0.1 Total real estate loans 20 0.4 9.3 0.2 1.8 11.7 Commercial 13 4.4 0.4 — 3.3 8.1 Agriculture 2 — 0.3 — — 0.3 Total 35 $ 4.8 $ 10.0 $ 0.2 $ 5.1 $ 20.1 Number of Notes Type of Concession Principal Balance at Restructure Date Year ended December 31, 2015 Interest only period Extension of terms or maturity Interest rate adjustment Other Commercial real estate 2 $ — $ 0.6 $ — $ 0.1 $ 0.7 Consumer 1 — — — — — Commercial 12 — 8.7 3.3 0.5 12.5 Total 15 $ — $ 9.3 $ 3.3 $ 0.6 $ 13.2 The following table presents information on the Company's troubled debt restructurings during the previous 12 months for which there was a payment default. Year ended December 31, 2017 Number of Notes Balance Commercial 1 $ 1.3 Total 1 $ 1.3
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, 2017 Other Assets Especially Mentioned Substandard Doubtful Total Criticized Loans Real estate: Commercial $ 78.0 $ 96.4 $ 10.3 $ 184.7 Construction: Land acquisition & development 3.2 16.4 — 19.6 Residential 2.3 1.7 0.5 4.5 Commercial 2.4 3.6 3.5 9.5 Total construction loans 7.9 21.7 4.0 33.6 Residential 3.9 12.5 1.9 18.3 Agricultural 4.3 19.1 — 23.4 Total real estate loans 94.1 149.7 16.2 260.0 Consumer: Indirect consumer 0.8 2.2 0.3 3.3 Other consumer 0.4 0.7 0.2 1.3 Total consumer loans 1.2 2.9 0.5 4.6 Commercial 54.7 56.3 11.1 122.1 Agricultural 5.1 8.3 0.4 13.8 Total $ 155.1 $ 217.2 $ 28.2 $ 400.5 As of December 31, 2016 Other Assets Especially Mentioned Substandard Doubtful Total Criticized Loans Real estate: Commercial $ 85.3 $ 85.3 $ 10.8 $ 181.4 Construction: Land acquisition & development 13.4 6.2 1.4 21.0 Residential 0.4 1.6 0.7 2.7 Commercial 1.6 6.3 0.7 8.6 Total construction loans 15.4 14.1 2.8 32.3 Residential 5.0 12.5 0.8 18.3 Agricultural 3.8 17.8 — 21.6 Total real estate loans 109.5 129.7 14.4 253.6 Consumer: Indirect consumer 0.8 1.5 0.1 2.4 Other consumer 0.7 1.0 0.3 2.0 Total consumer loans 1.5 2.5 0.4 4.4 Commercial 46.4 29.3 21.2 96.9 Agricultural 6.2 10.7 0.4 17.3 Total $ 163.6 $ 172.2 $ 36.4 $ 372.2

Allowance for Loan Losses (Tabl

Allowance for Loan Losses (Tables)	12 Months Ended
	Dec. 31, 2017
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, 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 (2.8 ) (0.2 ) — 11.1 Less loans charged-off (4.4 ) (11.3 ) (6.8 ) (0.4 ) — (22.9 ) Add back recoveries of loans previously charged-off 1.4 4.2 2.1 — — 7.7 Ending balance $ 31.6 $ 8.7 $ 30.6 $ 1.2 $ — $ 72.1 Individually evaluated for impairment $ 6.2 $ — $ 4.4 $ 0.2 $ — $ 10.8 Collectively evaluated for impairment 25.4 8.7 26.2 1.0 — 61.3 Ending balance $ 31.6 $ 8.7 $ 30.6 $ 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 Year ended December 31, 2016 Real Estate Consumer Commercial Agriculture Other Total Allowance for loan losses: Beginning balance $ 52.3 $ 5.1 $ 18.8 $ 0.6 $ — $ 76.8 Provision charged (credited) to operating expense (21.7 ) 8.4 21.9 1.4 — 10.0 Less loans charged-off (5.2 ) (8.6 ) (5.8 ) (0.2 ) — (19.8 ) Add back recoveries of loans previously charged-off 3.2 2.8 3.2 — — 9.2 Ending balance $ 28.6 $ 7.7 $ 38.1 $ 1.8 $ — $ 76.2 Individually evaluated for impairment $ 4.6 $ — $ 9.3 $ 0.1 $ — $ 14.0 Collectively evaluated for impairment 24.0 7.7 28.8 1.7 — 62.2 Ending balance $ 28.6 $ 7.7 $ 38.1 $ 1.8 $ — $ 76.2 Total loans: Individually evaluated for impairment $ 68.6 $ — $ 32.4 $ 3.7 $ — $ 104.7 Collectively evaluated for impairment 3,445.4 970.3 765.5 129.2 1.6 5,312.0 Total loans held for investment $ 3,514.0 $ 970.3 $ 797.9 $ 132.9 $ 1.6 $ 5,416.7 Year ended December 31, 2015 Real Estate Consumer Commercial Agriculture Other Total Allowance for loan losses: Beginning balance $ 53.9 $ 5.0 $ 14.3 $ 1.0 $ — $ 74.2 Provision charged (credited) to operating expense (0.6 ) 3.2 4.4 (0.2 ) — 6.8 Less loans charged-off (4.1 ) (5.7 ) (1.7 ) (0.2 ) — (11.7 ) Add back recoveries of loans previously charged-off 3.1 2.6 1.8 — — 7.5 Ending balance $ 52.3 $ 5.1 $ 18.8 $ 0.6 $ — $ 76.8 Individually evaluated for impairment $ 4.8 $ — $ 6.5 $ 0.3 $ — $ 11.6 Collectively evaluated for impairment 47.5 5.1 12.3 0.3 — 65.2 Ending balance $ 52.3 $ 5.1 $ 18.8 $ 0.6 $ — $ 76.8 Total loans: Individually evaluated for impairment $ 66.5 $ — $ 24.5 $ 1.0 $ — $ 92.0 Collectively evaluated for impairment 3,346.6 844.4 767.9 141.2 1.3 5,101.4 Total loans held for investment $ 3,413.1 $ 844.4 $ 792.4 $ 142.2 $ 1.3 $ 5,193.4

Premises and Equipment (Tables)

Premises and Equipment (Tables)	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Premises and Equipment and Related Accumulated Depreciation	Premises and equipment and related accumulated depreciation are as follows: December 31, 2017 2016 Land $ 49.4 $ 39.8 Buildings and improvements 257.1 218.3 Furniture and equipment 97.7 88.6 404.2 346.7 Less accumulated depreciation (162.3 ) (152.2 ) Premises and equipment, net $ 241.9 $ 194.5

Company-Owned Life Insurance (T

Company-Owned Life Insurance (Tables)	12 Months Ended
	Dec. 31, 2017
Investments, All Other Investments [Abstract]
Schedule of Life Insurance Corporate or Bank Owned	Company-owned life insurance consists of the following: December 31, 2017 2016 Key executive, principal shareholder $ 4.2 $ 4.0 Key executive split dollar 4.9 4.8 Group life 251.5 189.3 Total $ 260.6 $ 198.1

Other Real Estate Owned (Tables

Other Real Estate Owned (Tables)	12 Months Ended
	Dec. 31, 2017
Repossessed Assets [Abstract]
Other Real Estate Owned	Information with respect to the Company’s other real estate owned follows: Year Ended December 31, 2017 2016 2015 Balance at beginning of year $ 10.0 $ 6.3 $ 13.6 Acquisitions 1.2 1.1 — Additions 5.4 7.6 5.6 Valuation adjustments (0.4 ) (0.6 ) (0.2 ) Dispositions (6.1 ) (4.4 ) (12.7 ) Balance at end of year $ 10.1 $ 10.0 $ 6.3

Derivatives and Hedging Activ46

Derivatives and Hedging Activities (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 and December 31, 2016 : December 31, 2017 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 $ 7.5 $ — $ 7.5 $ 2.4 $ — $ 5.1 Mortgage related derivatives 1.3 — 1.3 — — 1.3 Total derivatives 8.8 — 8.8 2.4 — 6.4 Total assets $ 8.8 $ — $ 8.8 $ 2.4 $ — $ 6.4 Financial Liabilities Interest rate swap contracts $ 7.8 $ — $ 7.8 $ 2.4 $ 3.3 $ 2.1 Mortgage related derivatives 0.1 — 0.1 — — 0.1 Total derivatives 7.9 — 7.9 2.4 3.3 2.2 Repurchase agreements 643.0 — 643.0 — 643.0 — Total liabilities $ 650.9 $ — $ 650.9 $ 2.4 $ 646.3 $ 2.2 December 31, 2016 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 $ 1.3 $ — $ 1.3 $ 0.5 $ — $ 0.8 Mortgage related derivatives 1.4 — 1.4 — — 1.4 Total derivatives 2.7 — 2.7 0.5 — 2.2 Total assets $ 2.7 $ — $ 2.7 $ 0.5 $ — $ 2.2 Financial Liabilities Interest rate swap contracts $ 1.3 $ — $ 1.3 $ 0.5 $ — $ 0.8 Total derivatives 1.3 — 1.3 0.5 — 0.8 Repurchase agreements 537.6 — 537.6 — 537.6 — Total liabilities $ 538.9 $ — $ 538.9 $ 0.5 $ 537.6 $ 0.8
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, 2017 and December 31, 2016 : December 31, 2017 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 $ 7.5 $ — $ 7.5 $ 2.4 $ — $ 5.1 Mortgage related derivatives 1.3 — 1.3 — — 1.3 Total derivatives 8.8 — 8.8 2.4 — 6.4 Total assets $ 8.8 $ — $ 8.8 $ 2.4 $ — $ 6.4 Financial Liabilities Interest rate swap contracts $ 7.8 $ — $ 7.8 $ 2.4 $ 3.3 $ 2.1 Mortgage related derivatives 0.1 — 0.1 — — 0.1 Total derivatives 7.9 — 7.9 2.4 3.3 2.2 Repurchase agreements 643.0 — 643.0 — 643.0 — Total liabilities $ 650.9 $ — $ 650.9 $ 2.4 $ 646.3 $ 2.2 December 31, 2016 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 $ 1.3 $ — $ 1.3 $ 0.5 $ — $ 0.8 Mortgage related derivatives 1.4 — 1.4 — — 1.4 Total derivatives 2.7 — 2.7 0.5 — 2.2 Total assets $ 2.7 $ — $ 2.7 $ 0.5 $ — $ 2.2 Financial Liabilities Interest rate swap contracts $ 1.3 $ — $ 1.3 $ 0.5 $ — $ 0.8 Total derivatives 1.3 — 1.3 0.5 — 0.8 Repurchase agreements 537.6 — 537.6 — 537.6 — Total liabilities $ 538.9 $ — $ 538.9 $ 0.5 $ 537.6 $ 0.8
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, 2017 December 31, 2016 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 $ 344.2 $ 7.5 $ 53.6 $ 1.3 Interest rate lock commitments 60.7 1.3 73.4 1.1 Forward loan sales contracts — — 126.8 0.3 Total derivative assets $ 404.9 $ 8.8 $ 253.8 $ 2.7 Derivative Liabilities (included in accounts payable and accrued expenses on the consolidated balance sheets) Derivatives designated as hedges: Interest rate swap contracts $ — $ — $ 100.0 $ — Non-hedging interest rate derivatives: Interest rate swap contracts 344.2 7.8 53.6 1.3 Forward loan sales contracts 88.8 0.1 — — Total derivative liabilities $ 433.0 $ 7.9 $ 153.6 $ 1.3
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, 2017 2016 2015 Derivatives designated as hedges: Amount of loss recognized in other comprehensive income (effective portion) $ (1.1 ) $ (0.2 ) $ 0.2 Reclassification adjustment for derivative net (gains) losses included in income 1.1 — — Non-hedging interest rate derivatives: Amount of gain (loss) recognized in other non-interest income — 0.1 — Amount of net fee income recognized in other non-interest income 0.8 0.9 0.1 Amount of net gains (losses) recognized in mortgage banking revenues (1.7 ) 1.4 —

Mortgage Servicing Rights (Tabl

Mortgage Servicing Rights (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 2015 Balance at beginning of year $ 18.7 $ 15.9 $ 14.4 Acquisitions of mortgage servicing rights 3.5 — 0.3 Originations of mortgage servicing rights 5.6 5.8 3.6 Amortization expense (3.0 ) (3.0 ) (2.4 ) Balance at end of year 24.8 18.7 15.9 Less valuation reserve — (0.2 ) (0.2 ) Balance at end of year, net of valuation reserve $ 24.8 $ 18.5 $ 15.7 Principal balance of serviced loans underlying mortgage servicing rights $ 3,636.7 $ 3,127.5 $ 2,906.2 Mortgage servicing rights as a percentage of serviced loans 0.68 % 0.59 % 0.54 %

Deposits (Tables)

Deposits (Tables)	12 Months Ended
	Dec. 31, 2017
Deposits [Abstract]
Schedule of Deposits, By Type	Deposits are summarized as follows: December 31, 2017 2016 Non-interest bearing demand $ 2,900.0 $ 1,906.3 Interest bearing: Demand 2,787.5 2,276.5 Savings 3,095.4 2,141.8 Time, $100 and over 432.0 461.4 Time, other 720.0 590.1 Total interest bearing 7,034.9 5,469.8 Total deposits $ 9,934.9 $ 7,376.1
Schedule of Maturities of Time Deposits	Maturities of time deposits at December 31, 2017 are as follows: Time, $100 and Over Total Time Due within 3 months or less $ 88.6 $ 313.8 Due after 3 months and within 6 months 65.3 167.3 Due after 6 months and within 12 months 110.6 259.2 Due after 12 months 167.5 411.6 Total $ 432.0 $ 1,151.9

Long-Term Debt and Other Borr49

Long-Term Debt and Other Borrowed Funds (Tables)	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Summary of Long-Term Debt	A summary of long-term debt follows: December 31, 2017 2016 Parent Company: 6.81% subordinated term loan maturing January 9, 2018, principal due at maturity, interest payable quarterly $ — $ 20.0 Subsidiaries: 8.00% capital lease obligation with term ending October 25, 2029 1.4 1.5 6.24% note payable maturing September 6, 2032, principal due at maturity, interest payable monthly 1.6 1.5 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 — Total long-term debt $ 13.1 $ 28.0
Schedule of Maturities of Long-term Debt	Maturities of long-term debt at December 31, 2017 are as follows: 2018 $ 0.1 2019 0.1 2020 0.1 2021 0.1 2022 5.1 Thereafter 7.6 Total $ 13.1

Subordinated Debentures Held 50

Subordinated Debentures Held by Subsidiary Trusts (Tables)	12 Months Ended
	Dec. 31, 2017
Subordinated Borrowings [Abstract]
Schedule of Subordinated Borrowings	A summary of Subordinated Debenture issuances follows: Principal Amount Outstanding as of December 31, Issuance Maturity Date 2017 2016 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 Total subordinated debentures held by subsidiary trusts $ 82.5 $ 82.5

Earnings per Common Share (Tabl

Earnings per Common Share (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 2016 2015 Net income, basic and diluted $ 106.5 $ 95.6 $ 86.8 Weighted average common shares outstanding for basic earnings per share computation 51,429,366 44,511,774 45,184,091 Dilutive effects of stock-based compensation 473,843 398,622 462,327 Weighted average common shares outstanding for diluted earnings per common share computation 51,903,209 44,910,396 45,646,418 Basic earnings per common share $ 2.07 $ 2.15 $ 1.92 Diluted earnings per common share 2.05 2.13 1.90

Regulatory Capital (Tables)

Regulatory Capital (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 are presented in the following tables: Actual Adequately Capitalized Basel III Phase-In Schedule Adequately Capitalized Basel III Fully Phased-In Well Capitalized (1) Amount Ratio Amount Ratio Amount Ratio Amount Ratio December 31, 2017 Total risk-based capital: Consolidated $ 1,112.5 12.76 % $ 806.5 9.25 % $ 915.5 10.50 % $ 871.9 10.00 % FIB 1,066.6 12.29 802.7 9.25 911.2 10.50 867.8 10.00 Tier 1 risk-based capital: Consolidated 1,040.3 11.93 632.1 7.25 741.1 8.50 697.5 8.00 FIB 994.4 11.46 629.1 7.25 737.6 8.50 694.2 8.00 Common equity tier 1 risk-based capital: Consolidated 962.4 11.04 501.3 5.75 610.3 7.00 566.7 6.50 FIB 994.4 11.46 499.0 5.75 607.4 7.00 564.1 6.50 Leverage capital ratio: Consolidated 1,040.3 8.86 469.9 4.00 469.9 4.00 587.4 5.00 FIB 994.4 8.48 469.1 4.00 469.1 4.00 586.3 5.00 Actual Adequately Capitalized Basel III Phase-In Schedule Adequately Capitalized Basel III Fully Phased-In Well Capitalized (1) Amount Ratio Amount Ratio Amount Ratio Amount Ratio December 31, 2016 Total risk-based capital: Consolidated $ 974.5 15.13 % $ 555.7 8.63 % $ 676.1 10.50 % $ 643.9 10.00 % FIB 842.0 13.13 553.5 8.63 673.4 10.50 641.3 10.00 Tier 1 risk-based capital: Consolidated 894.3 13.89 426.9 6.63 547.3 8.50 515.1 8.00 FIB 765.8 11.94 425.2 6.63 545.1 8.50 513.1 8.00 Common equity tier 1 risk-based capital: Consolidated 814.3 12.65 330.3 5.13 450.7 7.00 418.5 6.50 FIB 765.8 11.94 329.0 5.13 448.9 7.00 416.9 6.50 Leverage capital ratio: Consolidated 894.3 10.11 353.8 4.00 353.8 4.00 442.3 5.00 FIB 765.8 8.69 352.3 4.00 352.3 4.00 440.4 5.00 (1) The ratios for the well capitalized requirement are only applicable to FIB. However, the Company manages its capital position as if the requirement applies to the consolidated entity and has presented the ratios as if they also applied on a consolidated basis.

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 , are as follows: Third Parties Related Entity Total For the year ending December 31: 2018 $ 3.5 $ 2.5 $ 6.0 2019 3.3 2.5 5.8 2020 2.6 2.5 5.1 2021 2.1 2.5 4.6 2022 1.6 2.5 4.1 Thereafter 9.4 9.1 18.5 Total $ 22.5 $ 21.6 $ 44.1

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Schedule of Income Tax Expense	Income tax expense consists of the following: Year ended December 31, 2017 2016 2015 Current: Federal $ 24.3 $ 40.3 $ 26.7 State 5.0 5.9 4.5 Total current 29.3 46.2 31.2 Deferred: Federal 18.4 2.9 11.1 State 2.5 0.5 1.4 Total deferred 20.9 3.4 12.5 Total income tax expense $ 50.2 $ 49.6 $ 43.7
Schedule of Effective Income Tax Rate Reconciliation	Total income tax expense differs from the amount computed by applying the statutory federal income tax rate of 35% in 2017 , 2016 and 2015 , respectively, to income before income taxes as a result of the following: Year ended December 31, 2017 2016 2015 Tax expense at the statutory tax rate $ 54.9 $ 50.8 $ 45.7 Increase (decrease) in tax resulting from: Tax-exempt income (4.5 ) (4.4 ) (4.1 ) State income tax, net of federal income tax benefit 4.9 4.3 3.8 Benefit of stock-based compensation plans (2.6 ) — — Federal tax credits (2.4 ) (2.1 ) (2.3 ) Benefit due to enactment of federal tax reform (2.2 ) — — Other, net 2.2 1.0 0.6 Tax expense at effective tax rate $ 50.2 $ 49.6 $ 43.7
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) with a tax rate of 21.0% and 35.0% as of December 31, 2017 and December 31, 2016 , respectively, relate to the following: December 31, 2017 2016 Deferred tax assets: Loans, principally due to allowance for loan losses $ 18.0 $ 30.1 Loan discount 6.7 3.1 Investment securities, unrealized losses 5.6 6.6 Employee benefits 11.5 9.3 Non-performing loan interest 1.1 1.2 Other real estate owned write-downs and carrying costs 0.6 1.3 Tax credit carryforwards (1) 3.7 — Net operating loss carryforwards (2) 8.7 — Other 4.7 2.5 Deferred tax assets 60.6 54.1 Deferred tax liabilities: Fixed assets, principally differences in bases and depreciation (6.7 ) (4.7 ) Deferred loan costs (1.7 ) (3.0 ) Investment in joint venture partnership, principally due to differences in depreciation of partnership assets (0.8 ) (1.2 ) Prepaid amounts (0.6 ) (1.5 ) Government agency stock dividends (1.6 ) (2.3 ) Goodwill and core deposit intangibles (38.1 ) (42.4 ) Mortgage servicing rights (5.7 ) (5.5 ) Other (1.4 ) (0.3 ) Deferred tax liabilities (56.6 ) (60.9 ) Net deferred tax assets (liabilities) $ 4.0 $ (6.8 ) (1) Based on filed tax returns and amounts expected to be reported in current year tax returns (December 31, 2017), we had remaining federal tax credit carryforwards of $3.7 million from acquired companies. The federal tax credits were primarily generated from low income housing and AMT tax credit carryforwards. These tax credits expire beginning in 2027 and ending in 2037 and their use is subject to annual limitations. (2) As of December 31, 2017, we had remaining federal net operating loss carryforwards of $20.1 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 2018 and ending in 2031. The use of these carryforwards is subject to annual limitations.

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Share-based Compensation, Stock Options, Activity	The following table summarizes stock option activity under the Company’s active stock option plans for the year ended December 31, 2017 : Number of Shares Weighted-Average Exercise Price Weighted-Average Remaining Contract Life Outstanding options, beginning of year 940,843 $ 16.77 Exercised (285,887 ) 18.15 Forfeited (12,701 ) 17.86 Outstanding options, end of year 642,255 $ 16.13 2.34 years Outstanding options exercisable, end of year 642,255 $ 16.13 2.34 years
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, 2017 : Number of Shares Weighted-Average Measurement Date Fair Value Restricted stock, beginning of year 317,055 $ 26.22 Granted 140,246 41.46 Vested (111,224 ) 26.14 Forfeited (53,571 ) 27.96 Restricted stock, end of year 292,506 $ 33.24

Other Comprehensive Income (Tab

Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2017
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, 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 gains included in net income (0.7 ) (0.3 ) (0.4 ) Change in unamortized loss on available-for-sale securities transferred into held-to-maturity 1.8 0.7 1.1 Change in net unrealized loss on derivatives (1.1 ) (0.4 ) (0.7 ) Reclassification adjustment for derivative net (gains) losses included in net income 1.1 0.4 0.7 Defined benefits post-retirement benefit plan: Change in net actuarial loss (gain) (1.2 ) (0.5 ) (0.7 ) Total other comprehensive loss $ (6.6 ) $ (2.8 ) $ (3.8 ) Year ended December 31, 2016 Before Tax Amount Tax Expense (Benefit) Net of Tax Amount Investment securities available-for sale: Change in net unrealized loss during period $ (19.4 ) $ (7.6 ) $ (11.8 ) Reclassification adjustment for net gains included in net income (0.3 ) (0.1 ) (0.2 ) Unamortized premium on available-to-sale securities transferred into held-for-maturity 1.9 0.7 1.2 Change in net unrealized gain on derivatives (0.2 ) (0.1 ) (0.1 ) Defined benefits post-retirement benefit plan: Change in net actuarial loss 4.0 1.6 2.4 Total other comprehensive loss $ (14.0 ) $ (5.5 ) $ (8.5 ) Year ended December 31, 2015 Before Tax Amount Tax Expense (Benefit) Net of Tax Amount Investment securities available-for sale: Change in net unrealized loss during period $ 3.1 $ 1.2 $ 1.9 Reclassification adjustment for net gains included in net income (0.2 ) (0.1 ) (0.1 ) Change in unamortized gain on available-for-sale securities transferred into held-to-maturity 1.6 0.6 1.0 Change in net unrealized gain on derivatives 0.2 0.1 0.1 Defined benefits post-retirement benefit plan: Change in net actuarial loss 0.1 0.1 — Total other comprehensive income $ 4.8 $ 1.9 $ 2.9
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, 2017 2016 Net unrealized gain (loss) on investment securities available-for-sale $ (13.2 ) $ (10.1 ) Net actuarial gain (loss) on defined benefit post-retirement benefit plans 1.3 2.0 Net unrealized gain (loss) on derivatives — — Net accumulated other comprehensive income (loss) $ (11.9 ) $ (8.1 )

Condensed Financial Informati57

Condensed Financial Information (Parent Company Only) (Tables)	12 Months Ended
	Dec. 31, 2017
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Schedule of Condensed Financial Statements	Following is condensed financial information of First Interstate BancSystem, Inc. December 31, 2017 2016 Condensed balance sheets: Cash and cash equivalents $ 42.3 $ 138.4 Investment in subsidiaries, at equity: Bank subsidiary 1,432.3 932.0 Total investment in subsidiaries 1,432.3 932.0 Advances to subsidiaries, net 25.7 — Other assets 61.7 35.4 Total assets $ 1,562.0 $ 1,105.8 Other liabilities $ 51.9 $ 18.0 Advances from subsidiaries, net — 2.7 Long-term debt — 20.0 Subordinated debentures held by subsidiary trusts 82.5 82.5 Total liabilities 134.4 123.2 Stockholders’ equity 1,427.6 982.6 Total liabilities and stockholders’ equity $ 1,562.0 $ 1,105.8 Years Ended December 31, 2017 2016 2015 Condensed statements of income: Dividends from subsidiaries $ 150.0 $ 140.0 $ 70.0 Other interest income 0.1 — — Other income, primarily management fees from subsidiaries 18.0 15.1 13.2 Total income 168.1 155.1 83.2 Salaries and benefits 21.8 18.8 16.3 Interest expense 4.7 4.1 3.8 Acquisition expenses 25.3 1.5 0.8 Other operating expenses, net 13.0 10.4 9.9 Total expenses 64.8 34.8 30.8 Earnings before income tax benefit 103.3 120.3 52.4 Income tax benefit (14.2 ) (7.7 ) (7.0 ) Income before undistributed earnings of subsidiaries 117.5 128.0 59.4 Undistributed earnings of subsidiaries (11.0 ) (32.4 ) 27.4 Net income $ 106.5 $ 95.6 $ 86.8 Years Ended December 31, 2017 2016 2015 Condensed statements of cash flows: Cash flows from operating activities: Net income $ 106.5 $ 95.6 $ 86.8 Adjustments to reconcile net income to cash provided by operating activities: Undistributed earnings of subsidiaries 11.0 32.4 (27.4 ) Stock-based compensation expense 3.9 4.4 3.9 Tax benefits from stock-based compensation — 2.1 1.4 Excess tax benefits from stock-based compensation — (1.6 ) (1.2 ) Other, net 14.7 (0.8 ) (11.3 ) Net cash provided by operating activities 136.1 132.1 52.2 Cash flows from investing activities: Capital distributions from nonbank subsidiaries 18.0 2.0 — Acquisition of intangible assets (28.0 ) — — Acquisition of bank holding company, net of cash and cash equivalents received (128.3 ) — (7.2 ) Investment in subsidiary (18.0 ) — — Net cash used in investing activities $ (156.3 ) $ 2.0 $ (7.2 ) Years Ended December 31, 2017 2016 2015 Cash flows from financing activities: Net (decrease) increase in advances from nonbank subsidiaries $ (28.4 ) $ 9.1 $ (2.0 ) Repayment of long-term debt — — (1.0 ) Proceeds from issuance of common stock, net of stock issuance costs 2.4 4.7 3.4 Excess tax benefits from stock-based compensation — 1.6 1.2 Purchase and retirement of common stock (1.3 ) (26.9 ) (20.6 ) Dividends paid to common stockholders (48.6 ) (39.4 ) (36.3 ) Net cash used in financing activities (75.9 ) (50.9 ) (55.3 ) Net change in cash and cash equivalents (96.1 ) 83.2 (10.3 ) Cash and cash equivalents, beginning of year 138.4 55.2 65.5 Cash and cash equivalents, end of year $ 42.3 $ 138.4 $ 55.2

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 Balance Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Investment securities available-for-sale: U.S. Treasury Notes $ 3.2 $ — $ 3.2 $ — Obligations of U.S. government agencies 561.5 — 561.5 — U.S. agency mortgage-backed securities & collateralized mortgage obligations 1,462.5 — 1,462.5 — Private mortgage-backed securities 90.7 — 90.7 — Corporate securities 87.9 — 87.9 — Other investments 3.0 — 3.0 — Loans held for sale 46.6 — 46.6 — Derivative assets: Interest rate swap contracts 7.5 — 7.5 — Interest rate lock commitments 1.3 — 1.3 — Derivative liabilities: Interest rate swap contracts 7.8 — 7.8 — Forward loan sales contracts 0.1 — 0.1 — Deferred compensation plan assets 12.2 — 12.2 — Deferred compensation plan liabilities 12.2 — 12.2 — Fair Value Measurements at Reporting Date Using As of December 31, 2016 Balance Quoted Prices in Active Markets for Identical Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Unobservable Inputs (Level 3) Investment securities available-for-sale: U.S. Treasury Notes $ 3.6 $ — $ 3.6 $ — Obligations of U.S. government agencies 391.3 — 391.3 — U.S. agency mortgage-backed securities & collateralized mortgage obligations 1,213.7 — 1,213.7 — Private mortgage-backed securities 0.1 — 0.1 — Other investments 3.0 — 3.0 — Loans held for sale 61.8 — 61.8 — Derivative assets: Interest rate swap contracts 1.3 — 1.3 — Interest rate lock commitments 1.1 — 1.1 — Forward loan sales contracts 0.3 — 0.3 — Derivative liabilities: Interest rate swap contracts 1.3 — 1.3 — Deferred compensation plan assets 10.6 — 10.6 — Deferred compensation plan liabilities 10.6 — 10.6 —
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, 2017 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 $ 32.6 $ — $ — $ 32.6 $ 22.2 Other real estate owned 1.3 — — 1.3 (1.6 ) Long-lived assets to be disposed of by sale 0.8 — — 0.8 — Fair Value Measurements at Reporting Date Using As of December 31, 2016 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 $ 39.3 $ — $ — $ 39.3 $ (25.8 ) Other real estate owned 2.1 — — 2.1 (1.7 ) Long-lived assets to be disposed of by sale 1.3 — — 1.3 (1.0 )
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, 2017 Fair Value Valuation Technique Unobservable Inputs Range (Weighted Average) Impaired loans $ 32.6 Appraisal Appraisal adjustment 0% - 78.11% (26.1%) Other real estate owned 1.3 Appraisal Appraisal adjustment 8% - 96% (11.55%) Long-lived assets to be disposed of by sale 0.8 Appraisal Appraisal adjustment 0% - 0% 0% As of December 31, 2016 Fair Value Valuation Technique Unobservable Inputs Range (Weighted Average) Impaired loans $ 39.3 Appraisal Appraisal adjustment 0% - 66% (31%) Other real estate owned 2.1 Appraisal Appraisal adjustment 8% - 96% (18%) Long-lived assets to be disposed of by sale 1.3 Appraisal Appraisal adjustment 0% - 9% (6%)
Fair Value, by Balance Sheet Grouping	A summary of the estimated fair values of financial instruments follows: ` Fair Value Measurements at Reporting Date Using As of December 31, 2017 Carrying Amount Estimated Fair Value Quoted Prices in Significant Other Significant Financial assets: Cash and cash equivalents $ 759.0 $ 759.0 $ 759.0 $ — $ — Investment securities available-for-sale 2,208.7 2,208.7 — 2,208.7 — Investment securities held-to-maturity 484.5 483.3 — 483.3 — Accrued interest receivable 38.0 38.0 — 38.0 — Mortgage servicing rights, net 24.8 40.1 — 40.1 — Net loans 7,542.2 7,298.8 — 7,266.2 32.6 Derivative assets 8.8 8.8 — 8.8 — Deferred compensation plan assets 12.2 12.2 — 12.2 — Total financial assets $ 11,078.2 $ 10,848.9 $ 759.0 $ 10,057.3 $ 32.6 Financial liabilities: Total deposits, excluding time deposits $ 8,783.0 $ 8,783.0 $ 8,783.0 $ — $ — Time deposits 1,151.9 1,137.9 — 1,137.9 — Securities sold under repurchase agreements 643.0 643.0 — 643.0 — Other borrowed funds 20.0 20.0 — 20.0 — Accrued interest payable 5.6 5.6 — 5.6 — Long-term debt 13.1 11.3 — 11.3 — Subordinated debentures held by subsidiary trusts 82.5 76.7 — 76.7 — Derivative liabilities 7.9 7.9 — 7.9 — Deferred compensation plan liabilities 12.2 12.2 — 12.2 — Total financial liabilities $ 10,719.2 $ 10,697.6 $ 8,783.0 $ 1,914.6 $ — Fair Value Measurements at Reporting Date Using As of December 31, 2016 Carrying Amount Estimated Fair Value Quoted Prices in Significant Other Significant Financial assets: Cash and cash equivalents $ 782.0 $ 782.0 $ 782.0 $ — $ — Investment securities available-for-sale 1,611.7 1,611.7 — 1,611.7 — Investment securities held-to-maturity 512.8 513.3 — 513.3 — Accrued interest receivable 29.9 29.9 — 29.9 — Mortgage servicing rights, net 18.5 35.7 — 35.7 — Net loans 5,402.3 5,309.9 — 5,270.6 39.3 Derivative assets 2.7 2.7 — 2.7 — Deferred compensation plan assets 10.6 10.6 — 10.6 — Total financial assets $ 8,370.5 $ 8,295.8 $ 782.0 $ 7,474.5 $ 39.3 Financial liabilities: Total deposits, excluding time deposits $ 6,324.5 $ 6,324.5 $ 6,324.5 $ — $ — Time deposits 1,051.6 1,044.7 — 1,044.7 — Securities sold under repurchase agreements 537.6 537.6 — 537.6 — Other borrowed funds — — — — — Accrued interest payable 5.4 5.4 — 5.4 — Long-term debt 28.0 27.5 — 27.5 — Subordinated debentures held by subsidiary trusts 82.5 73.6 — 73.6 — Derivative liabilities 1.3 1.3 — 1.3 — Deferred compensation plan liabilities 10.6 10.6 — 10.6 — Total financial liabilities $ 8,041.5 $ 8,025.2 $ 6,324.5 $ 1,700.7 $ —

Summary of Significant Accoun59

Summary of Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2017USD ($)subsidiarysegment	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Property, Plant and Equipment
Number of significant subsidiaries | subsidiary	1
Cash on deposit with Federal Reserve Bank	$ 552,000,000	$ 625,300,000
Compensating balances with Federal Reserve Bank	16,100,000	19,300,000
Mortgage loans held for sale	46,635,000	61,794,000
Deferred compensation plan assets	12,200,000	10,600,000
Deferred compensation plan liabilities	12,200,000	10,600,000
Impairment losses	0	200,000	$ 800,000
Write-downs of OREO	(400,000)	(600,000)	(200,000)
Carrying value of foreclosed OREO	2,700,000	2,300,000
Federal Reserve Bank stock	31,800,000	16,400,000
Federal Home Loan Bank stock	10,200,000	10,100,000
Income tax examination, penalties	$ 0	0	0
Number of operating segments | segment	1
Advertising expense	$ 3,500,000	2,800,000	3,500,000
Stock compensation expense	3,900,000	4,400,000	4,000,000
Tax benefits from stock-based compensation	$ 2,600,000	2,100,000	$ 1,400,000
Buildings and improvements | Minimum
Property, Plant and Equipment
Estimated useful lives	5 years
Buildings and improvements | Maximum
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
Property, Plant and Equipment
Estimated useful lives	15 years
Core Deposits Intangibles
Property, Plant and Equipment
Accumulated amortization	$ 36,900,000	$ 31,500,000

Acquisitions - Narrative (Deta

Acquisitions - Narrative (Details) $ / shares in Units, $ in Thousands	May 30, 2017USD ($)$ / sharesshares	Aug. 12, 2016USD ($)	Jun. 30, 2017USD ($)	Dec. 31, 2017USD ($)segment	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Nov. 17, 2016office	Dec. 31, 2014USD ($)
Business Acquisition [Line Items]
Cash received by Cascade shareholders (in dollars per share) | $ / shares	$ 1.91
Common stock issued (in shares) | shares	11,300,000
Price per share of common stock issued (in dollars per share) | $ / shares	$ 34.3
Cash paid related to Cascade options and restricted stock	$ 9,300
Cash paid for Cascade unvested restricted stock	$ 2,200
Shares issued for cascade unvested restricted stock | shares	168,000
Expense related to restricted awards excluded from consideration paid	$ 2,400
Restricted stock expense included In consideration paid	$ 5,500
Goodwill				$ 444,704	$ 212,820	$ 204,500		$ 205,600
Number of operating segments | segment				1
Estimated useful lives of related deposits	10 years
Acquisition related expenses				$ 27,156	$ 2,821	795
Legal and professional fees directly related to merger				6,500
Adjustment to pro forma net income for other costs				21,800
Cascade Bank
Business Acquisition [Line Items]
Number of banking offices | office							46
Core Deposits Intangibles
Business Acquisition [Line Items]
Finite-lived intangible assets acquired	$ 48,000	$ 2,500				$ 700
Cascade Bank
Business Acquisition [Line Items]
Acquisition related expenses				27,200
Legal and professional fees				$ 9,600
Revenue reported by Cascade			$ 12,900
Net income reported by Cascade			$ 3,000
Cascade Bank | Class A Common Stock
Business Acquisition [Line Items]
Common stock received by Cascade shareholders (in shares) | shares	0.14864
Common stock issued (in shares) | shares	11,252,750
Cascade Bank Acquisition, as Recorded by the Company
Business Acquisition [Line Items]
Cash paid as consideration	$ 155,000
Total consideration paid	541,000
Goodwill	231,900
Cascade Bank Acquisition, as Recorded by the Company | Core Deposits Intangibles
Business Acquisition [Line Items]
Finite-lived intangible assets acquired	$ 48,000
Flathead Bank
Business Acquisition [Line Items]
Cash paid as consideration		34,100
Flathead Bank | Core Deposits Intangibles
Business Acquisition [Line Items]
Finite-lived intangible assets acquired		$ 2,500
Estimated useful lives of related deposits		9 years

Acquisitions - Schedule of Rec

Acquisitions - Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Millions	May 30, 2017	Sep. 30, 2016
Cascade Bank Acquisition, As Recorded By Cascade Bank
Business Acquisition [Line Items]
Cash and cash equivalents	$ 246.8
Investment securities	476.7
Loans held for investment	2,111
Mortgage loans held for sale	10.3
Allowance for loan losses	(24)
Premises and equipment	46.6
Other real estate owned (OREO)	1.2
Core deposit intangible assets	0
Deferred tax assets, net	47.6
Other assets	98.6
Total assets acquired	3,014.8
Deposits	2,669.9
Accounts payable and accrued expense	62.2
Total liabilities assumed	2,732.1
Net assets acquired	282.7
Cascade Bank Acquisition, Fair Value Adjustments
Business Acquisition [Line Items]
Cash and cash equivalents	0
Investment securities	4.9
Loans held for investment	(31.7)
Mortgage loans held for sale	0
Allowance for loan losses	24
Premises and equipment	0.1
Other real estate owned (OREO)	0
Core deposit intangible assets	48
Deferred tax assets, net	(19)
Other assets	1.1
Total assets acquired	27.4
Deposits	(0.9)
Accounts payable and accrued expense	1.9
Total liabilities assumed	1
Net assets acquired	26.4
Cascade Bank Acquisition, as Recorded by the Company
Business Acquisition [Line Items]
Cash and cash equivalents	246.8
Investment securities	481.6
Loans held for investment	2,079.3
Mortgage loans held for sale	10.3
Allowance for loan losses	0
Premises and equipment	46.7
Other real estate owned (OREO)	1.2
Core deposit intangible assets	48
Deferred tax assets, net	28.6
Other assets	99.7
Total assets acquired	3,042.2
Deposits	2,669
Accounts payable and accrued expense	64.1
Total liabilities assumed	2,733.1
Net assets acquired	309.1
Cash	155
Total consideration paid	541
Reduction in goodwill		$ 0.3
Cascade Bank Acquisition, as Recorded by the Company | Class A Common Stock
Business Acquisition [Line Items]
Class A common stock	$ 386

Acquisitions - Schedule of Acq

Acquisitions - Schedule of Acquisition Related Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Salaries and wages	$ 122,751	$ 108,684	$ 101,451
Total acquisition related expenses	27,156	$ 2,821	$ 795
Cascade Bank
Business Acquisition [Line Items]
Legal and professional fees	9,600
Salaries and wages	5,100
Technology conversion and contract termination	10,200
Other	2,300
Total acquisition related expenses	$ 27,200

Acquisitions - Schedules of Lo

Acquisitions - Schedules of Loans Acquired (Details) $ in Millions	May 30, 2017USD ($)
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities Acquired During Period [Line Items]
Contractually required principal and interest payments	$ 49.7
Contractual cash flows not expected to be collected (non-accretable discount)	24.7
Cash flows expected to be collected	25
Interest component of cash flows expected to be collected (accretable discount)	1.9
Fair value of acquired credit-impaired loans	23.1
Contractually required principal and interest payments	2,098.1
Contractual cash flows not expected to be collected	23.3
Fair value at acquisition	$ 2,066.5

Acquisitions - Schedule of Una

Acquisitions - Schedule of Unaudited Pro Forma Revenue and Income (Details) - Cascade Bank - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Interest income	$ 420.8	$ 392.7
Non-interest income	153.3	165.9
Total revenues	574.1	558.6
Net income	$ 126.4	$ 72.3
EPS - basic (in dollars per share)	$ 2.03	$ 1.30
EPS - diluted (in dollars per share)	$ 2.01	$ 1.29

Goodwill and Core Deposit Int65

Goodwill and Core Deposit Intangibles - Schedule of Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Roll Forward]
Net carrying value at beginning of period	$ 212,820	$ 204,500	$ 205,600
Acquisitions and measurement period adjustments	231,900	8,300	(1,100)
Net carrying value at end of period	$ 444,704	$ 212,820	$ 204,500

Goodwill and Core Deposit Int66

Goodwill and Core Deposit Intangibles - Goodwill Narrative (Details) - USD ($)	Jul. 01, 2017	Jul. 01, 2016	Jul. 01, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Goodwill [Line Items]
Change in goodwill				$ 231,900,000	$ 8,300,000
Percent change in goodwill				109.00%
Goodwill				$ 444,704,000	$ 212,820,000	$ 204,500,000	$ 205,600,000
Impairment of goodwill	$ 0	$ 0	$ 0
MWFC Acquisition
Goodwill [Line Items]
Change in goodwill						$ (1,100,000)

Goodwill and Core Deposit Int67

Goodwill and Core Deposit Intangibles - Schedule of Core Deposit Intangibles Activity (Details) - USD ($) $ in Thousands	May 30, 2017	Aug. 12, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Finite-lived Intangible Assets [Roll Forward]
Current period amortization			$ 5,438	$ 3,427	$ 3,388
Core Deposits Intangibles
Finite-lived Intangible Assets [Roll Forward]
CDI, net, beginning of period			9,700	10,600	13,300
Established through acquisitions	$ 48,000	$ 2,500			700
Reductions due to sale of accounts			(3,200)	0	0
Current period amortization			5,400	3,400	3,400
Total CDI, net, at end of period			$ 49,100	$ 9,700	$ 10,600

Goodwill and Core Deposit Int68

Goodwill and Core Deposit Intangibles - Intangible Assets Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
Finite-Lived Intangible Assets [Line Items]
Purchase accounting adjustment from sale of custodial rights	$ 3.2
Core Deposits Intangibles
Finite-Lived Intangible Assets [Line Items]
Intangible assets, useful life (in years)	10 years
Proceeds from sale of custodial rights	$ 6.2

Goodwill and Core Deposit Int69

Goodwill and Core Deposit Intangibles - Schedule of Future Estimated CDI Amortization Expense (Details) - Core Deposits Intangibles - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Finite-Lived Intangible Assets [Line Items]
2,018	$ 6.9
2,019	6.6
2,020	6.2
2,021	5.7
2,022	5.3
Thereafter	18.4
Total	$ 49.1	$ 9.7	$ 10.6	$ 13.3

Investment Securities - Amorti

Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available-for-Sale:
Amortized Cost	$ 2,229,300	$ 1,625,000
Gross Unrealized Gains	3,900	6,700
Gross Unrealized Losses	(24,500)	(20,000)
Estimated Fair Value	2,208,712	1,611,698
Held-to-Maturity:
Amortized Cost	484,494	512,770
Gross Unrealized Gains	11,500	10,700
Gross Unrealized Losses	(12,700)	(10,100)
Estimated Fair Value	483,349	513,273
US Treasury notes
Available-for-Sale:
Amortized Cost	3,200	3,600
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	3,200	3,600
State, county and municipal securities
Held-to-Maturity:
Amortized Cost	172,400	160,200
Gross Unrealized Gains	2,600	2,700
Gross Unrealized Losses	(600)	(500)
Estimated Fair Value	174,400	162,400
Obligations of U.S. government agencies
Available-for-Sale:
Amortized Cost	569,500	397,400
Gross Unrealized Gains	0	300
Gross Unrealized Losses	(8,000)	(6,400)
Estimated Fair Value	561,500	391,300
Held-to-Maturity:
Amortized Cost	19,800	19,800
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(200)	(200)
Estimated Fair Value	19,600	19,600
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Amortized Cost	1,474,100	1,220,900
Gross Unrealized Gains	3,800	6,400
Gross Unrealized Losses	(15,400)	(13,600)
Estimated Fair Value	1,462,500	1,213,700
Held-to-Maturity:
Amortized Cost	230,500	279,600
Gross Unrealized Gains	8,800	7,800
Gross Unrealized Losses	(11,600)	(9,200)
Estimated Fair Value	227,700	278,200
Corporate securities
Available-for-Sale:
Amortized Cost	88,000
Gross Unrealized Gains	100
Gross Unrealized Losses	(300)
Estimated Fair Value	87,800
Held-to-Maturity:
Amortized Cost	61,600	53,000
Gross Unrealized Gains	100	200
Gross Unrealized Losses	(300)	(200)
Estimated Fair Value	61,400	53,000
Private mortgage-backed securities
Available-for-Sale:
Amortized Cost	91,500	100
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(800)	0
Estimated Fair Value	90,700	100
Other Investments
Available-for-Sale:
Amortized Cost	3,000	3,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	3,000	3,000
Other Investments
Held-to-Maturity:
Amortized Cost	200	200
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 200	$ 200

Investment Securities - Realiz

Investment Securities - Realized Gains (Losses) on Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Gross realized gains	$ 1,100	$ 400	$ 200
Gross realized losses	400	$ 100	$ 20
State, county and municipal securities
Schedule of Available-for-sale Securities
General obligation securities, amortized cost	133,500
State, county and municipal securities | Idaho, Montana, Oregon, South Dakota, Washington, and Wyoming
Schedule of Available-for-sale Securities
General obligation securities, amortized cost	$ 108,200

Investment Securities - Transf

Investment Securities - Transfer from AFS to HTM (Details) $ in Millions	Oct. 30, 2015USD ($)
Schedule of Available for Sale and Held-to-Maturity Securities
Remaining expected lives of available-for-sale securities transferred to held-to-maturity	4 years
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Schedule of Available for Sale and Held-to-Maturity Securities
Available-for-sale securities transferred into held-to-maturity, amortized cost	$ 100.3
Available-for-sale securities transferred into held-to-maturity, Fair value	100.1
Available-for-sale securities transferred into held-to-maturity, unrealized net losses	$ 0.2

Investment Securities - Gross

Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details)	12 Months Ended
	Dec. 31, 2017USD ($)security	Dec. 31, 2016USD ($)security
Fair Value
Less than 12 Months	$ 1,080,300,000	$ 1,063,400,000
12 Months or More	705,300,000	15,800,000
Total	1,785,600,000	1,079,200,000
Gross Unrealized Losses
Less than 12 Months	(10,700,000)	(19,500,000)
12 Months or More	(13,800,000)	(500,000)
Total	(24,500,000)	(20,000,000)
Fair Value
Less than 12 Months	180,600,000	203,400,000
12 Months or More	87,700,000	22,800,000
Total	268,300,000	226,200,000
Gross Unrealized Losses
Less than 12 Months	(9,700,000)	(8,800,000)
12 Months or More	(3,000,000)	(1,300,000)
Total	$ (12,700,000)	$ (10,100,000)
Available-for-Sale and Held-to-Maturity:
Investment securities in an unrealized loss position (number of securities) | security	581	396
Impairment losses	$ 0
Obligations of U.S. government agencies
Fair Value
Less than 12 Months	284,900,000	$ 316,500,000
12 Months or More	266,100,000	0
Total	551,000,000	316,500,000
Gross Unrealized Losses
Less than 12 Months	(3,400,000)	(6,400,000)
12 Months or More	(4,600,000)	0
Total	(8,000,000)	(6,400,000)
Fair Value
Less than 12 Months	9,700,000	19,600,000
12 Months or More	9,900,000	0
Total	19,600,000	19,600,000
Gross Unrealized Losses
Less than 12 Months	0	(200,000)
12 Months or More	(200,000)	0
Total	(200,000)	(200,000)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Fair Value
Less than 12 Months	670,100,000	746,300,000
12 Months or More	439,200,000	15,800,000
Total	1,109,300,000	762,100,000
Gross Unrealized Losses
Less than 12 Months	(6,200,000)	(13,100,000)
12 Months or More	(9,200,000)	(500,000)
Total	(15,400,000)	(13,600,000)
Fair Value
Less than 12 Months	76,400,000	108,800,000
12 Months or More	60,500,000	20,000,000
Total	136,900,000	128,800,000
Gross Unrealized Losses
Less than 12 Months	(9,100,000)	(7,900,000)
12 Months or More	(2,500,000)	(1,300,000)
Total	(11,600,000)	(9,200,000)
Private mortgage-backed securities
Fair Value
Less than 12 Months	74,000,000
12 Months or More	0
Total	74,000,000
Gross Unrealized Losses
Less than 12 Months	(800,000)
12 Months or More	0
Total	(800,000)
Corporate securities
Fair Value
Less than 12 Months	51,300,000
12 Months or More	0
Total	51,300,000
Gross Unrealized Losses
Less than 12 Months	(300,000)
12 Months or More	0
Total	(300,000)
Fair Value
Less than 12 Months	41,200,000	32,500,000
12 Months or More	5,000,000	0
Total	46,200,000	32,500,000
Gross Unrealized Losses
Less than 12 Months	(200,000)	(200,000)
12 Months or More	(100,000)	0
Total	(300,000)	(200,000)
US Treasury notes
Fair Value
Less than 12 Months		600,000
12 Months or More		0
Total		600,000
Gross Unrealized Losses
Less than 12 Months		0
12 Months or More		0
Total		0
State, county and municipal securities
Fair Value
Less than 12 Months	53,300,000	42,500,000
12 Months or More	12,300,000	2,800,000
Total	65,600,000	45,300,000
Gross Unrealized Losses
Less than 12 Months	(400,000)	(500,000)
12 Months or More	(200,000)	0
Total	$ (600,000)	$ (500,000)

Investment Securities - Maturi

Investment Securities - Maturities of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-sale amortized cost, within one year	$ 450,700
Available-for-sale amortized cost, after one year but within five years	1,422,700
Available-for-sale amortized cost, after five years but within ten years	277,600
Available-for-sale amortized cost, after ten years	78,300
Amortized Cost	2,229,300	$ 1,625,000
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-sale estimated fair value, within one year	447,500
Available-for-sale estimated fair value, after one year but within five years	1,409,300
Available-for-sale estimated fair value, after ten years	274,200
Available-for-sale estimated fair value, after five years but within ten years	77,700
Available-for-Sale, Estimated Fair Value	2,208,712	1,611,698
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-maturity amortized cost, within one year	76,100
Held-to-maturity amortized cost, after one year but within five years	266,200
Held-to-maturity amortized cost, after five years but within ten years	111,700
Held-to-maturity amortized cost, after ten years	30,500
Amortized Cost	484,494	512,770
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-maturity estimated fair value, within one year	76,700
Held-to-maturity estimated fair value, after one year but within five years	263,100
Held-to-maturity estimated fair value, after five years but within ten years	113,300
Held-to-maturity estimated fair value, after ten years	30,200
Estimated Fair Value	483,349	513,273
Available-for-sale Securities and Held-to-maturity Securities
Securities sold under agreements to repurchase	2,087,700	1,400,100
Pledged assets separately reported, securities pledged for repurchase agreements, at fair value	2,062,600	1,388,200
Callable within one year
Available-for-sale Securities and Held-to-maturity Securities
Investment securities primarily classified as available-for-sale, amortized costs, after one year but within five years	133,200
Investment securities primarily classified as available-for-sale, fair value, after one year but within five years	132,900
Mortgage Backed Securities, Variable Rate
Available-for-sale Securities and Held-to-maturity Securities
Available-for-sale securities, amortized cost basis	$ 247,900	$ 66,200

Loans - Schedule of Loans by C

Loans - Schedule of Loans by Class (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Loans and Leases Receivable
Loans held for investment	$ 7,567,720	$ 5,416,750
Mortgage loans held for sale	46,635	61,794
Total loans	7,614,355	5,478,544
Commercial
Loans and Leases Receivable
Loans held for investment	$ 2,822,900	1,834,400
Loans receivable, maturity	5 years
Land acquisition & development
Loans and Leases Receivable
Loans held for investment	$ 348,700	208,500
Residential
Loans and Leases Receivable
Loans held for investment	240,200	147,900
Commercial
Loans and Leases Receivable
Loans held for investment	119,400	125,600
Total construction loans
Loans and Leases Receivable
Loans held for investment	708,300	482,000
Residential
Loans and Leases Receivable
Loans held for investment	$ 1,487,400	1,027,400
Loans receivable, maturity	15 years
Agricultural
Loans and Leases Receivable
Loans held for investment	$ 158,200	170,200
Total real estate loans
Loans and Leases Receivable
Loans held for investment	5,176,800	3,514,000
Indirect consumer
Loans and Leases Receivable
Loans held for investment	784,700	752,400
Other consumer
Loans and Leases Receivable
Loans held for investment	175,100	148,100
Credit card consumer
Loans and Leases Receivable
Loans held for investment	74,600	69,800
Total consumer loans
Loans and Leases Receivable
Loans held for investment	1,034,400	970,300
Commercial
Loans and Leases Receivable
Loans held for investment	1,215,400	797,900
Agricultural
Loans and Leases Receivable
Loans held for investment	136,200	132,900
Other, including overdrafts
Loans and Leases Receivable
Loans held for investment	4,900	1,600
Loans held for investment
Loans and Leases Receivable
Loans held for investment	7,567,700	5,416,700
Total loans
Loans and Leases Receivable
Total loans	7,614,300	5,478,500
Home equity line of credit
Loans and Leases Receivable
Loans held for investment	$ 397,000	$ 321,500
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, 2017	Dec. 31, 2016
Deteriorated Loans Transferred in [Abstract]
Outstanding principal	$ 38.2	$ 35.8
Loans on accrual status	24.9	21.9
Total carrying value	$ 24.9	$ 21.9

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, 2017	Dec. 31, 2016	Dec. 31, 2015
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Rollforward [Abstract]
Beginning Balance	$ 6.8	$ 6.7	$ 5.8
Acquisitions	1.9	1.1	0.4
Accretion income	(2.9)	(2.5)	(2.9)
Additions	0.1	0	0.6
Reductions due to exit events	(1.5)	(1.1)	(0.5)
Reclassifications from nonaccretable differences	2.9	2.6	3.3
Ending Balance	$ 7.3	$ 6.8	$ 6.7

Loans - Schedule of Recorded I

Loans - Schedule of Recorded Investment in Past Due Loans by Class (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loans and Leases Receivable Recorded Investment, Past Due
Loans held for investment	$ 7,567,720	$ 5,416,750
Mortgage loans held for sale	46,635	61,794
Total loans	7,614,355	5,478,544
Interest income on non-accrual loans if accrued	3,500	3,400	$ 3,200
Commercial
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	3,700	8,700
Current Loans	2,792,400	1,799,500
Non-accrual Loans	26,800	26,200
Loans held for investment	2,822,900	1,834,400
Land acquisition & development
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	7,900	1,300
Current Loans	337,800	202,200
Non-accrual Loans	3,000	5,000
Loans held for investment	348,700	208,500
Residential
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	2,100	1,200
Current Loans	236,400	146,200
Non-accrual Loans	1,700	500
Loans held for investment	240,200	147,900
Commercial
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0	0
Current Loans	115,600	124,800
Non-accrual Loans	3,800	800
Loans held for investment	119,400	125,600
Total construction loans
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	10,000	2,500
Current Loans	689,800	473,200
Non-accrual Loans	8,500	6,300
Loans held for investment	708,300	482,000
Residential
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	15,100	6,000
Current Loans	1,464,100	1,015,000
Non-accrual Loans	8,200	6,400
Loans held for investment	1,487,400	1,027,400
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	500	600
Current Loans	154,300	165,300
Non-accrual Loans	3,400	4,300
Loans held for investment	158,200	170,200
Total real estate loans
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	29,300	17,800
Current Loans	5,100,600	3,453,000
Non-accrual Loans	46,900	43,200
Loans held for investment	5,176,800	3,514,000
Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	10,300	11,400
Current Loans	772,600	740,200
Non-accrual Loans	1,800	800
Loans held for investment	784,700	752,400
Other consumer
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	2,200	1,700
Current Loans	172,600	146,100
Non-accrual Loans	300	300
Loans held for investment	175,100	148,100
Credit card consumer
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	2,200	1,400
Current Loans	72,400	68,400
Non-accrual Loans	0	0
Loans held for investment	74,600	69,800
Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	14,700	14,500
Current Loans	1,017,600	954,700
Non-accrual Loans	2,100	1,100
Loans held for investment	1,034,400	970,300
Commercial
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	6,300	4,600
Current Loans	1,189,500	767,900
Non-accrual Loans	19,600	25,400
Loans held for investment	1,215,400	797,900
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	1,900	1,900
Current Loans	133,500	128,000
Non-accrual Loans	800	3,000
Loans held for investment	136,200	132,900
Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	0	300
Current Loans	4,900	1,300
Non-accrual Loans	0	0
Loans held for investment	4,900	1,600
Loans held for investment
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	52,200	39,100
Current Loans	7,446,100	5,304,900
Non-accrual Loans	69,400	72,700
Loans held for investment	7,567,700	5,416,700
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	46,600	61,800
Non-accrual Loans	0	0
Mortgage loans held for sale	46,600
Total loans
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	52,200	39,100
Current Loans	7,492,700	5,366,700
Non-accrual Loans	69,400	72,700
Total loans	7,614,300	5,478,500
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	2,900	7,300
Financing Receivables, 30 to 59 Days Past Due | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	7,300	600
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	2,100	900
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
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	9,400	1,500
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	13,300	4,000
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	300	300
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	25,900	13,100
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	7,800	8,400
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	1,600	1,300
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	900	500
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	10,300	10,200
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	3,900	3,200
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	1,800	1,500
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	41,900	28,000
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	41,900	28,000
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	500	1,100
Financing Receivables, 60 to 89 Days Past Due | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	300	400
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	300
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	300	700
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	1,400	1,300
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	300
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	2,200	3,400
Financing Receivables, 60 to 89 Days Past Due | Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	2,100	2,300
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	500	200
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	600	300
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	3,200	2,800
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	1,700	700
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	100	400
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	7,200	7,300
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	7,200	7,300
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	300	300
Financing Receivables, Equal to Greater than 90 Days Past Due | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Past Due
Total Loans 30 or More Days Past Due	300	300
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	300	300
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	400	700
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	200	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	1,200	1,300
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	400	700
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	100	200
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	700	600
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	1,200	1,500
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	700	700
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	300
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	3,100	3,800
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	$ 3,100	$ 3,800

Loans - Schedule of Recorded79

Loans - Schedule of Recorded Investment in Impaired Loans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	$ 107.9	$ 132.8	$ 120.4
Recorded Investment With No Allowance	51.4	57.9	56.3
Recorded Investment With Allowance	31.8	46.8	35.7
Total Recorded Investment	83.2	104.7	92
Related Allowance	10.8	14	11.6
Average Recorded Investment	94	101.1	80.4
Income Recognized	0.5	0.6	1
Impaired Loans, interest lost on nonaccrual loans	3.5	4.2	3.9
Commercial
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	45.6	57	58.2
Recorded Investment With No Allowance	20.9	24.4	27.9
Recorded Investment With Allowance	14.1	21.4	17.6
Total Recorded Investment	35	45.8	45.5
Related Allowance	3.9	2.8	3.4
Average Recorded Investment	40.4	46.9	39.2
Income Recognized	0.3	0.2	0.8
Land acquisition & development
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	10	12.1	15.5
Recorded Investment With No Allowance	3.4	4.3	7.2
Recorded Investment With Allowance	0.5	1.8	0.8
Total Recorded Investment	3.9	6.1	8
Related Allowance	0	0.8	0.3
Average Recorded Investment	5	8	8.3
Income Recognized	0	0.1	0.1
Residential
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	1.8	1.6	1
Recorded Investment With No Allowance	1.7	0.2	0.3
Recorded Investment With Allowance	0	0.6	0
Total Recorded Investment	1.7	0.8	0.3
Related Allowance	0	0	0
Average Recorded Investment	1.3	0.8	0.3
Income Recognized	0	0	0
Commercial
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	4.7	4.8	1.3
Recorded Investment With No Allowance	0.4	3.9	0.3
Recorded Investment With Allowance	3.5	0.7	0.7
Total Recorded Investment	3.9	4.6	1
Related Allowance	2.2	0.7	0.7
Average Recorded Investment	4.2	2.9	1.9
Income Recognized	0	0	0
Total construction loans
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	16.5	18.5	17.8
Recorded Investment With No Allowance	5.5	8.4	7.8
Recorded Investment With Allowance	4	3.1	1.5
Total Recorded Investment	9.5	11.5	9.3
Related Allowance	2.2	1.5	1
Average Recorded Investment	10.5	11.7	10.5
Income Recognized	0	0.1	0.1
Residential
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	11.5	8.2	7.1
Recorded Investment With No Allowance	8.2	4.1	3.6
Recorded Investment With Allowance	2	2.5	2.3
Total Recorded Investment	10.2	6.6	5.9
Related Allowance	0.1	0.3	0.4
Average Recorded Investment	8.4	6.2	4.1
Income Recognized	0	0	0
Agricultural
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	3.7	5.1	6.4
Recorded Investment With No Allowance	3.6	4.5	5.6
Recorded Investment With Allowance	0	0.2	0.2
Total Recorded Investment	3.6	4.7	5.8
Related Allowance	0	0	0
Average Recorded Investment	4.2	4.4	7.2
Income Recognized	0	0	0
Total real estate loans
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	77.3	88.8	89.5
Recorded Investment With No Allowance	38.2	41.4	44.9
Recorded Investment With Allowance	20.1	27.2	21.6
Total Recorded Investment	58.3	68.6	66.5
Related Allowance	6.2	4.6	4.8
Average Recorded Investment	63.5	69.2	61
Income Recognized	0.3	0.3	0.9
Commercial
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	29.5	40.3	29.6
Recorded Investment With No Allowance	12.4	13.2	10.8
Recorded Investment With Allowance	11.4	19.2	13.7
Total Recorded Investment	23.8	32.4	24.5
Related Allowance	4.4	9.3	6.5
Average Recorded Investment	28.1	30.1	18.5
Income Recognized	0.2	0.3	0.1
Agricultural
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	1.1	3.7	1.3
Recorded Investment With No Allowance	0.8	3.3	0.6
Recorded Investment With Allowance	0.3	0.4	0.4
Total Recorded Investment	1.1	3.7	1
Related Allowance	0.2	0.1	0.3
Average Recorded Investment	2.4	1.8	0.9
Income Recognized	$ 0	$ 0	$ 0

Loans - Schedule of Loans Rene

Loans - Schedule of Loans Renegotiated in Troubled Debt Restructurings (Details)	12 Months Ended
	Dec. 31, 2017USD ($)note	Dec. 31, 2016USD ($)note	Dec. 31, 2015USD ($)note
Loans and Leases Receivable, Troubled Debt Restructuring
Loans renegotiated in troubled debt restructurings	$ 44,500,000	$ 49,600,000
Loans renegotiated in troubled debt restructurings, non-accrual loans	31,900,000	27,300,000
Loans renegotiated in troubled debt restructurings, accrual loans	$ 12,600,000	$ 22,300,000
Number of notes under troubled debt restructurings | note	24	35	15
Concessions in troubled debt restructurings, amount	$ 12,900,000	$ 20,100,000	$ 13,200,000
Charge-offs directly related to modified loans	$ 0	$ 0	$ 0
Number of notes for which there was a payment default | note	1
Balance of notes for which there was a payment default	$ 1,300,000
Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note	5	18	2
Concessions in troubled debt restructurings, amount	$ 2,800,000	$ 7,900,000	$ 700,000
Commercial Construction
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note		1
Concessions in troubled debt restructurings, amount		$ 3,700,000
Residential
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note		1
Concessions in troubled debt restructurings, amount		$ 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	$ 800,000
Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note	6	20
Concessions in troubled debt restructurings, amount	$ 3,600,000	$ 11,700,000
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			$ 0
Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Number of notes under troubled debt restructurings | note	17	13	12
Concessions in troubled debt restructurings, amount	$ 9,200,000	$ 8,100,000	$ 12,500,000
Number of notes for which there was a payment default | note	1
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	1	2
Concessions in troubled debt restructurings, amount	$ 100,000	$ 300,000
Interest Only Period Concession
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	2,700,000	4,800,000	0
Interest Only Period Concession | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	1,500,000	400,000	0
Interest Only Period Concession | Commercial Construction
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		0
Interest Only Period Concession | Residential
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
Interest Only Period Concession | Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	1,500,000	400,000
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	1,200,000	4,400,000	0
Interest Only Period Concession | Agriculture
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0	0
Extension of terms or maturity
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	3,300,000	10,000,000	9,300,000
Extension of terms or maturity | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	400,000	5,500,000	600,000
Extension of terms or maturity | Commercial Construction
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		3,700,000
Extension of terms or maturity | Residential
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		100,000
Extension of terms or maturity | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	800,000
Extension of terms or maturity | Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	1,200,000	9,300,000
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	2,000,000	400,000	8,700,000
Extension of terms or maturity | Agriculture
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	100,000	300,000
Interest rate adjustment
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0	200,000	3,300,000
Interest rate adjustment | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0	200,000	0
Interest rate adjustment | Commercial Construction
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		0
Interest rate adjustment | Residential
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
Interest rate adjustment | Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0	200,000
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	3,300,000
Interest rate adjustment | Agriculture
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0	0
Other
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	6,900,000	5,100,000	600,000
Other | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	900,000	1,800,000	100,000
Other | Commercial Construction
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		0
Other | Residential
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount		0
Other | Agricultural
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	0
Other | Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	900,000	1,800,000
Other | Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount			0
Other | Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	6,000,000	3,300,000	$ 500,000
Other | Agriculture
Loans and Leases Receivable, Troubled Debt Restructuring
Concessions in troubled debt restructurings, amount	$ 0	$ 0

Loans - Schedule of Recorded81

Loans - Schedule of Recorded Investment in Criticized Loans by Class and Credit Quality Indicator (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	$ 7,567,720	$ 5,416,750
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2,822,900	1,834,400
Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	348,700	208,500
Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	240,200	147,900
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	119,400	125,600
Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	708,300	482,000
Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,487,400	1,027,400
Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	158,200	170,200
Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	5,176,800	3,514,000
Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	784,700	752,400
Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	175,100	148,100
Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,034,400	970,300
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,215,400	797,900
Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	136,200	132,900
Other Assets Especial Mentioned
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	155,100	163,600
Other Assets Especial Mentioned | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	78,000	85,300
Other Assets Especial Mentioned | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3,200	13,400
Other Assets Especial Mentioned | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2,300	400
Other Assets Especial Mentioned | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2,400	1,600
Other Assets Especial Mentioned | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	7,900	15,400
Other Assets Especial Mentioned | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3,900	5,000
Other Assets Especial Mentioned | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	4,300	3,800
Other Assets Especial Mentioned | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	94,100	109,500
Other Assets Especial Mentioned | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	800	800
Other Assets Especial Mentioned | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	400	700
Other Assets Especial Mentioned | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,200	1,500
Other Assets Especial Mentioned | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	54,700	46,400
Other Assets Especial Mentioned | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	5,100	6,200
Substandard
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	217,200	172,200
Substandard | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	96,400	85,300
Substandard | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	16,400	6,200
Substandard | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,700	1,600
Substandard | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3,600	6,300
Substandard | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	21,700	14,100
Substandard | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	12,500	12,500
Substandard | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	19,100	17,800
Substandard | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	149,700	129,700
Substandard | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2,200	1,500
Substandard | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	700	1,000
Substandard | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2,900	2,500
Substandard | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	56,300	29,300
Substandard | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	8,300	10,700
Doubtful
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	28,200	36,400
Doubtful | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	10,300	10,800
Doubtful | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0	1,400
Doubtful | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	500	700
Doubtful | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3,500	700
Doubtful | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	4,000	2,800
Doubtful | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,900	800
Doubtful | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0	0
Doubtful | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	16,200	14,400
Doubtful | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	300	100
Doubtful | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	200	300
Doubtful | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	500	400
Doubtful | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	11,100	21,200
Doubtful | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	400	400
Total Criticized Loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	400,500	372,200
Total Criticized Loans | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	184,700	181,400
Total Criticized Loans | Land acquisition & development
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	19,600	21,000
Total Criticized Loans | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	4,500	2,700
Total Criticized Loans | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	9,500	8,600
Total Criticized Loans | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	33,600	32,300
Total Criticized Loans | Residential
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	18,300	18,300
Total Criticized Loans | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	23,400	21,600
Total Criticized Loans | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	260,000	253,600
Total Criticized Loans | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3,300	2,400
Total Criticized Loans | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,300	2,000
Total Criticized Loans | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	4,600	4,400
Total Criticized Loans | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	122,100	96,900
Total Criticized Loans | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	$ 13,800	$ 17,300

Allowance for Loan Losses - Sc

Allowance for Loan Losses - Schedule of Allowance for Loan Losses by Portfolio Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	$ 76,214	$ 76,800	$ 74,200
Provision charged (credited) to operating expense	11,053	9,991	6,822
Less loans charged-off	(22,900)	(19,800)	(11,700)
Add back recoveries of loans previously charged-off	7,700	9,200	7,500
Ending balance	72,147	76,214	76,800
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				$ 10,800	$ 14,000	$ 11,600
Collectively evaluated for impairment				61,300	62,200	65,200
Ending balance	76,214	76,800	74,200	72,147	76,214	76,800
Total loans:
Individually evaluated for impairment				83,200	104,700	92,000
Collectively evaluated for impairment				7,484,500	5,312,000	5,101,400
Total loans held for investment				7,567,700	5,416,700	5,193,400
Allowance for loans losses				1,000	400
Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	28,600	52,300	53,900
Provision charged (credited) to operating expense	6,000	(21,700)	(600)
Less loans charged-off	(4,400)	(5,200)	(4,100)
Add back recoveries of loans previously charged-off	1,400	3,200	3,100
Ending balance	31,600	28,600	52,300
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				6,200	4,600	4,800
Collectively evaluated for impairment				25,400	24,000	47,500
Ending balance	28,600	52,300	53,900	31,600	28,600	52,300
Total loans:
Individually evaluated for impairment				58,300	68,600	66,500
Collectively evaluated for impairment				5,118,500	3,445,400	3,346,600
Total loans held for investment				5,176,800	3,514,000	3,413,100
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	7,700	5,100	5,000
Provision charged (credited) to operating expense	8,100	8,400	3,200
Less loans charged-off	(11,300)	(8,600)	(5,700)
Add back recoveries of loans previously charged-off	4,200	2,800	2,600
Ending balance	8,700	7,700	5,100
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				0	0	0
Collectively evaluated for impairment				8,700	7,700	5,100
Ending balance	7,700	5,100	5,000	8,700	7,700	5,100
Total loans:
Individually evaluated for impairment				0	0	0
Collectively evaluated for impairment				1,034,400	970,300	844,400
Total loans held for investment				1,034,400	970,300	844,400
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	38,100	18,800	14,300
Provision charged (credited) to operating expense	(2,800)	21,900	4,400
Less loans charged-off	(6,800)	(5,800)	(1,700)
Add back recoveries of loans previously charged-off	2,100	3,200	1,800
Ending balance	30,600	38,100	18,800
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				4,400	9,300	6,500
Collectively evaluated for impairment				26,200	28,800	12,300
Ending balance	38,100	18,800	14,300	30,600	38,100	18,800
Total loans:
Individually evaluated for impairment				23,800	32,400	24,500
Collectively evaluated for impairment				1,191,600	765,500	767,900
Total loans held for investment				1,215,400	797,900	792,400
Agriculture
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	1,800	600	1,000
Provision charged (credited) to operating expense	(200)	1,400	(200)
Less loans charged-off	(400)	(200)	(200)
Add back recoveries of loans previously charged-off	0	0	0
Ending balance	1,200	1,800	600
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Individually evaluated for impairment				200	100	300
Collectively evaluated for impairment				1,000	1,700	300
Ending balance	1,800	600	1,000	1,200	1,800	600
Total loans:
Individually evaluated for impairment				1,100	3,700	1,000
Collectively evaluated for impairment				135,100	129,200	141,200
Total loans held for investment				136,200	132,900	142,200
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				4,900	1,600	1,300
Total loans held for investment				$ 4,900	$ 1,600	$ 1,300

Premises and Equipment (Details

Premises and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment
Premises and equipment, gross	$ 404,200	$ 346,700
Less accumulated depreciation	(162,300)	(152,200)
Premises and equipment, net	241,862	194,457
Land
Property, Plant and Equipment
Premises and equipment, gross	49,400	39,800
Buildings and improvements
Property, Plant and Equipment
Premises and equipment, gross	257,100	218,300
Furniture and equipment
Property, Plant and Equipment
Premises and equipment, gross	$ 97,700	$ 88,600

Company-Owned Life Insurance (D

Company-Owned Life Insurance (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Company - Owned Life Insurance, By Type
Total	$ 260,551	$ 198,116
Key executive, principal shareholder
Schedule of Company - Owned Life Insurance, By Type
Cash surrender value of life insurance	4,200	4,000
Key executive split dollar
Schedule of Company - Owned Life Insurance, By Type
Cash surrender value of life insurance	4,900	4,800
FIB | Group life
Schedule of Company - Owned Life Insurance, By Type
Cash surrender value of life insurance	$ 251,500	$ 189,300

Other Real Estate Owned (Detail

Other Real Estate Owned (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Real Estate Acquired Through Foreclosure [Roll Forward]
Balance at beginning of year	$ 10,019	$ 6,300	$ 13,600
Acquisitions	1,200	1,100	0
Additions	5,400	7,600	5,600
Valuation adjustments	(400)	(600)	(200)
Dispositions	(6,100)	(4,400)	(12,700)
Balance at end of year	10,053	10,019	6,300
Write-downs of OREO	400	600	200
Write-down due to change in appraised value	300	550	100
Write-down due to change in management estimates	$ 100	$ 50	$ 100

Derivatives and Hedging Activ86

Derivatives and Hedging Activities - Notional Amounts and Estimated Fair Values of Derivatives (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Sep. 22, 2015
Derivative [Line Items]
Derivative Asset, Notional Amount	$ 404,900,000	$ 253,800,000
Derivative Assets, Estimated Fair Value	8,800,000	2,700,000
Derivative Liability, Notional Amount	433,000,000	153,600,000	$ 100,000,000
Derivative Liabilities, Estimated Fair Value	7,900,000	1,300,000
Not Designated as Hedging Instrument | Interest rate swap contracts
Derivative [Line Items]
Derivative Asset, Notional Amount	344,200,000	53,600,000
Derivative Assets, Estimated Fair Value	7,500,000	1,300,000
Derivative Liability, Notional Amount	344,200,000	53,600,000
Derivative Liabilities, Estimated Fair Value	7,800,000	1,300,000
Not Designated as Hedging Instrument | Interest rate lock commitments
Derivative [Line Items]
Derivative Asset, Notional Amount	60,700,000	73,400,000
Derivative Assets, Estimated Fair Value	1,300,000	1,100,000
Not Designated as Hedging Instrument | Forward loan sales contracts
Derivative [Line Items]
Derivative Asset, Notional Amount	0	126,800,000
Derivative Assets, Estimated Fair Value	0	300,000
Derivative Liability, Notional Amount	88,800,000	0
Derivative Liabilities, Estimated Fair Value	100,000	0
Designated as Hedging Instrument | Interest rate swap contracts
Derivative [Line Items]
Derivative Liability, Notional Amount	0	100,000,000
Derivative Liabilities, Estimated Fair Value	$ 0	$ 0

Derivatives and Hedging Activ87

Derivatives and Hedging Activities - Narrative (Details) - USD ($)	Sep. 06, 2017	Dec. 31, 2017	Dec. 31, 2016	Sep. 22, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Payment to terminate existing interest rate swap contract	$ 1,100,000
Derivative liability, notional amount		$ 433,000,000	$ 153,600,000	$ 100,000,000
Derivative, fixed interest rate				1.94%
Hedge ineffectiveness		$ 0	$ 0

Derivatives and Hedging Activ88

Derivatives and Hedging Activities - Schedule of Master Netting Arrangements (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial Assets
Gross Amounts Recognized	$ 8,800	$ 2,700
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	8,800	2,700
Financial Instruments	2,400	500
Fair Value of Financial Collateral in the Balance Sheet	0	0
Net Amount	6,400	2,200
Offsetting Derivative Liabilities [Abstract]
Gross Amounts Recognized	7,900	1,300
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	7,900	1,300
Financial Instruments	2,400	500
Fair Value of Financial Collateral in the Balance Sheet	3,300	0
Net Amount	2,200	800
Repurchase agreements
Gross Amounts Recognized	643,000	537,600
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	642,961	537,556
Financial Instruments	0	0
Fair Value of Financial Collateral in the Balance Sheet	643,000	537,600
Net Amount	0	0
Offsetting Derivative Liability, Securities Sold under Agreements to Repurchase, Securities Loaned [Abstract]
Gross Amounts Recognized	650,900	538,900
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	650,900	538,900
Financial Instruments	2,400	500
Fair Value of Financial Collateral in the Balance Sheet	646,300	537,600
Net Amount	2,200	800
Interest rate swap contracts
Financial Assets
Gross Amounts Recognized	7,500	1,300
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	7,500	1,300
Financial Instruments	2,400	500
Fair Value of Financial Collateral in the Balance Sheet	0	0
Net Amount	5,100	800
Offsetting Derivative Liabilities [Abstract]
Gross Amounts Recognized	7,800	1,300
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	7,800	1,300
Financial Instruments	2,400	500
Fair Value of Financial Collateral in the Balance Sheet	3,300	0
Net Amount	2,100	800
Mortgage related derivatives
Financial Assets
Gross Amounts Recognized	1,300	1,400
Gross Amounts Offset in the Balance Sheet	0	0
Net Amounts in the Balance Sheet	1,300	1,400
Financial Instruments	0	0
Fair Value of Financial Collateral in the Balance Sheet	0	0
Net Amount	1,300	$ 1,400
Offsetting Derivative Liabilities [Abstract]
Gross Amounts Recognized	100
Gross Amounts Offset in the Balance Sheet	0
Net Amounts in the Balance Sheet	100
Financial Instruments	0
Fair Value of Financial Collateral in the Balance Sheet	0
Net Amount	$ 100

Derivatives and Hedging Activ89

Derivatives and Hedging Activities - Pre-tax Gains and Losses on Derivative Contracts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Designated as Hedging Instrument
Derivative [Line Items]
Amount of loss recognized in other comprehensive income (effective portion)	$ (1.1)	$ (0.2)	$ 0.2
Reclassification adjustment for derivative net (gains) losses included in income	1.1	0	0
Not Designated as Hedging Instrument | Interest Rate Swap
Derivative [Line Items]
Amount of gain (loss) recognized in other non-interest income	0	0.1	0
Amount of net fee income recognized in other non-interest income	0.8	0.9	0.1
Amount of net gains (losses) recognized in mortgage banking revenues	$ (1.7)	$ 1.4	$ 0

Mortgage Servicing Rights - Sc

Mortgage Servicing Rights - Schedule of Mortgage Servicing Rights (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Servicing Asset at Amortized Value, Balance [Roll Forward]
Balance at end of year, net of valuation reserve	$ 24,770	$ 18,457
Mortgage loans
Servicing Asset at Amortized Value, Balance [Roll Forward]
Servicing asset at amortized cost, before valuation reserve, beginning	18,700	15,900	$ 14,400
Acquisitions of mortgage servicing rights	3,500	0	300
Originations of mortgage servicing rights	5,600	5,800	3,600
Amortization expense	(3,000)	(3,000)	(2,400)
Servicing asset at amortized cost, before valuation reserve, ending	24,800	18,700	15,900
Less valuation reserve	0	(200)	(200)
Balance at end of year, net of valuation reserve	24,800	18,500	15,700
Principal balance of serviced loans underlying mortgage servicing rights	$ 3,636,700	$ 3,127,500	$ 2,906,200
Mortgage servicing rights as a percentage of serviced loans	0.0068	0.0059	0.0054

Mortgage Servicing Rights - Na

Mortgage Servicing Rights - Narrative (Details) - Mortgage loans - USD ($)	12 Months Ended
	Dec. 31, 2017	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	$ 40,100,000	$ 35,700,000
Servicing asset at fair value, assumptions used to estimate fair value, weighted average life	7 years 3 months 17 days	8 years
Impairment reversals	$ 90,000	$ 40,000	$ 100,000
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	9.50%	9.60%
Servicing assets at fair value, assumptions used to estimate fair value, prepayment speed	0.50%	0.50%
Maximum
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	11.30%	11.40%
Servicing assets at fair value, assumptions used to estimate fair value, prepayment speed	1.80%	1.50%

Deposits (Details)

Deposits (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deposits, by Type [Abstract]
Non-interest bearing demand	$ 2,900,042,000	$ 1,906,257,000
Interest bearing:
Demand	2,787,500,000	2,276,500,000
Savings	3,095,400,000	2,141,800,000
Time, $100 and over	432,000,000	461,400,000
Time, other	720,000,000	590,100,000
Total interest bearing	7,034,829,000	5,469,853,000
Total deposits	9,934,871,000	7,376,110,000
Brokered time deposits	0	0
Time deposits obtained through Certificate of Deposit Account Registry Service (CDARS)	94,200,000	25,700,000
Time deposits of $250,000 or more	182,000,000	208,500,000
FDIC deposit insurance limit	250,000
Time, $100 and Over
Due within 3 months or less	88,600,000
Due after 3 months and within 6 months	65,300,000
Due after 6 months and within 12 months	110,600,000
Due after 12 months	167,500,000
Total	432,000,000	461,400,000
Total Time
Due within 3 months or less	313,800,000
Due after 3 months and within 6 months	167,300,000
Due after 6 months and within 12 months	259,200,000
Due after 12 months	411,600,000
Total	1,151,900,000
Interest expense, time deposits, $100,000 or more	$ 4,300,000	$ 3,700,000	$ 3,700,000

Long-Term Debt and Other Borr93

Long-Term Debt and Other Borrowed Funds (Details) - USD ($)	Jan. 08, 2018	Jan. 29, 2015	Jan. 10, 2008	Dec. 31, 2017	Nov. 16, 2017	Dec. 31, 2016	Dec. 31, 2014
Debt Instrument
Long-term debt and capital lease obligations				$ 13,100,000		$ 28,000,000
Fiscal Year Maturities of Long-term Debt
2,018				100,000
2,019				100,000
2,020				100,000
2,021				100,000
2,022				5,100,000
Thereafter				7,600,000
Long-term debt and capital lease obligations				13,100,000		28,000,000
Advances from FHLB				0		0
Other borrowings				20,009,000		6,000
Parent Company
Debt Instrument
Long-term debt				0		$ 20,000,000
Subordinated term loan | Subordinated Term Loan, 6.81% | Parent Company
Debt Instrument
Stated interest rate						6.81%
Long-term debt				$ 0		$ 20,000,000
Fiscal Year Maturities of Long-term Debt
Repayments of Unsecured Debt			$ 20,000,000
Subordinated term loan | Note Payable, 6.24% | Parent Company
Debt Instrument
Stated interest rate							6.24%
Capital lease obligations | Capital Lease Obligation, 8.00% | Subsidiaries
Debt Instrument
Stated interest rate				8.00%
Capital Lease Obligations				$ 1,400,000		1,500,000
Notes payable | Note Payable, 6.24% | Subsidiaries
Debt Instrument
Stated interest rate				6.24%
Long-term debt				$ 1,600,000		1,500,000
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%	1.00%
Long-term debt				$ 5,100,000	$ 5,100,000	$ 0
Federal Home Loan Bank | Notes Payable to FHLB | Subsidiaries
Fiscal Year Maturities of Long-term Debt
FHLB, advances, maximum amount available				$ 1,313,300,000
Secured Debt
Debt Instrument
Stated interest rate		2.28%
Fiscal Year Maturities of Long-term Debt
Ownership percentage in subsidiary				99.90%
Repayments of Unsecured Debt		$ 5,000,000
Federal Funds Purchased
Fiscal Year Maturities of Long-term Debt
Line of credit facility, maximum borrowing capacity				$ 185,000,000
Federal Reserve Bank Advances
Fiscal Year Maturities of Long-term Debt
Line of credit facility, maximum borrowing capacity				$ 464,800,000
Subsequent Event | Subordinated term loan | Subordinated Term Loan, 6.81%
Debt Instrument
Stated interest rate	6.81%
Fiscal Year Maturities of Long-term Debt
Repayments of Unsecured Debt	$ 20,000,000
Subsequent Event | Subordinated term loan | Subordinated Term Loan, 6.81% | Parent Company
Debt Instrument
Stated interest rate	6.81%
Fiscal Year Maturities of Long-term Debt
Repayments of Unsecured Debt	$ 20,000,000

Subordinated Debentures Held 94

Subordinated Debentures Held by Subsidiary Trusts (Details) $ in Thousands	1 Months Ended				12 Months Ended
	Jan. 31, 2008USD ($)	Dec. 31, 2007USD ($)	Nov. 30, 2007USD ($)	Oct. 31, 2007USD ($)	Dec. 31, 2017USD ($)trusts	Dec. 31, 2016USD ($)
Subordinated Borrowing
Number of company sponsored wholly-owned business trusts | trusts					6
Subordinated debentures held by subsidiary trusts					$ 82,477	$ 82,477
First Interstate Statutory Trust II | Junior Subordinated Deferrable Interest Debentures Issued by FIST II
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts				$ 10,300	$ 10,300	10,300
Debt instrument, interest rate at period end					3.94%
First Interstate Statutory Trust II | Junior Subordinated Deferrable Interest Debentures Issued by FIST II | LIBOR
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			$ 15,500		$ 15,500	15,500
Debt instrument, interest rate at period end					4.34%
First Interstate Statutory Trust I | Junior Subordinated Deferrable Interest Debentures Issued by FIST I | Debt Instrument, Redemption, At Issuance through Year Five
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		$ 20,600			$ 20,600	20,600
Debt instrument, interest rate at period end					3.99%
First Interstate Statutory Trust III | Junior Subordinated Deferrable Interest Debentures Issued by FIST III | Debt Instrument, Redemption, At Issuance through Year Five
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		$ 15,500			$ 15,500	15,500
Debt instrument, interest rate at period end					4.39%
First Interstate Statutory Trust IV | Junior Subordinated Deferrable Interest Debentures Issued by FIST IV | LIBOR
Subordinated Borrowing
Debt instrument, basis spread on variable rate		2.70%
First Interstate Statutory Trust V | Junior Subordinated Deferrable Interest Debentures Issued by FIST V
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts	$ 10,300				$ 10,300	10,300
Debt instrument, interest rate at period end					4.44%
First Interstate Statutory Trust V | Junior Subordinated Deferrable Interest Debentures Issued by FIST V | Debt Instrument, Redemption, At Issuance through Year Five
Subordinated Borrowing
Stated interest rate	6.78%
First Interstate Statutory Trust V | Junior Subordinated Deferrable Interest Debentures Issued by FIST V | LIBOR | Debt Instrument, Redemption, After Period Five
Subordinated Borrowing
Debt instrument, basis spread on variable rate	2.75%
First Interstate Statutory Trust VI | Junior Subordinated Deferrable Interest Debentures Issued by FIST VI
Subordinated Borrowing
Subordinated debentures held by subsidiary trusts	$ 10,300				$ 10,300	$ 10,300
Debt instrument, interest rate at period end					4.44%
First Interstate Statutory Trust VI | Junior Subordinated Deferrable Interest Debentures Issued by FIST VI | LIBOR
Subordinated Borrowing
Debt instrument, basis spread on variable rate	2.75%
Trust Preferred Securities Subject to Mandatory Redemption | Trusts
Subordinated Borrowing
Amount of financial instruments subject to mandatory redemption					$ 80,000
Business trust term (in years)					30 years
Common Stock Subject to Mandatory Redemption | Trusts
Subordinated Borrowing
Amount of financial instruments subject to mandatory redemption					$ 2,500
Subordinated Debentures Subject to Mandatory Redemption | Trusts
Subordinated Borrowing
Business trust term (in years)					30 years

Capital Stock and Dividend Re95

Capital Stock and Dividend Restrictions (Details) $ / shares in Units, $ in Thousands	May 30, 2017USD ($)shares	Dec. 31, 2017USD ($)votesshares	Dec. 31, 2016USD ($)$ / sharesshares	Dec. 31, 2015USD ($)	Sep. 25, 2017USD ($)	Jan. 26, 2017shares
Class of Stock
Common stock, shares authorized		200,000,000
Aggregate purchase price of shares repurchased and retired | $		$ 1,348	$ 26,854	$ 20,647
Common stock issued (in shares)	11,300,000
Class A Common Stock
Class of Stock
Common stock, shares authorized		100,000,000
Common stock, voting rights per share (vote per share) | votes		1
Common stock, shares, outstanding		33,560,202	21,613,885
Common shares purchased and retired		0	975,877
Aggregate purchase price of shares repurchased and retired | $			$ 25,500
Weighted average price of shares repurchased and retired (in dollars per share) | $ / shares			$ 26.16
Class B Common Stock
Class of Stock
Common stock, shares authorized		100,000,000
Common stock, voting rights per share (vote per share) | votes		5
Common stock, shares, outstanding		22,905,357	23,312,291
Cascade Bank | Class A Common Stock
Class of Stock
Common stock, shares authorized						11,839,179
Common stock issued (in shares)	11,252,750
Class A common stock | $	$ 386,000
Board of Directors
Class of Stock
Aggregate value of shares issued to directors | $		$ 500	$ 500
Board of Directors | Class A Common Stock
Class of Stock
Shares issued to directors during period		14,926	16,347
Secondary Public Offering | Class A Common Stock
Class of Stock
Common stock, value authorized | $					$ 250,000

Earnings per Common Share Compu

Earnings per Common Share Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Income (Loss) Available to Common Stockholders, Basic [Abstract]
Net income, basic and diluted	$ 106,521	$ 95,636	$ 86,795
Weighted Average Number of Shares Outstanding, Basic and Diluted [Abstract]
Weighted average common shares outstanding for basic earnings per share computation	51,429,366	44,511,774	45,184,091
Dilutive effect of stock-based compensation (in shares)	473,843	398,622	462,327
Weighted average common shares outstanding for diluted earnings per common share computation	51,903,209	44,910,396	45,646,418
Earnings Per Share, Basic and Diluted [Abstract]
Basic earnings per common share (in dollars per share)	$ 2.07	$ 2.15	$ 1.92
Diluted earnings per common share (in dollars per share)	$ 2.05	$ 2.13	$ 1.90
Restricted Stock
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)	83,635	7,215	12,890
Performance Shares
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)	113,874	155,637	142,512

Regulatory Capital - Schedule

Regulatory Capital - Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Total risk-based capital:
Actual amount	$ 1,112.5	$ 974.5
Actual ratio (percent)	12.76%	15.13%
Adequately capitalized Basel III phase-in schedule, amount	$ 806.5	$ 555.7
Adequately capitalized Basel III phase-in schedule, ratio (percent)	9.25%	8.63%
Adequately capitalized Basel III fully phased-in, amount	$ 915.5	$ 676.1
Adequately capitalized Basel III fully phased-in, ratio (percent)	10.50%	10.50%
Well capitalized, amount	$ 871.9	$ 643.9
Well capitalized, ratio (percent)	10.00%	10.00%
Tier 1 risk-based capital:
Actual amount	$ 1,040.3	$ 894.3
Actual ratio (percent)	11.93%	13.89%
Adequately capitalized Basel III phase-in schedule, amount	$ 632.1	$ 426.9
Adequately capitalized Basel III phase-in schedule, ratio (percent)	7.25%	6.63%
Adequately capitalized Basel III fully phased-in, amount	$ 741.1	$ 547.3
Adequately capitalized Basel III fully phased-in, ratio (percent)	8.50%	8.50%
Well capitalized, amount	$ 697.5	$ 515.1
Well capitalized, ratio (percent)	8.00%	8.00%
Common Equity Tier One Capital [Abstract]
Actual amount	$ 962.4	$ 814.3
Actual ratio (percent)	11.04%	12.65%
Adequately capitalized Basel III phase-in schedule, amount	$ 501.3	$ 330.3
Adequately capitalized Basel III phase-in schedule, ratio (percent)	5.75%	5.13%
Adequately capitalized Basel III fully phased-in, amount	$ 610.3	$ 450.7
Adequately capitalized Basel III fully phased-in, ratio (percent)	7.00%	7.00%
Well capitalized, amount	$ 566.7	$ 418.5
Well capitalized, ratio (percent)	6.50%	6.50%
Leverage capital ratio:
Actual amount	$ 1,040.3	$ 894.3
Actual ratio (percent)	8.86%	10.11%
Adequately capitalized, amount	$ 469.9	$ 353.8
Adequately capitalized, ratio (percent)	4.00%	4.00%
Well capitalized, amount	$ 587.4	$ 442.3
Well capitalized, ratio (percent)	5.00%	5.00%
Capital Conservation Buffer	4.76%
FIB
Total risk-based capital:
Actual amount	$ 1,066.6	$ 842
Actual ratio (percent)	12.29%	13.13%
Adequately capitalized Basel III phase-in schedule, amount	$ 802.7	$ 553.5
Adequately capitalized Basel III phase-in schedule, ratio (percent)	9.25%	8.63%
Adequately capitalized Basel III fully phased-in, amount	$ 911.2	$ 673.4
Adequately capitalized Basel III fully phased-in, ratio (percent)	10.50%	10.50%
Well capitalized, amount	$ 867.8	$ 641.3
Well capitalized, ratio (percent)	10.00%	10.00%
Tier 1 risk-based capital:
Actual amount	$ 994.4	$ 765.8
Actual ratio (percent)	11.46%	11.94%
Adequately capitalized Basel III phase-in schedule, amount	$ 629.1	$ 425.2
Adequately capitalized Basel III phase-in schedule, ratio (percent)	7.25%	6.63%
Adequately capitalized Basel III fully phased-in, amount	$ 737.6	$ 545.1
Adequately capitalized Basel III fully phased-in, ratio (percent)	8.50%	8.50%
Well capitalized, amount	$ 694.2	$ 513.1
Well capitalized, ratio (percent)	8.00%	8.00%
Common Equity Tier One Capital [Abstract]
Actual amount	$ 994.4	$ 765.8
Actual ratio (percent)	11.46%	11.94%
Adequately capitalized Basel III phase-in schedule, amount	$ 499	$ 329
Adequately capitalized Basel III phase-in schedule, ratio (percent)	5.75%	5.13%
Adequately capitalized Basel III fully phased-in, amount	$ 607.4	$ 448.9
Adequately capitalized Basel III fully phased-in, ratio (percent)	7.00%	7.00%
Well capitalized, amount	$ 564.1	$ 416.9
Well capitalized, ratio (percent)	6.50%	6.50%
Leverage capital ratio:
Actual amount	$ 994.4	$ 765.8
Actual ratio (percent)	8.48%	8.69%
Adequately capitalized, amount	$ 469.1	$ 352.3
Adequately capitalized, ratio (percent)	4.00%	4.00%
Well capitalized, amount	$ 586.3	$ 440.4
Well capitalized, ratio (percent)	5.00%	5.00%
Capital Conservation Buffer		4.29%

Commitments and Contingencies

Commitments and Contingencies - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrecorded Unconditional Purchase Obligation
Operating leases, rent expense	$ 2.5	$ 1.9	$ 2.7
Construction Contracts
Unrecorded Unconditional Purchase Obligation
Commitments under construction contracts	$ 1.1
Related Entity
Unrecorded Unconditional Purchase Obligation
Ownership percentage in partnership	50.00%
Obligation to repurchase loans sold
Unrecorded Unconditional Purchase Obligation
Amount of sold residential mortgage loans with recourse provisions	$ 1.9	$ 2.7

Commitments and Contingencies99

Commitments and Contingencies - Operating Lease Commitments (Details) $ in Millions	Dec. 31, 2017USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2,018	$ 6
2,019	5.8
2,020	5.1
2,021	4.6
2,022	4.1
Thereafter	18.5
Operating leases, future minimum payments due	44.1
Third Parties
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2,018	3.5
2,019	3.3
2,020	2.6
2,021	2.1
2,022	1.6
Thereafter	9.4
Operating leases, future minimum payments due	22.5
Related Entity
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2,018	2.5
2,019	2.5
2,020	2.5
2,021	2.5
2,022	2.5
Thereafter	9.1
Operating leases, future minimum payments due	$ 21.6

Financial Instruments with O100

Financial Instruments with Off-Balance Sheet Risk (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Entity Information
Credit extension commitments	$ 2,179.5	$ 1,600
Unused credit card lines	775	424.5
Unused Credit Card Lines
Entity Information
Credit extension commitments	635.3	544.9
Standby Letters of Credit
Entity Information
Credit extension commitments	$ 50.5	$ 50.6

Income Taxes - Schedule of Inc

Income Taxes - Schedule of Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
Federal	$ 24,300	$ 40,300	$ 26,700
State	5,000	5,900	4,500
Total current	29,300	46,200	31,200
Deferred:
Federal	18,400	2,900	11,100
State	2,500	500	1,400
Total deferred	20,917	3,369	12,449
Total income tax expense	$ 50,201	$ 49,623	$ 43,662

Income Taxes - Reconciliation

Income Taxes - Reconciliation of Tax Expense at Effective Tax Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Expense, Continuing Operations Income Tax Reconciliation [Abstract]
Tax expense at the statutory tax rate	$ 54,900	$ 50,800	$ 45,700
Increase (decrease) in tax resulting from:
Tax-exempt income	(4,500)	(4,400)	(4,100)
State income tax, net of federal income tax benefit	4,900	4,300	3,800
Benefit of stock-based compensation plans	(2,600)	0	0
Federal tax credits	(2,400)	(2,100)	(2,300)
Benefit due to enactment of federal tax reform	(2,200)	0	0
Other, net	2,200	1,000	600
Total income tax expense	$ 50,201	$ 49,623	$ 43,662

Income Taxes - Deferred Tax As

Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Loans, principally due to allowance for loan losses	$ 18	$ 30.1
Loan discount	6.7	3.1
Investment securities, unrealized losses	5.6	6.6
Employee benefits	11.5	9.3
Non-performing loan interest	1.1	1.2
Other real estate owned write-downs and carrying costs	0.6	1.3
Tax credit carryforwards	3.7	0
Net operating loss carryforwards	8.7	0
Other	4.7	2.5
Deferred tax assets	60.6	54.1
Deferred tax liabilities:
Fixed assets, principally differences in bases and depreciation	(6.7)	(4.7)
Deferred loan costs	(1.7)	(3)
Investment in joint venture partnership, principally due to differences in depreciation of partnership assets	(0.8)	(1.2)
Prepaid amounts	(0.6)	(1.5)
Government agency stock dividends	(1.6)	(2.3)
Goodwill and core deposit intangibles	(38.1)	(42.4)
Mortgage servicing rights	(5.7)	(5.5)
Other	(1.4)	(0.3)
Deferred tax liabilities	(56.6)	(60.9)
Net deferred tax assets (liabilities)	4
Net deferred tax assets (liabilities)		$ (6.8)
Federal
Operating Loss Carryforwards [Line Items]
Federal tax credit carryforwards	3.7
Net operating loss carryforwards	$ 20.1

Income Taxes - Narrative (Deta

Income Taxes - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Effective income tax rate reconciliation, at federal statutory income tax rate	35.00%	35.00%	35.00%
Current net income tax receivable	$ 5.7	$ 5.8
Benefit due to enactment of federal tax reform	$ 2.2	$ 0	$ 0

Stock-Based Compensation - Nar

Stock-Based Compensation - Narrative (Details)	12 Months Ended
	Dec. 31, 2017USD ($)planshares	Dec. 31, 2016USD ($)shares	Dec. 31, 2015USD ($)
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	$ 3,900,000	$ 4,400,000	$ 4,000,000
Tax benefits from stock-based compensation	2,600,000	2,100,000	1,400,000
Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	0	40,000	480,000
Tax benefits from stock-based compensation	0	20,000	180,000
Intrinsic value of fully-vested stock options outstanding	15,400,000
Total intrinsic value of options exercised	6,300,000	5,600,000	3,900,000
Actual tax benefit realized for tax deduction	2,000,000	2,100,000	1,500,000
Proceeds from stock options exercised	2,400,000	4,700,000	3,400,000
Common stock redeemed for payment of stock option exercises, aggregate value	2,800,000	3,100,000	2,500,000
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense	3,300,000	3,900,000	2,900,000
Tax benefits from stock-based compensation	$ 600,000	$ 1,500,000	$ 1,100,000
Stock issued during period, shares, restricted stock award | shares	140,246
Unrecognized compensation cost related to nonvested restricted stock awards	$ 5,800,000
Unrecognized compensation cost related to nonvested restricted stock to be recognized over weighted average period (in years)	1 year 10 months 5 days
Restricted Stock | 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	35,634
Restricted Stock | 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	17,817
Restricted Stock | 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	17,817
Restricted Stock | 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	104,612
Restricted Stock | First Anniversary
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage	33.33%
Restricted Stock | Second Anniversary
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage	33.33%
Restricted Stock | Third Anniversary
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Annual vesting percentage	33.33%
2006 Plan | Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for grant | shares	1,954,046

Stock-Based Compensation - Sch

Stock-Based Compensation - Schedule of Stock Options Activity (Details)	12 Months Ended
	Dec. 31, 2017$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding options, beginning of year, number of shares | shares	940,843
Exercised, number of shares | shares	(285,887)
Forfeited, number of shares | shares	(12,701)
Outstanding options, end of year, number of shares | shares	642,255
Outstanding options exercisable, end of year, number of shares | shares	642,255
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) | $ / shares	$ 16.77
Exercised, weighted average exercise price (in dollars per share) | $ / shares	18.15
Forfeited, weighted average exercise price (in dollars per share) | $ / shares	17.86
Outstanding options, end of year, weighted average exercise price (in dollars per share) | $ / shares	16.13
Outstanding options exercisable, end of year, weighted average exercise price (in dollars per share) | $ / shares	$ 16.13
Outstanding options, end of year weighted average remaining contractual term (in years)	2 years 4 months 3 days
Outstanding options exercisable, end of year, weighted average remaining contractual term (in years)	2 years 4 months 3 days

Stock-Based Compensation - 107

Stock-Based Compensation - Schedule of Restricted Stock Activity (Details) - Restricted Stock	12 Months Ended
	Dec. 31, 2017$ / sharesshares
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 | shares	317,055
Granted, number of shares | shares	140,246
Vested, number of shares | shares	(111,224)
Forfeited, number of shares | shares	(53,571)
Restricted stock, end of year, number of shares | shares	292,506
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) | $ / shares	$ 26.22
Granted, weighted average grant date fair value (in dollars per share) | $ / shares	41.46
Vested, weighted average grant date fair value (in dollars per share) | $ / shares	26.14
Forfeited, weighted average grant date fair value (in dollars per share) | $ / shares	27.96
Restricted stock, end of year, weighted-average measurement date fair value (in dollars per share) | $ / shares	$ 33.24

Employee Benefit Plans - Profi

Employee Benefit Plans - Profit Sharing and Savings Plans (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits
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	3 years
Compensation expense	$ 1,600,000	$ 2,700,000	$ 700,000
Savings Plan, Employee Elected
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits
Compensation expense	5,500,000	$ 4,800,000	$ 4,700,000
Maximum annual contribution per employee, amount per dollar of employee contribution	$ 1.25
Maximum annual contribution per employee, percent	4.00%

Employee Benefit Plans - Postr

Employee Benefit Plans - Postretirement Healthcare Plan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure
Amortization of gain, weighted average remaining service period (in years)		4 years
Stockholders’ equity	$ 1,427,616	$ 982,593	$ 950,493	$ 908,924
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	$ 2,200	2,800
Stockholders’ equity	1,300
Reduction in accumulated post-retirement benefit obligation due to curtailment		2,800
Reduction in net periodic benefit cost due to curtailment		300
Unfunded benefit obligation	700	700
Net periodic benefit cost	$ 400	$ 200	$ 400
Assumptions used calculating net periodic benefit cost, discount rate	2.40%	2.40%
Assumptions used calculating net periodic benefit cost, rate of compensation increase	6.00%	6.00%
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
2,018	$ 130
2,019	130
2,020	100
2,021	70
2,022	50
2023 through 2027	100
Net actuarial gains	2,100
Unamortized negative prior service costs	100
Estimated amount amortized from accumulated other comprehensive loss into net periodic benefit costs	$ 800

Other Comprehensive Income - C

Other Comprehensive Income - Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Before Tax Amount
Change in net unrealized loss during period	$ (6,545)	$ (19,379)	$ 3,151
Reclassification adjustment for net gains included in net income	(718)	(330)	(137)
Change in unamortized loss on available-for-sale securities transferred into held-to-maturity	1,800	1,900	1,600
Change in net unrealized loss on derivatives	(1,100)	(200)	200
Reclassification adjustment for derivative net (gains) losses included in net income	1,132	0	0
Change in net actuarial loss (gain)	(1,200)	4,000	100
Total other comprehensive loss	(6,600)	(14,000)	4,800
Tax Expense (Benefit)
Change in net unrealized loss during period	(2,700)	(7,600)	1,200
Reclassification adjustment for net gains included in net income	(300)	(100)	(100)
Unamortized premium on available-to-sale securities transferred into held-for-maturity	700	700	600
Change in net unrealized gain on derivatives	(400)	(100)	100
Reclassification adjustment for derivative net (gains) losses included in net income	400
Change in net actuarial loss	(500)	1,600	100
Total other comprehensive income (loss)	(2,800)	(5,500)	1,900
Net of Tax Amount
Change in net unrealized loss during period	(3,800)	(11,800)	1,900
Reclassification adjustment for net gains included in net income	(400)	(200)	(100)
Change in unamortized gain on available-for-sale securities transferred into held-to-maturity	1,100	1,200	1,000
Change in net unrealized gain on derivatives	(700)	(100)	100
Reclassification adjustment for derivative net (gains) losses included in net income	700
Change in net actuarial loss	(700)	2,400	0
Other comprehensive income (loss), net of tax	$ (3,835)	$ (8,534)	$ 2,940

Other Comprehensive Income - A

Other Comprehensive Income - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Change in net unrealized loss during period	$ (6,545)	$ (19,379)	$ 3,151
Reclassification adjustment for net gains included in net income	(718)	(330)	(137)
Accumulated other comprehensive income (loss)	1,427,616	982,593	950,493	$ 908,924
Net unrealized gain (loss) on investment securities available-for-sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	(13,200)	(10,100)
Net actuarial gain (loss) on defined benefit post-retirement benefit plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	1,300	2,000
Net unrealized gain (loss) on derivatives
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	0	0
Total other comprehensive income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive income (loss)	$ (11,963)	$ (8,128)	$ 406	$ (2,534)

Condensed Financial Informat112

Condensed Financial Information (Parent Company Only) - Condensed Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed balance sheets:
Cash and cash equivalents	$ 758,986	$ 782,023
Investment in subsidiaries, at equity:
Other assets	145,747	81,705
Total assets	12,213,255	9,063,895
Subordinated debentures held by subsidiary trusts	82,477	82,477
Total liabilities	10,785,639	8,081,302
Stockholders’ equity	1,427,616	982,593	$ 950,493	$ 908,924
Total liabilities and stockholders’ equity	12,213,255	9,063,895
Parent Company
Condensed balance sheets:
Cash and cash equivalents	42,300	138,400
Investment in subsidiaries, at equity:
Equity method investments	1,432,300	932,000
Advances from subsidiaries, net	25,700	0
Other assets	61,700	35,400
Total assets	1,562,000	1,105,800
Other liabilities	51,900	18,000
Advances from subsidiaries, net	0	2,700
Long-term debt	0	20,000
Subordinated debentures held by subsidiary trusts	82,500	82,500
Total liabilities	134,400	123,200
Stockholders’ equity	1,427,600	982,600
Total liabilities and stockholders’ equity	1,562,000	1,105,800
Bank subsidiary | Parent Company
Investment in subsidiaries, at equity:
Equity method investments	$ 1,432,300	$ 932,000

Condensed Financial Informat113

Condensed Financial Information (Parent Company Only) - Condensed Statements of Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions
Salaries and benefits	$ 122,751	$ 108,684	$ 101,451
Interest expense	27,934	17,661	18,060
Acquisition related expenses	27,156	2,821	795
Earnings before income tax benefit	156,722	145,259	130,457
Income tax benefit	50,201	49,623	43,662
Net income	106,521	95,636	86,795
Parent Company
Condensed Financial Statements, Captions
Dividends from subsidiaries	150,000	140,000	70,000
Other interest income	100	0	0
Other income, primarily management fees from subsidiaries	18,000	15,100	13,200
Total income	168,100	155,100	83,200
Salaries and benefits	21,800	18,800	16,300
Interest expense	4,700	4,100	3,800
Acquisition related expenses	25,300	1,500	800
Other operating expenses, net	13,000	10,400	9,900
Total expenses	64,800	34,800	30,800
Earnings before income tax benefit	103,300	120,300	52,400
Income tax benefit	(14,200)	(7,700)	(7,000)
Income before undistributed earnings of subsidiaries	117,500	128,000	59,400
Undistributed earnings of subsidiaries	(11,000)	(32,400)	27,400
Net income	$ 106,500	$ 95,600	$ 86,800

Condensed Financial Informat114

Condensed Financial Information (Parent Company Only) - Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 106,521	$ 95,636	$ 86,795
Adjustments to reconcile net income to cash provided by operating activities:
Stock-based compensation expense	3,868	4,376	3,959
Tax benefits from stock-based compensation	0	2,146	1,443
Excess tax benefits from stock-based compensation	0	(1,566)	(1,184)
Net cash provided by operating activities	154,683	118,070	115,400
Cash flows from investing activities:
Acquisition of intangible assets	(28,013)	0	0
Acquisition of bank and bank holding company, net of cash and cash equivalents, received	91,779	18,554	(1,636)
Net cash provided by (used in) investing activities	(130,565)	(143,092)	(95,339)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of stock issuance costs	2,431	4,683	3,369
Excess tax benefits from stock-based compensation	0	1,566	1,184
Purchase and retirement of common stock	(1,348)	(26,854)	(20,647)
Dividends paid to common stockholders	(48,583)	(39,353)	(36,290)
Net cash used in financing activities	(47,155)	26,588	(38,274)
Net change in cash and cash equivalents	(23,037)	1,566	(18,213)
Cash and cash equivalents at beginning of period	782,023	780,457	798,670
Cash and cash equivalents at end of period	758,986	782,023	780,457
Parent Company
Cash flows from operating activities:
Net income	106,500	95,600	86,800
Adjustments to reconcile net income to cash provided by operating activities:
Undistributed earnings of subsidiaries	11,000	32,400	(27,400)
Stock-based compensation expense	3,900	4,400	3,900
Tax benefits from stock-based compensation	0	2,100	1,400
Excess tax benefits from stock-based compensation	0	(1,600)	(1,200)
Other, net	14,700	(800)	(11,300)
Net cash provided by operating activities	136,100	132,100	52,200
Cash flows from investing activities:
Acquisition of bank and bank holding company, net of cash and cash equivalents, received	(128,300)	0	(7,200)
Investment in subsidiary	(18,000)	0	0
Net cash provided by (used in) investing activities	(156,300)	2,000	(7,200)
Cash flows from financing activities:
Net (decrease) increase in advances from nonbank subsidiaries	(28,400)	9,100	(2,000)
Repayment of long-term debt	0	0	(1,000)
Proceeds from issuance of common stock, net of stock issuance costs	2,400	4,700	3,400
Excess tax benefits from stock-based compensation	0	1,600	1,200
Purchase and retirement of common stock	(1,300)	(26,900)	(20,600)
Dividends paid to common stockholders	(48,600)	(39,400)	(36,300)
Net cash used in financing activities	(75,900)	(50,900)	(55,300)
Net change in cash and cash equivalents	(96,100)	83,200	(10,300)
Cash and cash equivalents at beginning of period	138,400	55,200	65,500
Cash and cash equivalents at end of period	42,300	138,400	55,200
Nonbank subsidiaries | Parent Company
Cash flows from investing activities:
Capital distributions from nonbank subsidiaries	18,000	2,000	0
Acquisition of intangible assets	(28,000)	$ 0	$ 0
BOTC Acquisition
Cash flows from financing activities:
Stock Issued	$ 386,000

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) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	$ 8.8	$ 2.7
Derivative liabilities	7.9	1.3
Deferred compensation plan assets	12.2	10.6
Deferred compensation plan liabilities	12.2	10.6
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
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 Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for sale	46.6	61.8
Derivative assets		1.3
Derivative liabilities	7.8	1.3
Deferred compensation plan assets	12.2	10.6
Deferred compensation plan liabilities	12.2	10.6
Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
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
US Treasury Notes | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
US Treasury Notes | Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	3.2	3.6
US Treasury Notes | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Obligations of U.S. government agencies | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Obligations of U.S. government agencies | Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	561.5	391.3
Obligations of U.S. government agencies | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations | Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	1,462.5	1,213.7
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Private mortgage-backed securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Private mortgage-backed securities | Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	90.7	0.1
Private mortgage-backed securities | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Corporate securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0
Corporate securities | Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	87.9
Corporate securities | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0
Other Investments | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Other Investments | Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	3	3
Other Investments | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	0	0
Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	2,208.7	1,611.7
Derivative assets	8.8	2.7
Derivative liabilities	7.9	1.3
Deferred compensation plan assets	12.2	10.6
Deferred compensation plan liabilities	12.2	10.6
Estimated Fair Value | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held for sale	46.6	61.8
Deferred compensation plan assets	12.2	10.6
Deferred compensation plan liabilities	12.2	10.6
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]
Investment securities available-for-sale	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]
Investment securities available-for-sale	2,208.7	1,611.7
Derivative assets	8.8	2.7
Derivative liabilities	7.9	1.3
Deferred compensation plan assets	12.2	10.6
Deferred compensation plan liabilities	12.2	10.6
Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	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 | US Treasury Notes | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	3.2	3.6
Estimated Fair Value | Obligations of U.S. government agencies | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	561.5	391.3
Estimated Fair Value | U.S. agency residential mortgage-backed securities & collateralized mortgage obligations | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	1,462.5	1,213.7
Estimated Fair Value | Private mortgage-backed securities | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	90.7	0.1
Estimated Fair Value | Corporate securities | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	87.9
Estimated Fair Value | Other Investments | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment securities available-for-sale	3	3
Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	7.5	1.3
Derivative liabilities	7.8	1.3
Interest Rate Swap | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
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 Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	7.5
Interest Rate Swap | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
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 Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	7.5	1.3
Derivative liabilities	7.8	1.3
Interest rate lock commitments | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
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 Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	1.3	1.1
Interest rate lock commitments | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
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 Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets	1.3	1.1
Forward loan sales contracts | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		0
Derivative liabilities	0
Forward loan sales contracts | Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		0.3
Derivative liabilities	0.1
Forward loan sales contracts | Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		0
Derivative liabilities	0
Forward loan sales contracts | Estimated Fair Value | Fair Value Measured on a Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets		$ 0.3
Derivative liabilities	$ 0.1

Fair Value Measurements - Fair

Fair Value Measurements - Fair Value Inputs, Quantitative Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Impaired loans | Minimum | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value inputs, weighted average range (as a percent)	0.00%	0.00%
Impaired loans | Maximum | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value inputs, weighted average range (as a percent)	78.11%	66.00%
Impaired loans | Weighted Average | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value inputs, weighted average range (as a percent)	26.10%	31.00%
Other real estate owned | Minimum | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value inputs, weighted average range (as a percent)	8.00%	8.00%
Other real estate owned | Maximum | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value inputs, weighted average range (as a percent)	96.00%	96.00%
Other real estate owned | Weighted Average | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value inputs, weighted average range (as a percent)	11.55%	18.00%
Long-lived assets to be disposed of by sale | Minimum | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value inputs, weighted average range (as a percent)	0.00%	0.00%
Long-lived assets to be disposed of by sale | Maximum | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value inputs, weighted average range (as a percent)	0.00%	9.00%
Long-lived assets to be disposed of by sale | Weighted Average | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair value inputs, weighted average range (as a percent)	0.00%	6.00%
Fair Value Measured on a Non-recurring Basis
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Impaired loans	$ 22.2	$ (25.8)
Other real estate owned	(1.6)	(1.7)
Long-lived assets to be disposed of by sale	0	(1)
Estimated Fair Value | Fair Value Measured on a Non-recurring Basis
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Impaired loans	32.6	39.3
Other real estate owned	1.3	2.1
Long-lived assets to be disposed of by sale	0.8	1.3
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value Measured on a Non-recurring Basis
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Impaired loans	0	0
Other real estate owned	0	0
Long-lived assets to be disposed of by sale	0	0
Significant Other Observable Inputs (Level 2) | Fair Value Measured on a Non-recurring Basis
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Impaired loans	0	0
Other real estate owned	0	0
Long-lived assets to be disposed of by sale	0	0
Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Impaired loans	32.6	39.3
Other real estate owned	1.3	2.1
Long-lived assets to be disposed of by sale	0.8	1.3
Significant Unobservable Inputs (Level 3) | Estimated Fair Value | Fair Value Measured on a Non-recurring Basis
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Impaired loans	$ 32.6	$ 39.3

Fair Value Measurements - Narr

Fair Value Measurements - Narrative (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Related allowance	$ 10,800,000	$ 14,000,000	$ 11,600,000
Fair Value Measured on a Non-recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	22,200,000	(25,800,000)
Carrying value of assets to be disposed of	800,000	2,400,000
Write down of assets to be disposed of	0	1,100,000
Fair value of assets to be disposed of	800,000	1,300,000
Fair Value Measured on a Non-recurring Basis | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	32,600,000	39,300,000
Related allowance	10,700,000	14,000,000
Partial loan charge-off	11,400,000	11,900,000
Carrying Amount | Fair Value Measured on a Non-recurring Basis | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	54,700,000	65,200,000
Estimated Fair Value | Fair Value Measured on a Non-recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	32,600,000	39,300,000
Estimated Fair Value | Fair Value Measured on a Non-recurring Basis | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	$ 32,600,000	$ 39,300,000

Fair Value Measurements - S118

Fair Value Measurements - Schedule of Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial assets:
Held-to-maturity investment securities, estimated fair value	$ 483,349	$ 513,273
Derivative assets	8,800	2,700
Deferred compensation plan assets	12,200	10,600
Financial liabilities:
Derivative liabilities	7,900	1,300
Deferred compensation plan liabilities	12,200	10,600
Carrying Amount
Financial assets:
Cash and cash equivalents	759,000	782,000
Investment securities available-for-sale	2,208,700	1,611,700
Held-to-maturity investment securities, estimated fair value	484,500	512,800
Accrued interest receivable	38,000	29,900
Mortgage servicing rights, net	24,800	18,500
Net loans	7,542,200	5,402,300
Derivative assets	8,800	2,700
Deferred compensation plan assets	12,200	10,600
Total financial assets	11,078,200	8,370,500
Financial liabilities:
Total deposits, excluding time deposits	8,783,000	6,324,500
Time deposits	1,151,900	1,051,600
Securities sold under repurchase agreements	643,000	537,600
Other borrowed funds	20,000	0
Accrued interest payable	5,600	5,400
Long-term debt	13,100	28,000
Subordinated debentures held by subsidiary trusts	82,500	82,500
Derivative liabilities	7,900	1,300
Deferred compensation plan liabilities	12,200	10,600
Total financial liabilities	10,719,200	8,041,500
Estimated Fair Value
Financial assets:
Cash and cash equivalents	759,000	782,000
Investment securities available-for-sale	2,208,700	1,611,700
Held-to-maturity investment securities, estimated fair value	483,300	513,300
Accrued interest receivable	38,000	29,900
Mortgage servicing rights, net	40,100	35,700
Net loans	7,298,800	5,309,900
Derivative assets	8,800	2,700
Deferred compensation plan assets	12,200	10,600
Total financial assets	10,848,900	8,295,800
Financial liabilities:
Total deposits, excluding time deposits	8,783,000	6,324,500
Time deposits	1,137,900	1,044,700
Securities sold under repurchase agreements	643,000	537,600
Other borrowed funds	20,000	0
Accrued interest payable	5,600	5,400
Long-term debt	11,300	27,500
Subordinated debentures held by subsidiary trusts	76,700	73,600
Derivative liabilities	7,900	1,300
Deferred compensation plan liabilities	12,200	10,600
Total financial liabilities	10,697,600	8,025,200
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	759,000	782,000
Investment securities available-for-sale	0	0
Held-to-maturity investment securities, estimated fair value	0	0
Accrued interest receivable	0	0
Mortgage servicing rights, net	0	0
Net loans	0	0
Derivative assets	0	0
Deferred compensation plan assets	0	0
Total financial assets	759,000	782,000
Financial liabilities:
Total deposits, excluding time deposits	8,783,000	6,324,500
Time deposits	0	0
Securities sold under repurchase agreements	0	0
Other borrowed funds	0	0
Accrued interest payable	0	0
Long-term debt	0	0
Subordinated debentures held by subsidiary trusts	0	0
Derivative liabilities	0	0
Deferred compensation plan liabilities	0	0
Total financial liabilities	8,783,000	6,324,500
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0	0
Investment securities available-for-sale	2,208,700	1,611,700
Held-to-maturity investment securities, estimated fair value	483,300	513,300
Accrued interest receivable	38,000	29,900
Mortgage servicing rights, net	40,100	35,700
Net loans	7,266,200	5,270,600
Derivative assets	8,800	2,700
Deferred compensation plan assets	12,200	10,600
Total financial assets	10,057,300	7,474,500
Financial liabilities:
Total deposits, excluding time deposits	0	0
Time deposits	1,137,900	1,044,700
Securities sold under repurchase agreements	643,000	537,600
Other borrowed funds	20,000	0
Accrued interest payable	5,600	5,400
Long-term debt	11,300	27,500
Subordinated debentures held by subsidiary trusts	76,700	73,600
Derivative liabilities	7,900	1,300
Deferred compensation plan liabilities	12,200	10,600
Total financial liabilities	1,914,600	1,700,700
Significant Unobservable Inputs (Level 3) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0	0
Investment securities available-for-sale	0	0
Held-to-maturity investment securities, estimated fair value	0	0
Accrued interest receivable	0	0
Mortgage servicing rights, net	0	0
Net loans	32,600	39,300
Derivative assets	0	0
Deferred compensation plan assets	0	0
Total financial assets	32,600	39,300
Financial liabilities:
Total deposits, excluding time deposits	0	0
Time deposits	0	0
Securities sold under repurchase agreements	0	0
Other borrowed funds	0	0
Accrued interest payable	0	0
Long-term debt	0	0
Subordinated debentures held by subsidiary trusts	0	0
Derivative liabilities	0	0
Deferred compensation plan liabilities	0	0
Total financial liabilities	$ 0	$ 0

Related Party Transactions (Det

Related Party Transactions (Details)	12 Months Ended
	Dec. 31, 2017USD ($)director	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Percentage of aircraft owned	66.00%
Number of directors | director	5
Education, Communication, Strategic Enterprise Planning and Corporate Governance Consultation
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Aggregate ownership interest of chairman and board of directors	15.00%
Shareholder
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Ownership percentage threshold for related party	5.00%
Executive Officers, Directors, Shareholders Greater than Five Percent, and Related Entities and Individuals to Such Persons
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Loans and leases receivable, related parties	$ 54,600,000	$ 53,300,000
New loans and advances on existing loans	15,000,000
Loan repayments	15,700,000
Amount of loans removed due to changes in related parties	2,000,000
Board of Directors Chairman, Wholly-Owned Entity | Aircraft Usage and Related Activities
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Related party transaction, expenses from transactions with related party	0	108,000	$ 332,000
Related party transactions, other revenues	0	53,000	64,000
Board of Directors Chairman | Airplane Usage by Company
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Related party transaction, expenses from transactions with related party	45,000	121,000	28,000
Related party transactions, other revenues	17,000	19,000	23,000
One director and one greater than 10% shareholder | Airplane Usage by Company
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Related party transaction, expenses from transactions with related party	17,000	175,000
One director and one greater than 10% shareholder | Payment Guarantee
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Loans and leases receivable, related parties	1,900,000	2,000,000
Entity Majority Owned by Shareholders and Directors | Education, Communication, Strategic Enterprise Planning and Corporate Governance Consultation
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Related party transaction, expenses from transactions with related party	$ 73,000	$ 100,000	$ 210,000

Recent Authoritative Accounting

Recent Authoritative Accounting Guidance - New Accounting Pronouncements and Changes in Accounting Principles (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reduction in income tax expense	$ 2.6	$ 0	$ 0
Stock Option Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reduction in income tax expense	2
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reduction in income tax expense	0.6
Accounting Standards Update 2018-02 | Adjustments for New Accounting Principle, Early Adoption
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stranded amounts resulting from remeasurement of deferred tax assets and liabilities	$ 3.1

Subsequent Events Subsequent Ev

Subsequent Events Subsequent Events (Details) - Subsequent Event - USD ($) $ / shares in Units, $ in Millions	Jan. 08, 2018	Jan. 30, 2018
Class A Common Stock
Subsequent Event [Line Items]
Dividends (in dollars per share)		$ 0.28
Subordinated term loan | Subordinated Term Loan, 6.81%
Subsequent Event [Line Items]
Redemption of unsecured subordinated term loan	$ 20
Stated interest rate	6.81%