First Interstate BancSystem (FIBK) 10-K 2 Mar 15 2014 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2014	Jun. 30, 2014	Jan. 31, 2015
Entity Registrant Name	FIRST INTERSTATE BANCSYSTEM INC
Entity Central Index Key	860413
Current Fiscal Year End Date	-19
Entity Filer Category	Accelerated Filer
Document Type	10-K
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
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		$614,072,224
Class A Common Stock
Entity Common Stock, Shares Outstanding			21,947,201
Class B Common Stock
Entity Common Stock, Shares Outstanding			23,859,483

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Assets
Cash and due from banks	$147,894	$141,663
Federal funds sold	543	672
Interest bearing deposits in banks	650,233	392,492
Total cash and cash equivalents	798,670	534,827
Investment securities:
Available-for-sale	1,711,924	1,947,706
Held-to-maturity (estimated fair values of $584,533 and $205,926 at December 31, 2014 and 2013, respectively)	575,186	203,837
Total investment securities	2,287,110	2,151,543
Loans held for investment	4,856,615	4,303,992
Mortgage loans held for sale	40,828	40,861
Total loans	4,897,443	4,344,853
Less allowance for loan losses	74,200	85,339
Net loans	4,823,243	4,259,514
Premises and equipment, net of accumulated depreciation	195,212	179,690
Goodwill	205,574	183,673
Company-owned life insurance	153,821	122,175
Other real estate owned (“OREO”)	13,554	15,504
Accrued interest receivable	27,063	26,450
Mortgage servicing rights, net of accumulated amortization and impairment reserve	14,038	13,546
Deferred tax asset, net	4,874	12,154
Core deposit intangibles, net of accumulated amortization	13,282	4,519
Other assets	73,495	61,056
Total assets	8,609,936	7,564,651
Deposits:
Non-interest bearing	1,791,364	1,491,683
Interest bearing	5,214,848	4,642,067
Total deposits	7,006,212	6,133,750
Securities sold under repurchase agreements	502,250	457,437
Accounts payable and accrued expenses	66,164	47,523
Accrued interest payable	5,833	4,963
Long-term debt	38,067	36,917
Other borrowed funds	9	3
Subordinated debentures held by subsidiary trusts	82,477	82,477
Total liabilities	7,701,012	6,763,070
Stockholders’ equity:
Nonvoting noncumulative preferred stock without par value; authorized 100,000 shares; no shares issued or outstanding as of December 31, 2014 and 2013	0	0
Common stock	323,596	285,535
Retained earnings	587,862	532,087
Accumulated other comprehensive loss, net	-2,534	-16,041
Total stockholders’ equity	908,924	801,581
Total liabilities and stockholders’ equity	$8,609,936	$7,564,651

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, except Share data, unless otherwise specified
Assets:
Held-to-maturity investment securities, estimated fair value	$584,533	$205,926
Stockholders' Equity:
Nonvoting, noncumulative preferred stock, par value	$0	$0
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_Inc

Consolidated Statements of Income (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Interest income:
Interest and fees on loans	$231,469	$220,687	$230,882
Interest and dividends on investment securities:
Taxable	29,900	31,237	36,847
Exempt from federal taxes	4,357	4,728	4,923
Interest on deposits in banks	1,334	992	1,235
Interest on federal funds sold	7	18	13
Total interest income	267,067	257,662	273,900
Interest expense:
Interest on deposits	13,779	15,800	22,306
Interest on securities sold under repurchase agreements	237	294	579
Interest on long-term debt	2,016	1,936	1,981
Interest in preferred stock pending redemption	0	159	131
Interest on subordinated debentures held by subsidiary trusts	2,574	2,506	5,117
Total interest expense	18,606	20,695	30,114
Net interest income	248,461	236,967	243,786
Provision for loan losses	-6,622	-6,125	40,750
Net interest income after provision for loan losses	255,083	243,092	203,036
Non-interest income:
Other service charges, commissions and fees	40,742	35,977	34,226
Income from the origination and sale of loans	23,940	34,254	41,790
Wealth management revenues	18,996	17,085	14,314
Service charges on deposit accounts	16,567	16,837	17,412
Investment securities gains, net	61	1	348
Other income	11,095	7,525	6,771
Total non-interest income	111,401	111,679	114,861
Non-interest expense:
Salaries and wages	96,513	94,002	89,833
Employee benefits	30,130	30,338	29,345
Occupancy, net	17,796	16,587	15,786
Furniture and equipment	13,816	12,554	12,859
Outsourced technology services	9,423	9,029	8,826
FDIC insurance premiums	4,608	5,057	6,470
Professional fees	4,882	4,773	4,044
OREO expense, net of income	-272	2,291	9,400
Mortgage servicing rights amortization	2,361	2,787	3,501
Mortgage servicing rights impairment recovery	-136	-99	-771
Core deposit intangibles amortization	2,251	1,418	1,420
Other expenses	47,480	43,332	45,922
Loss Contingency Expense	4,000	0	3,000
Acquisition Expense	4,017	0	0
Total non-interest expense	236,869	222,069	229,635
Income before income tax expense	129,615	132,702	88,262
Income tax expense	45,214	46,566	30,038
Net income	84,401	86,136	58,224
Preferred stock dividends	0	0	3,300
Net income available to common shareholders	$84,401	$86,136	$54,924
Basic earnings per common share	$1.89	$1.98	$1.28
Diluted earnings per common share	$1.87	$1.96	$1.27

Consolidated_Statements_of_Com

Consolidated Statements of Comprehensive Income (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$84,401	$86,136	$58,224
Investment securities available-for-sale:
Change in net unrealized gains (losses) during the period	21,147	-52,949	-4,648
Reclassification adjustment for net gains included in income	-61	-1	-348
Change in unamortized gain (loss) on available-for-sale investment securities transferred into held-to-maturity	-548	0	56
Defined benefit post-retirement benefits plans:
Change in net actuarial loss	1,731	137	-77
Other comprehensive income (loss), before tax	22,269	-52,813	-5,017
Deferred tax benefit (expense) related to other comprehensive income (loss)	-8,762	20,781	1,974
Other comprehensive income (loss), net of tax	13,507	-32,032	-3,043
Total comprehensive income	$97,908	$54,104	$55,181

Consolidated_Statements_of_Sto

Consolidated Statements of Stockholders' Equity (USD $)	Total	Preferred Stock	Common Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)
In Thousands, unless otherwise specified
Beginning Balance at Dec. 31, 2011	$771,020	$50,000	$266,842	$435,144	$19,034
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	58,224	0	0	58,224	0
Other comprehensive loss, net of tax	-3,043	0	0	0	-3,043
Common stock transactions:
Common shares purchased and retired	263	0	-263	0	0
Common shares issued	299	0	299	0	0
Non-vested common shares issued	0	0	0	0	0
Non-vested common shares forfeited or canceled	0	0	0	0	0
Stock options exercised, net of shares tendered in payment of option price and income tax withholding amounts	1,612	0	1,612	0	0
Tax benefit of stock-based compensation	360	0	360	0	0
Stock-based compensation expense	2,485	0	2,485	0	0
Stock Redeemed or Called During Period, Value	-50,000	-50,000	0	0	0
Cash dividends declared:
Common (per share)	-26,208	0	0	-26,208	0
Preferred (per share)	-3,300	0	0	-3,300	0
Ending Balance at Dec. 31, 2012	751,186	0	271,335	463,860	15,991
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	86,136	0	0	86,136	0
Other comprehensive loss, net of tax	-32,032	0	0	0	-32,032
Common stock transactions:
Common shares purchased and retired	-448	0	-448	0	0
Common shares issued	543	0	543	0	0
Non-vested common shares issued	0	0	0	0	0
Non-vested common shares forfeited or canceled	0	0	0	0	0
Stock options exercised, net of shares tendered in payment of option price and income tax withholding amounts	9,271	0	9,271	0	0
Tax benefit of stock-based compensation	1,898	0	1,898	0	0
Stock-based compensation expense	2,936	0	2,936	0	0
Cash dividends declared:
Common (per share)	-17,909	0	0	-17,909	0
Ending Balance at Dec. 31, 2013	801,581	0	285,535	532,087	-16,041
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	84,401	0	0	84,401	0
Other comprehensive loss, net of tax	13,507	0	0	0	13,507
Common stock transactions:
Common shares purchased and retired	-9,739	0	-9,739	0	0
Common shares issued	36,294	0	36,294	0	0
Non-vested common shares issued	0	0	0	0	0
Non-vested common shares forfeited or canceled	0	0	0	0	0
Stock options exercised, net of shares tendered in payment of option price and income tax withholding amounts	6,299	0	6,299	0	0
Tax benefit of stock-based compensation	2,193	0	2,193	0	0
Stock-based compensation expense	3,014	0	3,014	0	0
Cash dividends declared:
Common (per share)	-28,626	0		-28,626	0
Ending Balance at Dec. 31, 2014	$908,924	$0	$323,596	$587,862	($2,534)

Consolidated_Statements_of_Sto1

Consolidated Statements of Stockholders' Equity (Parenthetical) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Common stock transactions:
Stock options exercised	739,290
Common stock
Common stock transactions:
Common shares purchased and retired	388,101	25,677	18,351
Common shares issued	1,402,811	26,096	23,991
Non-vested common shares issued	148,278	120,873	122,912
Non-vested common shares forfeited	29,261	30,648	15,232
Stock options exercised	499,625	774,096	192,829
Shares tendered in payment of option price and income tax withholding amounts	239,665	392,411	183,805
Preferred Stock Including Additional Paid in Capital [Member]
Common stock transactions:
Stock Redeemed or Called During Period, Shares	0	0	5,000
Retained Earnings
Cash dividends declared:
Common (in dollars per share)		0.61	0.45
Preferred (in dollars per share)	0.00%	6.75%	6.75%

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities:
Net income	$84,401	$86,136	$58,224
Adjustments to reconcile net income from operations to net cash provided by operating activities:
Provision for loan losses	6,622	6,125	-40,750
Net gain on disposal of property and equipment	-1,256	-217	-424
Depreciation and amortization	16,855	16,245	17,112
Net premium amortization on investment securities	14,690	14,655	11,700
Net gain on investment securities transactions	-61	-1	-348
Net gain on sale of mortgage loans held for sale	-17,475	-24,482	-29,606
Net gain on sale of OREO	-1,849	-3,232	-1,041
Write-down of OREO and other assets pending disposal	326	3,512	6,724
Mortgage servicing rights impairment recovery	-136	-99	-771
Deferred income tax expense	5,345	11,276	8,762
Net increase in cash surrender value of company-owned life insurance policies	-3,600	-446	-1,849
Stock-based compensation expense	3,014	2,936	2,485
Tax benefits from stock-based compensation	2,193	1,898	360
Excess tax benefits from stock-based compensation	-2,205	-2,031	-273
Originations of loans held for sale	-903,373	-1,557,288	-1,197,744
Proceeds from Sale of Mortgage Loans Held-for-sale	923,350	1,603,770	1,209,866
Changes in operating assets and liabilities:
Decrease in accrued interest receivable	470	2,419	3,105
Decrease (increase) in other assets	-6,956	3,440	4,498
Increase (decrease) in accrued interest payable	286	-1,539	-1,621
Increase (decrease) in accounts payable and accrued expenses	13,991	-602	5,913
Net cash provided by operating activities	121,388	150,225	135,822
Purchases of investment securities:
Held-to-maturity	-21,627	-16,370	-68,305
Available-for-sale	-664,821	-741,579	-1,246,068
Proceeds from maturities, paydowns, calls and sales of investment securities:
Held-to-maturity	47,784	19,465	12,192
Available-for-sale	613,930	722,447	1,252,266
Payments to Acquire Life Insurance Policies	-15,000	-45,000	0
Proceeds from sales of mortgage servicing rights	266	470	907
Extensions of credit to customers, net of repayments	-216,730	-178,580	-128,919
Recoveries of loans charged-off	9,478	11,466	8,116
Proceeds from sales of OREO	12,381	28,397	42,814
Capital contribution to equity method investment	0	0	-900
Capital distribution from unconsolidated subsidiary	0	0	1,238
Payments to Acquire Businesses, Net of Cash Acquired	35,556	0	0
Capital expenditures, net of proceeds from sales	2,941	-5,653	-14,420
Net cash used in investing activities	-195,842	-204,937	-141,079
Cash flows from financing activities:
Net increase (decrease) in deposits	357,083	-106,661	413,440
Net increase (decrease) in repurchase agreements	43,892	-48,348	-10,458
Net increase (decrease) in short-term borrowings	-12,720	-29	25
Proceeds from Issuance of Unsecured Debt	68	0	0
Repayments of long-term debt	-48	-243	-40
Payments for Repurchase of Preferred Stock and Preference Stock	0	-50,000	0
Repayment of junior subordinated debentures held by subsidiary trusts	-20,439	0	-41,238
Proceeds from issuance of common stock	6,919	9,814	1,911
Payments of Stock Issuance Costs	298	0	0
Excess tax benefits from stock-based compensation	2,205	2,031	273
Purchase and retirement of common stock	-9,739	-448	-263
Dividends paid to common stockholders	-28,626	-17,909	-26,208
Dividends paid to preferred stockholders	0	0	-3,300
Net cash used in financing activities	338,297	-211,793	334,142
Net increase (decrease) in cash and cash equivalents	263,843	-266,505	328,885
Cash and cash equivalents at beginning of year	534,827	801,332	472,447
Cash and cash equivalents at end of year	798,670	534,827	801,332
Supplemental disclosures of cash flow information:
Cash paid during the year for income taxes	26,650	39,879	17,540
Cash paid during the year for interest expense	$17,736	$22,234	$31,735

Summary_of_Significant_Account

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2014
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 Montana, Wyoming and western South Dakota. 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, 2014, 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 2013 and 2012 to conform to the 2014 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, 2014 and 2013, the Company had cash of $631,562 and $392,413, respectively, on deposit with the Federal Reserve Bank. In addition, the Company maintained compensating balances with the Federal Reserve Bank of approximately $7,507 and $1,412 as of December 31, 2014 and 2013, 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 (losses). 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.
	The Company invests in securities on behalf of certain officers and directors of the Company who have elected to participate in the Company’s deferred compensation plans. These securities are included in other assets and are carried at their fair value based on quoted market prices. Net realized and unrealized holding gains and losses are included in other non-interest income and employee benefits expense.
	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 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 investment 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.
	Included in loans are certain residential mortgage loans originated for sale. These loans are carried at the lower of aggregate cost or estimated market value. Market value is estimated based on binding contracts or quotes or bids from third party investors. Residential mortgages held for sale were $40,828 and $40,861 as of December 31, 2014 and 2013, respectively. Gains and losses on sales of mortgage loans are determined using the specific identification method and are included in income from the origination and sale of loans.
	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 collectibility 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 analyses 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, if any.
	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 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 $24,652 as of December 31, 2014 and $22,401 as of December 31, 2013. Amortization expense related to core deposit intangibles recorded as of December 31, 2014 is expected to total $3,336, $3,099, $1,825, $1,318, 1,118, and 2,587 in 2015, 2016, 2017, 2018, 2019, and thereafter, respectively.
	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.
	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. Impairment losses of $102, $616 and $70 were recognized in other non-interest expense in 2014, 2013 and 2012, 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 $224, $3,512 and $6,724 were recorded in 2014, 2013 and 2012, respectively.
	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, 2014, restricted equity securities of the Federal Reserve Bank and the Federal Home Loan Bank of $16,187 and $10,662, respectively, were included in other assets at cost. As of December 31, 2013, restricted equity securities of the Federal Reserve Bank and the Federal Home Loan Bank were $13,357 and $7,003, 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 2014, 2013 or 2012.
	Derivatives and Hedging Activities. During 2014, the Company entered into derivative interest rate swap agreements as part of its interest rate risk management strategy. The interest rate swaps are recognized as assets or liabilities on the Company's balance sheet at fair value. Fair value estimations are obtained from third parties and are based on pricing models. Currently, none of the Company’s derivatives are designated in qualifying hedging relationships. As such, all changes in the fair value of the Company’s derivatives are recognized directly in earnings. As of December 31, 2014, interest rate swap derivative assets of $61 were included in other assets and interest rate swap derivative liabilities of $59 were included in other liabilities on the Company's consolidated balance sheet. During 2014, the Company recorded insignificant amounts of non-interest income related to changes in the fair value of derivatives. The Company does not enter into derivative agreements for trading or speculative purposes.
	Income from Fiduciary Activities. Consistent with industry practice, income for trust services is recognized on the basis of cash received. However, use of this method in lieu of accrual basis accounting does not materially affect reported earnings.
	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 time restricted stock awards and non-vested performance restricted stock awards granted prior to 2014. 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 income statement. With few exceptions, the Company is no longer subject to U.S. federal and state examinations by tax authorities for years before 2011. The Company had no accrued interest or penalties as of December 31, 2014, 2013 or 2012.
	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, 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 $3,734, $3,532, and $3,555 in 2014, 2013 and 2012, 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. Stock-based compensation expense of $3,014, $2,936 and $2,485 for the years ended December 31, 2014, 2013 and 2012, 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, 2014, 2013 and 2012 were $1,153, $1,122 and $950, respectively. All compensation cost for stock-based awards is expensed at the Parent Company.
	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

Acquisitions	12 Months Ended
	Dec. 31, 2014
Business Combinations [Abstract]
Mergers, Acquisitions and Dispositions Disclosures	On February 10, 2014, the Company entered into an agreement and plan of merger to acquire all of the outstanding stock of Mountain West Financial Corp ("MWFC"), a Montana-based bank holding company that operated one wholly-owned subsidiary bank, Mountain West Bank, NA ("MWB"), with branches located in five of the Company's current market areas in Montana. The acquisition was completed on July 31, 2014, and the Company merged MWB with its existing bank subsidiary, First Interstate Bank ("FIB"), on October 17, 2014. The acquisition allowed the Company to gain market share in several of its current market areas. The Company also benefited from cost savings related to the merger of MWB with FIB.
	Under the terms of the agreement and plan of merger, each outstanding share of Mountain West common stock was canceled and converted into the right to receive 0.2552 shares of the Company's Class A common stock plus $7.125 in cash, or, at the stockholder's election, an amount in all cash or all stock intended to be substantially equal in value to the combination of stock and cash merger consideration described above. Consideration for the acquisition of $74,451 consisted of cash of $38,479 and the issuance of 1,378,230 shares of the Company's Class A common stock valued at $26.10 per share, the closing price of the Company's Class A common stock as quoted on the NASDAQ stock market on the acquisition date. The acquisition was accounted for using the acquisition method with the cash portion of the purchase price funded from cash on hand. In conjunction with the acquisition, the Company recognized acquisition costs of $4,017.
	The assets and liabilities of MWFC were recorded in the Company's consolidated financial statements at their estimated fair values as of the acquisition date. The excess value of the consideration paid over the fair value of assets acquired and liabilities assumed was recorded as goodwill. Goodwill arising from the acquisition consists largely of the synergies and economies of scale expected from combining the operations of MWFC and the Company. This acquisition was accounted for as a tax-free exchange; therefore, goodwill recorded in conjunction with this acquisition is not deductible for income tax purposes.
	The following table summarizes the consideration paid, fair values of MWFC assets acquired and liabilities assumed and the resulting goodwill. The amount reported below for net deferred tax asset is provisional pending completion of the Company's review of tax items.
		As Recorded			Fair Value				As Recorded
	As of July 31, 2014	by MWFC			Adjustments				by the Company
	Assets acquired:
	Cash and cash equivalents	$	74,035		$	—			$	74,035
	Investment securities	104,945			(34		)	-1	104,911
	Loans	378,558			(18,286		)	-2	360,272
	Allowance for loan losses	(11,598		)	11,598			-3	—
	Premises and equipment	35,283			(5,847		)	-4	29,436
	Company-owned life insurance	13,046			—				13,046
	Deferred tax asset, net	6,491			1,135			-5	7,626
	Core deposit intangible	—			11,014			-6	11,014
	Other assets	16,559			(5,300		)	-7	11,259
	Total assets acquired	617,319			(5,720		)		611,599
	Liabilities assumed:
	Deposits	515,538			(159		)	-8	515,379
	Other liabilities	20,501			2,730			-9	23,231
	Subordinated debentures held by subsidiary trusts	20,439			—			-10	20,439
	Total liabilities assumed	556,478			2,571				559,049
	Net assets acquired	$	60,841		$	(8,291	)		52,550
	Consideration paid:
	Cash								38,479
	Class A common stock								35,972
	Total consideration								74,451
	Goodwill								$	21,901
	Explanation of fair value adjustments:
	-1	Write down of the book value of investment securities 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. Except for collateral dependent loans acquired with deteriorated credit quality, the fair value of 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 the 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 MWB allowance for loan losses at acquisition date as the credit risk is accounted for in the fair value adjustment for loans receivable described in (2) above.
	-4	Write down of the book value of premises and equipment to their estimated fair values based upon appraisals obtained from an independent third party appraiser.
	-5	Adjustment represents the net deferred tax assets resulting from fair value adjustments related to acquired assets, assumed liabilities, core deposit intangible assets and other purchase accounting adjustments.
	-6	Adjustment represents the value of the core deposit base assumed in the acquisition based upon a valuation obtained from an independent third party valuation expert.
	-7	Adjustment consists of a reduction in the value of equity method investments, accrued interest receivable and accrued net income taxes receivable, and the write-off of pre-existing goodwill and computer software costs.
	-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.
	-9	Adjustment represents increases in the book values of other liabilities to their estimated fair values at the acquisition date. The adjustment primarily consists of a $2,000 increase to an acquired operating lease obligation and a $473 increase to Federal Home Loan Bank borrowings based upon interest interest rates of similar obligations with similar characteristics on the date of acquisition.
	-10	Recorded value of junior subordinated debentures held by subsidiary trusts approximated fair value as of the acquisition date due to the short-term nature of the instruments. These debentures were redeemed at par value in December 2014.
	Fair values of assets acquired and liabilities assumed as part of the MWFC acquisition were estimated using relevant market information and significant other inputs and generally fall within Levels 2 and 3 of the fair value hierarchy.
	The core deposit intangible asset of $11,014 is being amortized using an accelerated method over the estimated useful lives of the related deposits of ten years.
	The Company acquired certain loans that are subject to Accounting Standards Codification ("ASC") Topic 310-30 "Loans and Debt Securities Acquired with Deteriorated Credit Quality." ASC Topic 310-30 provides recognition, measurement and disclosure guidance for acquired loans that have evidence of deterioration in credit quality since origination for which is it probable, at acquisition, the Company will be unable to collect all contractual amounts owned. For loans that meet the criteria stipulated in ASC Topic 310-30, the excess of all cash flows expected at acquisition over the initial fair value of the loans acquired ("accretable yield") is amortized to interest income over the expected remaining lives of the underlying loans using the effective interest method. The accretable yield will fluctuate due to changes in (i) estimated lives of underling credit-impaired loans, (ii) assumptions regarding future principal and interest amounts collected, and (iii) indices used to fair value variable rate loans.
	Information regarding acquired credit-impaired loans as of the July 31, 2014 acquisition date is as follows:
	Contractually required principal and interest payments	$	112,882
	Contractual cash flows not expected to be collected ("non-accretable discount")	74,760
	Cash flows expected to be collected	38,122
	Interest component of cash flows expected to be collected ("accretable discount")	5,233
	Fair value of acquired credit-impaired loans	$	32,889
	Information regarding acquired loans not deemed credit-impaired at the acquisition date is as follows:
	Contractually required principal and interest payments	$	445,345
	Contractual cash flows not expected to be collected	(15,090		)
	Fair value at acquisition	327,383
	The accompanying consolidated statements of income include the results of operations of the acquired entity from the July 31, 2014 acquisition date. Operations of the acquired entity were immediately integrated with the Company's operations and the acquired bank was merged with the Company's existing banking subsidiary in October 2014. Post-acquisition revenues and net income of the acquired entity were not captured separately subsequent to the merger. As such, the Company has determined it is not practical to report the post-acquisition date revenues and net income of the acquired entity that were included in the Company's consolidated income statement for the year ended December 31, 2014.
	The following table presents unaudited pro forma consolidated revenues and net income as if the acquisition had occurred as of January 1, 2013.
	Year ended December 31, (unaudited)	2014			2013
	Interest income	$	280,102		$	280,455
	Non-interest income	114,984			120,005
	Total revenues	$	395,086		$	400,460
	Net income	$	87,129		$	84,264
	The unaudited pro forma net income presented in the table above for 2014 was adjusted to exclude acquisition-related costs, including change in control expenses related to employee benefit plans, stock option cancellation fees and legal and professional expenses, of $5,052, net of tax. Pro forma net income presented in the table above for 2013 was adjusted to include the aforementioned acquisition-related costs. The unaudited pro forma net income presented in the table above for 2014 and 2013 includes adjustments for scheduled amortization of core deposit intangible assets acquired in the acquisition. No adjustments were made for operating costs savings and other business synergies expected as a result of the acquisition, or accretion or amortization of fair value adjustments other than core deposit intangible assets.

Investment_Securities

Investment Securities	12 Months Ended
	Dec. 31, 2014
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, 2014	Amortized		Gross			Gross			Estimated
		Cost		Unrealized			Unrealized			Fair
				Gains			Losses			Value
	Available-for-Sale
	Obligations of U.S. government agencies	$	725,408	$	895		$	(5,370	)	$	720,933
	U.S. agency residential mortgage-backed securities &	982,764		11,526			(3,624		)	990,666
	collateralized mortgage obligations
	Private mortgage-backed securities	322		5			(2		)	325
	Total	$	1,708,494	$	12,426		$	(8,996	)	$	1,711,924
	December 31, 2014	Amortized		Gross			Gross			Estimated
		Cost		Unrealized			Unrealized			Fair
				Gains			Losses			Value
	Held-to Maturity
	State, county and municipal securities	$	188,941	$	5,949		$	(386	)	$	194,504
	Corporate securities	32,565		54			(75		)	32,544
	U.S agency residential mortgage-backed securities &	353,176		5,563			(1,758		)	356,981
	collateralized mortgage obligations
	Other investments	504		—			—			504
	Total	$	575,186	$	11,566		$	(2,219	)	$	584,533
	Gross gains of $274 and gross losses of $213 were realized on the disposition of available-for-sale securities in 2014.
	December 31, 2013	Amortized		Gross			Gross			Estimated
		Cost		Unrealized			Unrealized			Fair
				Gains			Losses			Value
	Available-for-Sale
	Obligations of U.S. government agencies	$	774,055	$	1,432		$	(12,249	)	$	763,238
	U.S. agency residential mortgage-backed securities &	1,197,295		11,905			(25,147		)	1,184,053
	collateralized mortgage obligations
	Private mortgage-backed securities	407		9			(1		)	415
	Total	$	1,971,757	$	13,346		$	(37,397	)	$	1,947,706
	December 31, 2013	Amortized		Gross			Gross			Estimated
		Cost		Unrealized			Unrealized			Fair
				Gains			Losses			Value
	Held-to Maturity
	State, county and municipal securities	$	185,818	$	4,043		$	(2,049	)	$	187,812
	Corporate securities	18,019		103			(8		)	$	18,114
	Total	$	203,837	$	4,146		$	(2,057	)	$	205,926
	Gross gains of $49 and $351 were realized on the disposition of available-for-sale securities in 2013 and 2012, respectively. Gross losses of $48 and $3 were realized on the disposition of available-for-sale securities in 2013 and 2012, respectively.
	As of December 31, 2014, the Company had general obligation securities with amortized costs of $131,845 included in state, county and municipal securities, of which $68,927 were issued by political subdivisions or agencies within the states of Montana, Wyoming and South Dakota.
	On June 27, 2014, the Company transferred available-for-sale U.S. agency residential mortgage-backed securities and collateralized mortgage obligations with amortized costs and fair values of $396,640 and $388,808, respectively, into the held-to-maturity category. Unrealized net losses of $7,832 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 4.3 years.
	The following table shows 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, 2014 and 2013.
		Less than 12 Months					12 Months or More						Total
	December 31, 2014	Fair		Gross			Fair			Gross			Fair		Gross
		Value		Unrealized			Value			Unrealized			Value		Unrealized
				Losses						Losses					Losses
	Available-for-Sale
	Obligations of U.S. government agencies	$	135,888	$	(702	)	$	309,283		$	(4,668	)	$	445,171	$	(5,370	)
	U.S. agency residential mortgage-backed	219,214		(887		)	151,380			(2,737		)	370,594		(3,624		)
	securities & collateralized mortgage
	obligations
	Private mortgage-backed securities	—		—			90			(2		)	90		(2		)
	Total	$	355,102	$	(1,589	)	$	460,753		$	(7,407	)	$	815,855	$	(8,996	)
		Less than 12 Months					12 Months or More						Total
	December 31, 2014	Fair		Gross			Fair			Gross			Fair		Gross
		Value		Unrealized			Value			Unrealized			Value		Unrealized
				Losses						Losses					Losses
	Held-to-Maturity
	State, county and municipal securities	$	7,979	$	(13	)	$	20,097		$	(373	)	$	28,076	$	(386	)
	U.S. agency residential mortgage-backed	61,201		(1,758		)	—			—			61,201		(1,758		)
	securities & collateralized mortgage
	obligations
	Corporate securities	14,755		(75		)	—			—			14,755		(75		)
	Total	$	83,935	$	(1,846	)	$	20,097		$	(373	)	$	104,032	$	(2,219	)
	Less than 12 Months					12 Months or More						Total
December 31, 2013	Fair		Gross			Fair			Gross			Fair		Gross
	Value		Unrealized			Value			Unrealized			Value		Unrealized
			Losses						Losses					Losses
Available-for-Sale
Obligations of U.S. government agencies	$	458,385	$	(10,355	)	$	59,362		$	(1,894	)	$	517,747	$	(12,249	)
U.S. agency residential mortgage-backed	634,199		(17,273		)	166,930			(7,874		)	801,129		(25,147		)
securities & collateralized mortgage
obligations
Private mortgage-backed securities	—		—			104			(1		)	104		(1		)
Total	$	1,092,584	$	(27,628	)	$	226,396		$	(9,769	)	$	1,318,980	$	(37,397	)
	Less than 12 Months					12 Months or More						Total
December 31, 2013	Fair		Gross			Fair			Gross			Fair		Gross
	Value		Unrealized			Value			Unrealized			Value		Unrealized
			Losses						Losses					Losses
Held-to-Maturity
State, county and municipal securities	$	37,550	$	(1,319	)	$	14,296		$	(730	)	$	51,846	$	(2,049	)
Corporate securities	7,294		(8		)	—			—			7,294		(8		)
Total	$	44,844	$	(1,327	)	$	14,296		$	(730	)	$	59,140	$	(2,057	)
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. As of December 31, 2014, the Company had 154 individual investment securities that were in an unrealized loss position. As of December 31, 2013, the Company had 229 individual investment securities that were in an unrealized loss position. Unrealized losses as of December 31, 2014 and 2013 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, 2014, 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 such securities before a recovery in cost. No impairment losses were recorded during 2014, 2013 or 2012.
Maturities of investment securities at December 31, 2014 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, 2014	Amortized		Estimated				Amortized			Estimated
	Cost		Fair Value				Cost			Fair Value
Within one year	$	252,677	$	254,560			$	85,751		$	86,983
After one year but within five years	1,218,143		1,219,799				283,000			286,783
After five years but within ten years	188,520		188,015				150,601			153,287
After ten years	49,154		49,550				55,834			57,480
Total	$	1,708,494	$	1,711,924			$	575,186		$	584,533
At December 31, 2014, the Company had investment securities callable within one year with amortized costs and estimated fair values of $176,023 and $175,892, 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, 2014, the Company had callable structured notes with amortized costs and estimated fair values of $29,995 and $30,012, respectively. These callable structured notes, which are classified as available-for-sale and included in the after one year but within five years category in the table above, have fixed interest rates that increase at various intervals as market rates increase.
Maturities of securities do not reflect rate repricing opportunities present in adjustable rate mortgage-backed securities. At December 31, 2014 and 2013, the Company had variable rate mortgage-backed securities with amortized costs of $40,666 and $46,315, respectively, classified as available-for-sale in the table above.
There are no significant concentrations of investments at December 31, 2014, (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 $1,351,787 and $1,288,750 at December 31, 2014 and 2013, respectively, were pledged to secure public deposits and securities sold under repurchase agreements. The approximate fair value of securities pledged at December 31, 2014 and 2013 was $1,356,341 and $1,278,663, 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, 2014
Receivables [Abstract]
Loans	LOANS
	The following table presents loans by class as of the dates indicated:
	December 31,	2014				2013
	Real estate loans:
	Commercial	$	1,639,422			$	1,449,174
	Construction:
	Land acquisition & development	220,443				205,911
	Residential	96,580				76,488
	Commercial	101,246				69,236
	Total construction loans	418,269				351,635
	Residential	999,903				867,912
	Agricultural	167,659				173,534
	Total real estate loans	3,225,253				2,842,255
	Consumer:
	Indirect consumer	552,863				476,012
	Other consumer	144,141				133,039
	Credit card	65,467				62,536
	Total consumer loans	762,471				671,587
	Commercial	740,073				676,544
	Agricultural	124,859				111,872
	Other, including overdrafts	3,959				1,734
	Loans held for investment	4,856,615				4,303,992
	Mortgage loans held for sale	40,828				40,861
	Total loans	$	4,897,443			$	4,344,853
	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 $298,692 and $272,415 as of December 31, 2014 and 2013, 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 and 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, accrued interest receivable and accrual status of loans acquired with credit impairment as of December 31, 2014 and 2013.
	December 31,	2014			2013
	Outstanding balance	$	41,910		$	—
	Carrying value
	Loans on accrual status	31,870			—
	Loans on non-accrual status	—			—
	Total carrying value	$	31,870		$	—
	The following table summarizes changes in the accretable yield for loans acquired credit impaired for year ended December 31, 2014 and 2013:
	Year Ended December 31,	2014			2013
	Beginning balance	$	—		$	—
	Acquisition	5,233			—
	Accretion income	(735		)	—
	Reductions due to exit events	(201		)	—
	Reclassifications from (to) nonaccretable differences	1,484			—
	Ending balance	$	5,781		$	—
	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, 2014	Past Due			Past Due			Past Due			Past Due			Loans			Loans			Loans
	Real estate
	Commercial	$	4,692		$	1,609		$	331		$	6,632		$	1,605,421		$	27,369		$	1,639,422
	Construction:
	Land acquisition & development	839			383			—			1,222			210,969			8,252			220,443
	Residential	—			475			—			475			95,833			272			96,580
	Commercial	100			—			—			100			98,582			2,564			101,246
	Total construction loans	939			858			—			1,797			405,384			11,088			418,269
	Residential	6,969			645			1,762			9,376			987,735			2,792			999,903
	Agricultural	1,624			236			—			1,860			158,957			6,842			167,659
	Total real estate loans	14,224			3,348			2,093			19,665			3,157,497			48,091			3,225,253
	Consumer:
	Indirect consumer	3,235			482			6			3,723			548,757			383			552,863
	Other consumer	988			140			32			1,160			142,432			549			144,141
	Credit card	369			284			315			968			64,484			15			65,467
Total consumer loans	4,592			906			353			5,851			755,673			947			762,471
Commercial	3,659			994			147			4,800			722,575			12,698			740,073
Agricultural	1,125			—			—			1,125			123,288			446			124,859
Other, including overdrafts	—			—			—			—			3,959			—			3,959
Loans held for investment	23,600			5,248			2,593			31,441			4,762,992			62,182			4,856,615
Mortgage loans originated for sale	—			—			—			—			40,828			—			40,828
Total loans	$	23,600		$	5,248		$	2,593		$	31,441		$	4,803,820		$	62,182		$	4,897,443
										Total Loans
	30 - 59			60 - 89			> 90			30 or More
	Days			Days			Days			Days			Current			Non-accrual			Total
As of December 31, 2013	Past Due			Past Due			Past Due			Past Due			Loans			Loans			Loans
Real estate
Commercial	$	5,924		$	2,472		$	22		$	8,418		$	1,391,823		$	48,933		$	1,449,174
Construction:
Land acquisition & development	1,062			468			38			1,568			188,074			16,269			205,911
Residential	933			250			—			1,183			73,933			1,372			76,488
Commercial	584			—			—			584			68,427			225			69,236
Total construction loans	2,579			718			38			3,335			330,434			17,866			351,635
Residential	3,630			206			1,162			4,998			856,800			6,114			867,912
Agricultural	328			646			—			974			163,986			8,574			173,534
Total real estate loans	12,461			4,042			1,222			17,725			2,743,043			81,487			2,842,255
Consumer:
Indirect consumer	3,303			430			9			3,742			471,906			364			476,012
Other consumer	925			130			1			1,056			131,508			475			133,039
Credit card	364			187			515			1,066			61,451			19			62,536
Total consumer loans	4,592			747			525			5,864			664,865			858			671,587
Commercial	2,791			1,186			563			4,540			660,035			11,969			676,544
Agricultural	453			672			—			1,125			110,622			125			111,872
Other, including overdrafts	—			—			—			—			1,734			—			1,734
Loans held for investment	20,297			6,647			2,310			29,254			4,180,299			94,439			4,303,992
Mortgage loans originated for sale	—			—			—			—			40,861			—			40,861
Total loans	$	20,297		$	6,647		$	2,310		$	29,254		$	4,221,160		$	94,439		$	4,344,853
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 $3,970, $4,630 and $8,537 during the years ended December 31, 2014, 2013 and 2012, respectively.
The Company considers impaired loans to include all loans, except consumer loans, that are risk rated as doubtful, or have been placed on non-accrual status or renegotiated in troubled debt restructurings. The following tables present information on the Company’s recorded investment in impaired loans as of dates indicated:
	December 31, 2014
	Unpaid			Recorded			Recorded			Total			Related
	Total			Investment			Investment			Recorded			Allowance
	Principal			With No			With			Investment
	Balance			Allowance			Allowance
Real estate:
Commercial	$	41,603		$	28,143		$	11,246		$	39,389		$	1,608
Construction:
Land acquisition & development	12,511			7,262			1,615			8,877			574
Residential	459			272			—			272			—
Commercial	2,729			253			2,442			2,695			904
Total construction loans	15,699			7,787			4,057			11,844			1,478
Residential	2,959			2,452			341			2,793			143
Agricultural	8,844			6,444			2,305			8,749			732
Total real estate loans	69,105			44,826			17,949			62,775			3,961
Commercial	16,904			11,882			2,644			14,526			1,190
Agricultural	1,231			342			837			1,179			641
Total	$	87,240		$	57,050		$	21,430		$	78,480		$	5,792
	December 31, 2013
	Unpaid			Recorded			Recorded			Total			Related
	Total			Investment			Investment			Recorded			Allowance
	Principal			With No			With			Investment
	Balance			Allowance			Allowance
Real estate:
Commercial	$	64,780		$	29,216		$	33,937		$	63,153		$	5,210
Construction:
Land acquisition & development	23,906			9,901			7,226			17,127			1,434
Residential	1,816			1,095			277			1,372			26
Commercial	397			279			84			363			85
Total construction loans	26,119			11,275			7,587			18,862			1,545
Residential	9,448			5,081			967			6,048			249
Agricultural	8,895			6,429			2,370			8,799			335
Total real estate loans	109,242			52,001			44,861			96,862			7,339
Commercial	15,448			10,684			2,901			13,585			1,504
Agricultural	177			39			86			125			86
Total	$	124,867		$	62,724		$	47,848		$	110,572		$	8,929
	December 31, 2012
	Unpaid			Recorded			Recorded			Total			Related
	Total			Investment			Investment			Recorded			Allowance
	Principal			With No			With			Investment
	Balance			Allowance			Allowance
Real estate:
Commercial	$	84,300		$	39,049		$	34,774		$	73,823		$	4,112
Construction:
Land acquisition & development	28,558			15,891			7,173			23,064			1,457
Residential	3,018			1,976			710			2,686			251
Commercial	10,447			7,785			340			8,125			69
Total construction loans	42,023			25,652			8,223			33,875			1,777
Residential	13,271			6,152			4,495			10,647			1,677
Agricultural	5,559			1,834			3,227			5,061			784
Total real estate loans	145,153			72,687			50,719			123,406			8,350
Commercial	12,770			9,036			3,206			12,242			1,919
Agricultural	589			509			28			537			28
Total	$	158,512		$	82,232		$	53,953		$	136,185		$	10,297
The following tables present the average recorded investment in and income recognized on impaired loans for the periods indicated:
	Year Ended December 31,
	2014							2013							2012
	Average Recorded Investment			Income Recognized				Average Recorded Investment			Income Recognized				Average Recorded Investment			Income Recognized
Real estate:
Commercial	$	54,701		$	876			$	66,330		$	1,092			$	78,670		$	1,339
Construction:
Land acquisition & development	13,056			43				19,523			487				44,457			110
Residential	822			—				1,893			—				8,431			4
Commercial	1,529			8				3,936			4				16,401			—
Total construction loans	15,407			51				25,352			491				69,289			114
Residential	4,537			5				8,104			17				13,703			26
Agricultural	8,774			78				8,230			8				6,936			41
Total real estate loans	83,419			1,010				108,016			1,608				168,598			1,520
Commercial	13,789			50				15,047			68				15,741			84
Agricultural	447			23				313			16				942			27
Total	$	97,655		$	1,083			$	123,376		$	1,692			$	185,281		$	1,631
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 2014, 2013 and 2012 would have been approximately $4,951, $5,786 and $8,463, respectively.
Collateral dependent impaired loans are recorded at the fair value 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 $44,227 as of December 31, 2014, of which $23,275 were included in non-accrual loans and $20,952 were on accrual status. The Company had loans renegotiated in troubled debt restructurings of $59,792 as of December 31, 2013, of which $38,011 were included in non-accrual loans and $21,781 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, 2014					Interest only period			Extension of terms or maturity			Interest rate adjustment			Other
Real estate:
Commercial		13			$	4,753		$	672		$	84		$	1,047		$	6,556
Residential		1			—			—			—			15			15
Total real estate loans		14			4,753			672			84			1,062			6,571
Consumer		1			—			113			—			—			113
Commercial		5			476			—			—			30			506
Total		20			$	5,229		$	785		$	84		$	1,092		$	7,190
		Number of Notes			Type of Concession												Principal Balance at Restructure Date
Year ended December 31, 2013					Interest only period			Extension of terms or maturity			Interest rate adjustment			Other
Real estate:
Commercial		19			$	543		$	1,378		$	11,420		$	2,310		$	15,651
Construction:
Land acquisition & development		8			528			7,308			1,952			—			9,788
Residential		3			—			408			411			—			819
Total construction loans		11			528			7,716			2,363			—			10,607
Residential		5			—			708			—			79			787
Agriculture		1			—			—			188			—			188
Total real estate loans		36			1,071			9,802			13,971			2,389			27,233
Consumer		1			—			—			27			—			27
Commercial		6			613			178			265			87			1,143
Total		43			$	1,684		$	9,980		$	14,263		$	2,476		$	28,403
		Number of Notes			Type of Concession												Principal Balance at Restructure Date
Year ended December 31, 2012					Interest only period			Extension of terms or maturity			Interest rate adjustment			Other
Real estate:
Commercial		16			$	—		$	959		$	4,504		$	8,611		$	14,074
Construction:
Commercial		1			—			—			—			3,155			3,155
Land acquisition & development		5			—			1,000			1,757			623			3,380
Residential		2			—			280			233			—			513
Total construction loans		8			—			1,280			1,990			3,778			7,048
Residential		2			568			25			—			—			593
Agriculture		1			—			154			—			—			154
Total real estate loans		27			568			2,418			6,494			12,389			21,869
Consumer		1			—			69			—			—			69
Commercial		10			387			217			—			218			822
Total		38			$	955		$	2,704		$	6,494		$	12,607		$	22,760
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 2014, 2013 or 2012.
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. As of December 31, 2014, 2013 and 2012, 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, 2014, and 2013, 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, 2014, 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, 2014	Other Assets			Substandard			Doubtful			Total
	Especially									Criticized
	Mentioned									Loans
Real estate:
Commercial	$	84,533		$	83,448		$	15,246		$	183,227
Construction:
Land acquisition & development	11,826			15,016			2,507			29,349
Residential	2,029			2,666			—			4,695
Commercial	39			253			2,442			2,734
Total construction loans	13,894			17,935			4,949			36,778
Residential	10,473			10,848			1,121			22,442
Agricultural	10,122			12,328			612			23,062
Total real estate loans	119,022			124,559			21,928			265,509
Consumer:
Indirect consumer	916			1,590			121			2,627
Other consumer	553			1,085			432			2,070
Credit card	—			348			1,263			1,611
Total consumer loans	1,469			3,023			1,816			6,308
Commercial	25,766			32,433			10,273			68,472
Agricultural	7,827			3,660			837			12,324
Total	$	154,084		$	163,675		$	34,854		$	352,613
As of December 31, 2013	Other Assets			Substandard			Doubtful			Total
	Especially									Criticized
	Mentioned									Loans
Real estate:
Commercial	$	79,747		$	86,426		$	24,840		$	191,013
Construction:
Land acquisition & development	13,211			19,677			7,329			40,217
Residential	1,859			1,649			277			3,785
Commercial	—			409			84			493
Total construction loans	15,070			21,735			7,690			44,495
Residential	7,500			7,188			4,184			18,872
Agricultural	13,597			10,245			2,370			26,212
Total real estate loans	115,914			125,594			39,084			280,592
Consumer:
Indirect consumer	875			1,524			115			2,514
Other consumer	573			969			268			1,810
Credit card	—			392			2,010			2,402
Total consumer loans	1,448			2,885			2,393			6,726
Commercial	33,318			23,833			3,745			60,896
Agricultural	8,401			1,788			86			10,275
Total	$	159,081		$	154,100		$	45,308		$	358,489
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, 2014
Receivables [Abstract]
Allowance For Loan Losses	ALLOWANCE FOR LOAN LOSSES
	The following table presents a summary of changes in the allowance for loan losses by portfolio segment:
	Year ended December 31, 2014	Real Estate			Consumer			Commercial			Agriculture			Other		Total
	Allowance for loan losses:
	Beginning balance	$	63,923		$	6,193		$	14,747		$	476		$	—	$	85,339
	Provision charged (credited) to operating	(10,348		)	1,382			1,809			535			—		(6,622		)
	expense
	Less loans charged-off	(3,014		)	(4,887		)	(6,030		)	(64		)	—		(13,995		)
	Add back recoveries of loans previously charged-off	3,323			2,347			3,781			27			—		9,478
	Ending balance	$	53,884		$	5,035		$	14,307		$	974		$	—	$	74,200
	Individually evaluated for impairment	$	3,961		$	—		$	1,190		$	641		$	—	$	5,792
	Collectively evaluated for impairment	49,923			5,035			13,117			333			—		68,408
	Ending balance	$	53,884		$	5,035		$	14,307		$	974		$	—	$	74,200
	Total loans:
	Individually evaluated for impairment	$	62,775		$	—		$	14,526		$	1,179		$	—	$	78,480
	Collectively evaluated for impairment	3,203,306			762,471			725,547			123,680			3,959		4,818,963
	Total loans	$	3,266,081		$	762,471		$	740,073		$	124,859		$	3,959	$	4,897,443
	Year ended December 31, 2013	Real Estate			Consumer			Commercial			Agriculture			Other		Total
	Allowance for loan losses:
	Beginning balance	$	75,782		$	7,141		$	17,085		$	503		$	—	$	100,511
	Provision charged (credited) to operating	(7,722		)	1,605			41			(49		)	—		(6,125		)
	expense
	Less loans charged-off	(10,224		)	(4,612		)	(5,672		)	(5		)	—		(20,513		)
	Add back recoveries of loans previously charged-off	6,087			2,059			3,293			27			—		11,466
	Ending balance	$	63,923		$	6,193		$	14,747		$	476		$	—	$	85,339
	Individually evaluated for impairment	$	7,339		$	—		$	1,504		$	86		$	—	$	8,929
	Collectively evaluated for impairment	56,584			6,193			13,243			390			—		76,410
	Ending balance	$	63,923		$	6,193		$	14,747		$	476		$	—	$	85,339
	Total loans:
	Individually evaluated for impairment	$	96,862		$	—		$	13,585		$	125		$	—	$	110,572
	Collectively evaluated for impairment	2,786,254			671,587			662,959			111,747			1,734		4,234,281
	Total loans	$	2,883,116		$	671,587		$	676,544		$	111,872		$	1,734	$	4,344,853
	Year ended December 31, 2012	Real Estate			Consumer			Commercial			Agriculture			Other		Total
	Allowance for loan losses:
	Beginning balance	$	87,396		$	8,594		$	15,325		$	1,266		$	—	$	112,581
	Provision charged (credited) to operating	28,651			1,922			10,845			(668		)	—		40,750
	expense
	Less loans charged-off	(43,506		)	(5,320		)	(11,990		)	(120		)	—		(60,936		)
	Add back recoveries of loans previously charged-off	3,241			1,945			2,905			25			—		8,116
	Ending balance	$	75,782		$	7,141		$	17,085		$	503		$	—	$	100,511
	Individually evaluated for impairment	$	8,350		$	—		$	1,919		$	28		$	—	$	10,297
	Collectively evaluated for impairment	67,432			7,141			15,166			475			—		90,214
	Ending balance	$	75,782		$	7,141		$	17,085		$	503		$	—	$	100,511
	Total loans:
	Individually evaluated for impairment	$	123,406		$	—		$	12,242		$	537		$	—	$	136,185
	Collectively evaluated for impairment	2,660,420			636,794			676,511			113,090			912		4,087,727
	Total loans	$	2,783,826		$	636,794		$	688,753		$	113,627		$	912	$	4,223,912
	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 environmental 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 factors and the estimated impact of current economic, environmental and regulatory conditions 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, 2014, the Company's allowance for loans losses included $287 related to acquired credit impaired loans.

Premises_and_Equipment

Premises and Equipment	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Premises and Equipment	PREMISES AND EQUIPMENT
	Premises and equipment and related accumulated depreciation are as follows:
	December 31,	2014			2013
	Land	$	40,620		$	37,581
	Buildings and improvements	209,328			196,950
	Furniture and equipment	74,959			68,870
		324,907			303,401
	Less accumulated depreciation	(129,695		)	(123,711		)
	Premises and equipment, net	$	195,212		$	179,690
	The Parent Company and a FIB branch office lease premises from an affiliated partnership. See Note 16—Commitments and Contingencies.

Company_Owned_Life_Insurance

Company - Owned Life Insurance	12 Months Ended
	Dec. 31, 2014
Investments, All Other Investments [Abstract]
Company-Owned Life Insurance	COMPANY-OWNED LIFE INSURANCE
	Company-owned life insurance consists of the following:
	December 31,	2014		2013
	Key executive, principal shareholder	$	3,798	$	3,660
	Key executive split dollar	4,707		4,628
	Group life	145,316		113,887
	Total	$	153,821	$	122,175
	The Company maintains key executive life insurance policies on certain principal shareholders. Under these policies, the Company receives benefits payable upon the death of the insured. The net cash surrender value of key executive, principal shareholder insurance policies was $3,798 and $3,660 at December 31, 2014 and 2013, respectively.
	The Company also has life insurance policies covering selected other key officers. The net cash surrender value of these policies was $4,707 and $4,628 at December 31, 2014 and 2013, 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 $132,111 and $113,887 at December 31, 2014 and 2013, 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.
	The Company obtained group life insurance policies covering certain key employees of MWB as part of the MWFC acquisition. The net cash surrender value of these policies was $13,205 at December 31, 2014. Under these policies, the Company receives all benefits payable upon death of the insured.

Other_Real_Estate_Owned

Other Real Estate Owned	12 Months Ended
	Dec. 31, 2014
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,	2014			2013			2012
	Balance at beginning of year	$	15,504		$	32,571		$	37,452
	Acquisitions	3,608			—			—
	Additions	5,198			11,545			43,541
	Capitalized improvements	—			65			75
	Valuation adjustments	(224		)	(3,512		)	(6,724		)
	Dispositions	(10,532		)	(25,165		)	(41,773		)
	Balance at end of year	$	13,554		$	15,504		$	32,571
	Write-downs of $224 during 2014 included adjustments of $93 directly related to receipt of updated appraisals and adjustments of $131 based on other sources, including management estimates of the current fair value of properties. Write-downs of $3,512 during 2013 included adjustments of $1,083 directly related to receipt of updated appraisals and adjustments of $2,429 based on other sources, including management estimates of the current fair value of properties. Write-downs of $6,724 during 2012 included adjustments of $702 directly related to receipt of updated appraisals and adjustments of $6,022 based on other sources, including management estimates of the current fair value of properties.

Mortgage_Servicing_Rights

Mortgage Servicing Rights	12 Months Ended
	Dec. 31, 2014
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,	2014			2013			2012
	Balance at beginning of year	$	14,018		$	13,224		$	13,450
	Sales of mortgage servicing rights	—			—			(735		)
	Originations of mortgage servicing rights	2,717			3,581			4,563
	Amortization expense	(2,361		)	(2,787		)	(3,501		)
	Write-off of permanent impairment	—			—			(553		)
	Balance at end of year	14,374			14,018			13,224
	Less valuation reserve	(336		)	(472		)	(571		)
	Balance at end of year	$	14,038		$	13,546		$	12,653
	Principal balance of serviced loans underlying mortgage servicing rights	$	2,615,311		$	2,416,621		$	2,146,351
	Mortgage servicing rights as a percentage of serviced loans	0.54		%	0.56		%	0.59		%
	At December 31, 2014, the estimated fair value and weighted average remaining life of the Company’s mortgage servicing rights were $21,434 and 6.9 years, respectively. The fair value of mortgage servicing rights was determined using discount rates ranging from 9.5% to 21.0% and monthly prepayment speeds ranging from 0.6% to 2.4% depending upon the risk characteristics of the underlying loans. The Company reversed impairment of $136, $99 and $771 in 2014, 2013 and 2012, respectively. Permanent impairment of $553 was charged against the carrying value of mortgage servicing rights in 2012. No permanent impairment was recorded in 2014 or 2013.
	During 2012, the Company sold mortgage servicing rights with carrying values aggregating $735. A gain of $19 on the sale was recorded as other income. In conjunction with the sale, the Company entered into an agreement with the purchaser whereby the Company continues to sub-service the loans underlying the sold mortgage servicing rights.

Deposits

Deposits	12 Months Ended
	Dec. 31, 2014
Deposits [Abstract]
Deposits	DEPOSITS
	Deposits are summarized as follows:
	December 31,	2014		2013
	Non-interest bearing demand	$	1,791,364	$	1,491,683
	Interest bearing:
	Demand	2,133,273		1,848,806
	Savings	1,843,355		1,602,544
	Time, $100 and over	520,125		492,051
	Time, other	718,095		698,666
	Total interest bearing	5,214,848		4,642,067
	Total deposits	$	7,006,212	$	6,133,750
	The Company had no brokered time deposits as of December 31, 2014 and 2013.
	Other time deposits include deposits obtained through the Company’s participation in the Certificate of Deposit Account Registry Service (“CDARS”). CDARS deposits totaled $40,491 and $51,526 as of December 31, 2014 and 2013, respectively.
	As of December 31, 2014 and 2013, the Company had time deposits of $228,410 and $207,504, respectively, that met or exceeded the FDIC insurance limit of $250.
	Maturities of time deposits at December 31, 2014 are as follows:
		Time, $100		Total Time
		and Over
	2015	$	337,404	$	803,986
	2016	90,509		209,192
	2017	58,387		127,367
	2018	20,971		62,084
	2019	12,854		35,577
	Thereafter	—		14
	Total	$	520,125	$	1,238,220
	Interest expense on time deposits of $100 or more was $4,003, $4,880 and $6,951 for the years ended December 31, 2014, 2013 and 2012, respectively.

LongTerm_Debt_and_Other_Borrow

Long-Term Debt and Other Borrowed Funds	12 Months Ended
	Dec. 31, 2014
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,	2014				2013
	Parent Company:
	6.81% subordinated term loan maturing January 9, 2018, principal due at maturity, interest payable quarterly	$	20,000			$	20,000
	Subsidiaries:
	Variable rate subordinated term loan maturing February 28, 2018, principal due at maturity, interest payable quarterly (rate of 2.24% at December 31, 2014)	15,000				15,000
	4.86% note payable to FHLB, maturing October 31, 2015	225				225
	8.00% capital lease obligation with term ending October 25, 2029	1,643				1,692
	6.24% note payable maturing September 2032, principal due at maturity, interest payable monthly	1,199				—
	Total long-term debt	$	38,067			$	36,917
	Maturities of long-term debt at December 31, 2014 are as follows:
	2015			$	285
	2016			65
	2017			71
	2018			35,077
	2019			83
	Thereafter			2,486
	Total			$	38,067
	On January 10, 2008, the Company borrowed $20,000 on a 6.81% unsecured subordinated term loan maturing January 9, 2018, with interest payable quarterly and principal due at maturity. The unsecured subordinated term loan qualifies as tier 2 capital under regulatory capital adequacy guidelines.
	During February 2008, the Company borrowed $15,000 on a variable rate unsecured subordinated term loan maturing February 28, 2018, with interest payable quarterly and principal due at maturity. The Company may elect at various dates either prime or LIBOR plus 2.00%. The interest rate on the subordinated term loan was 2.24% as of December 31, 2014. The unsecured subordinated term loan qualifies as tier 2 capital under regulatory capital adequacy guidelines.
	The note payable to FHLB is secured by a blanket assignment of the Company’s qualifying residential and commercial real estate loans. The Company has available lines of credit with the FHLB of approximately $701,768, subject to collateral availability. As of December 31, 2014 and 2013, FHLB advances of $225 were included in long-term debt. As of December 31, 2014 and 2013 there were no short-term advances outstanding with the FHLB.
	The Company has a capital lease obligation on a banking office. The balance of the obligation was $1,643 and $1,692 as of December 31, 2014 and 2013, 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 the MWFC acquisition, the Company assumed a 6.24% fixed rate note payable maturing in September, 2032, with interest payable monthly and principal due at maturity. The balance of the obligation was $1,199 as of December 31, 2014.
	The Company had other borrowed funds of $9 and $3 as of December 31, 2014 and 2013, respectively, consisting of demand notes issued to the United States Treasury, secured by investment securities and bearing no interest.
	The Company has federal funds lines of credit with third parties amounting to $115,000, 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 $364,205 secured by a blanket pledge of indirect consumer loans.

Subordinated_Debentures_Held_b

Subordinated Debentures Held by Subsidiary Trusts	12 Months Ended
	Dec. 31, 2014
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 $80,000 of 30-year floating rate mandatorily redeemable capital trust preferred securities (“Trust Preferred Securities”) to third-party investors. The Trusts also issued, in aggregate, $2,477 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	2014		2013
	October 2007	January 1, 2038	$	10,310	$	10,310
	November 2007	December 15, 2037	15,464		15,464
	December 2007	December 15, 2037	20,619		20,619
	December 2007	April 1, 2038	15,464		15,464
	January 2008	April 1, 2038	10,310		10,310
	January 2008	April 1, 2038	10,310		10,310
	Total subordinated debentures held by subsidiary trusts		$	82,477	$	82,477
	In October 2007, the Company issued $10,310 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, 2014 the interest rate on the Subordinated Debentures was 2.49%.
	In November 2007, the Company issued $15,464 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, 2014, the interest rate on the Subordinated Debentures was 2.99%.
	In December 2007, the Company issued $20,619 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, 2014, the interest rate on the Subordinated Debentures was 2.64%.
	In December 2007, the Company issued $15,464 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, 2014 the interest rate on the Subordinated Debentures was 2.94%.
	In January 2008, the Company issued $10,310 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, 2014 the interest rate on the Subordinated Debentures was 2.99%.
	In January 2008, the Company issued $10,310 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, 2014, the interest rate on the Subordinated Debentures was 2.99%.
	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.
	In conjunction with the acquisition of MWFC on July 31, 2014, the Company acquired two business trusts, Mountain West Statutory Trust III and Mountain West Statutory Trust IV (collectively, the “Mountain West Trusts”). The Mountain West Trusts were formed for the exclusive purpose of issuing an aggregate of $19,825 of 30-year floating rate mandatorily redeemable capital trust preferred securities (“Mountain West Trust Preferred Securities”) to third-party investors. The Trusts also issued, in aggregate, $614 of common equity securities to MWFC. Proceeds from the issuance of the Trust Preferred Securities and common equity securities were invested in 30-year junior subordinated deferrable interest debentures (“Mountain West Subordinated Debentures”) issued by the MWFC.
	On December 15, 2014, the Company redeemed $14,433 of Mountain West Subordinated Debentures bearing a cumulative floating interest rate equal to LIBOR plus 1.85% per annum. The redemption price of $14,433 was equal to the $1 liquidation amount of each debenture plus all accrued and unpaid distributions to the date of redemption. The redemption of the Mountain West Subordinated Debentures caused a mandatory redemption of $14,000 of Mountain West trust Preferred Securities and $433 of common equity securities.
	On December 26, 2014, the Company redeemed $6,006 of Mountain West Subordinated Debentures bearing a cumulative floating interest rate equal to LIBOR plus 3.10% per annum. The redemption price of $6,006 was equal to the $1 liquidation amount of each debenture plus all accrued and unpaid distributions to the date of redemption. The redemption of the Mountain West Subordinated Debentures caused a mandatory redemption of $5,825 of Mountain West Trust Preferred Securities and $181 of common equity securities.

Capital_Stock_and_Dividend_Res

Capital Stock and Dividend Restrictions	12 Months Ended
	Dec. 31, 2014
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 21,928,932 shares of Class A common stock and 23,859,483 shares of Class B common stock outstanding as of December 31, 2014. The Company had 19,868,018 shares of Class A common stock and 24,287,045 shares of Class B common stock outstanding as of December 31, 2013.
	On July 31, 2014, the Company issued 1,378,230 shares of its Class A common stock with an aggregate value of $35,972 as partial consideration for the acquisition of MWFC. In addition, the Company issued 24,581 shares of its Class A common stock with an aggregate value of $620 to directors for their service on the Company's board of directors during 2014. The Company issued 26,096 shares of its Class A common stock with an aggregate value of $543 to directors for their service on the Company's board of directors during 2013.
	During 2014, the Company repurchased and retired 362,121 shares of its Class A common stock in a combination of open market and privately negotiated transactions at an aggregate purchase price of $9,078, or a weighted average price of $25.07 per share. The repurchases were made pursuant to a stock repurchase program approved by the Company's Board of Directors. All other stock repurchases during 2014 and 2013 were redemptions of vested restricted shares tendered in lieu of cash for payment of income tax withholding amounts by participants of the Company's 2006 Equity Compensation Plan.
	On January 24, 2014, the Company filed a Registration Statement on Form S-8 to register an additional 1,500,000 share of Class A common stock to be issued pursuant to the Company's 2006 Equity Compensation Plan, as amended and restated.
	As of December 31, 2012, the Company had 5,000 shares of 6.75% Series A noncumulative redeemable preferred stock (“Series A Preferred Stock”) issued with an aggregate value of $50,000. The Series A Preferred Stock ranked senior to the Company’s common stock with respect to dividend and liquidation rights and had no voting rights. Holders of the Series A Preferred Stock were entitled to receive, if and when declared, noncumulative dividends at an annual rate of $675 per share, based on a 360 day year. The Company redeemed all of the Series A Preferred Stock on January 18, 2013 at an aggregate redemption price of $50,150, or $10,000 per share plus all accrued and unpaid dividends. Upon notice to holders of the redemption in December 2012, the Series A Preferred Stock was reclassified from stockholders' equity to a liability.
	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, 2014
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,	2014		2013		2012
	Net income	$	84,401	$	86,136	$	58,224
	Less preferred stock dividends	—		—		3,300
	Net income available to common shareholders, basic and diluted	$	84,401	$	86,136	$	54,924
	Weighted average common shares outstanding for basic earnings per share computation	44,615,060		43,566,681		42,965,987
	Dilutive effects of stock-based compensation	595,501		477,921		126,991
	Weighted average common shares outstanding for diluted earnings per common share computation	45,210,561		44,044,602		43,092,978
	Basic earnings per common share	$	1.89	$	1.98	$	1.28
	Diluted earnings per common share	1.87		1.96		1.27
	The Company had 5,000, 21,372 and 2,427,823 stock options outstanding as of December 31, 2014, 2013 and 2012, respectively, that were not included in the computation of diluted earnings per common share because their effect would be anti-dilutive. The Company had 88,797, 37,734 and 41,240 shares of unvested restricted stock as of December 31, 2014, 2013 and 2012, 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, 2014
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, 2014, 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, 2014 and 2013 are presented in the following table:
		Actual				Adequately Capitalized				Well Capitalized
		Amount		Ratio		Amount		Ratio		Amount		Ratio
	December 31, 2014
	Total risk-based capital:
	Consolidated	$	897,769	16.2	%	$	444,685	8	%	NA		NA
	FIB	832,907		15.1		442,468		8		$	553,085	10	%
	Tier 1 risk-based capital:
	Consolidated	807,229		14.5		222,343		4		NA		NA
	FIB	754,708		13.7		221,234		4		$	331,851	6
	Leverage capital ratio:
	Consolidated	807,229		9.6		335,897		4		NA		NA
	FIB	754,708		9.2		330,006		4		$	412,507	5
		Actual				Adequately Capitalized				Well Capitalized
		Amount		Ratio		Amount		Ratio		Amount		Ratio
	December 31, 2013
	Total risk-based capital:
	Consolidated	$	829,443	16.7	%	$	396,210	8	%	NA		NA
	FIB	723,955		14.7		394,038		8		$	492,548	10	%
	Tier 1 risk-based capital:
	Consolidated	739,246		14.9		198,105		4		NA		NA
	FIB	650,093		13.2		197,019		4		$	295,529	6
	Leverage capital ratio:
	Consolidated	739,246		10.1		293,414		4		NA		NA
	FIB	650,093		8.9		292,199		4		$	365,248	5
	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 revise 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 are effective for the Company on January 1, 2015, subject to a phase-in period for certain provisions.

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	COMMITMENTS AND CONTINGENCIES
	Legal Proceedings:
	FIB was a defendant in a lender liability lawsuit, Kelly Logging Inc. v. First Interstate Bank (the “case"), which was tried in in the Montana Fourth Judicial District, Missoula County in Missoula, Montana (the “court") in August, 2014. On August 14, 2014, a jury awarded damages to Kelly Logging of $17,047, which included $287 in compensatory damages and $16,760 in punitive damages. On October 1, 2014, a non-final judgment was entered in this matter in the amount of $17,047 plus reasonable attorney fees and interest. The non-final judgment is subject to the court's mandatory review of the jury's punitive damages award. The court held oral argument on the punitive damages award and an evidentiary hearing on Kelly Logging's attorneys' fees and costs. No decisions on the punitive damages award or Kelly Logging’s fees and costs have been rendered but FIB's other post-trial motions have been deemed denied for failure of the court to rule on them within the time allowed by the Montana Rules of Civil Procedure.
	The Company intends to continue to defend itself vigorously in this litigation and believes it has valid bases in law and fact to appeal the verdict. The Montana Supreme Court has previously reduced an excessive punitive damage award to an amount within the upper limit of the federal due process guidelines and such guidelines are expected to be applied by the court and, if necessary, by the Montana Supreme Court in this case. Although the Company believes it has meritorious defenses and appellate issues for this litigation, these proceedings are subject to many uncertainties and, given their complexity and scope, the final outcome cannot be predicted and could have a material adverse effect on the consolidated financial condition, results of operations or liquidity of the Company. During 2014, the Company accrued $4,000 of litigation-related loss contingency expense, which takes into consideration the federal due process guidelines related to punitive damage awards and reasonable estimates of the plaintiff's attorneys fees and interest.
	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.
	Other Committments:
	The Company had commitments under construction contracts of $4,047 as of December 31, 2014.
	The Company had commitments to purchase available-for-sale residential mortgage-backed investment securities of $16,840 as of December 31, 2014.
	The Parent Company and the Billings office of FIB are the anchor tenants in a building owned by a partnership in which FIB is one of two partners, and has a 50% partnership interest.
	The Company leases certain premises and equipment from third parties under operating leases. Total rental expense to third parties was $1,190 in 2014, $1,403 in 2013 and $1,423 in 2012.
	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, 2014, are as follows:
		Third		Related		Total
		Parties		Partnership
	For the year ending December 31:
	2015	$	2,132	$	676	$	2,808
	2016	2,236		676		2,912
	2017	1,728		499		2,227
	2018	1,537		188		1,725
	2019	1,487		—		1,487
	Thereafter	9,941		—		9,941
	Total	$	19,061	$	2,039	$	21,100
	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 $4,486 and $5,871 of sold residential mortgage loans with recourse provisions still in effect as of December 31, 2014 and 2013, 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, 2014, 2013 and 2012. 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.

Financial_Instruments_with_Off

Financial Instruments with Off-Balance Sheet Risk	12 Months Ended
	Dec. 31, 2014
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 sheet. 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 $1,415,119 at December 31, 2014, which included $476,752 on unused credit card lines and $363,609 with commitment maturities beyond one year. Commitments to extend credit to borrowers approximated $1,238,269 at December 31, 2013, which included $401,021 on unused credit card lines and $304,789 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, 2014 and 2013, the Company had outstanding stand-by letters of credit of $58,950 and $53,508, 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, 2014
Income Tax Disclosure [Abstract]
Income Taxes	INCOME TAXES
	Income tax expense consists of the following:
	Year ended December 31,	2014			2013			2012
	Current:
	Federal	$	34,941		$	30,757		$	18,458
	State	4,928			4,533			2,818
	Total current	39,869			35,290			21,276
	Deferred:
	Federal	4,870			10,056			7,697
	State	475			1,220			1,065
	Total deferred	5,345			11,276			8,762
	Total income tax expense	$	45,214		$	46,566		$	30,038
	Total income tax expense differs from the amount computed by applying the statutory federal income tax rate of 35 percent in 2014, 2013 and 2012 to income before income taxes as a result of the following:
	Year ended December 31,	2014			2013			2012
	Tax expense at the statutory tax rate	$	45,365		$	46,446		$	30,892
	Increase (decrease) in tax resulting from:
	Tax-exempt income	(4,255		)	(3,620		)	(3,498		)
	State income tax, net of federal income tax benefit	3,541			3,741			2,524
	Other, net	563			(1		)	120
	Tax expense at effective tax rate	$	45,214		$	46,566		$	30,038
	The tax effects of temporary differences between the financial statement carrying amounts and tax bases of assets and liabilities that give rise to significant portions of the net deferred tax asset (liability) relate to the following:
	December 31,	2014			2013
	Deferred tax assets:
	Loans, principally due to allowance for loan losses	$	28,506		$	30,903
	Lease Costs	719			—
	Loss Contingencies	1,785			—
	Loan Discount	7,022			—
	Investment securities, unrealized losses	2,728			9,473
	Employee benefits	8,256			6,813
	Other real estate owned write-downs and carrying costs	2,536			4,731
	Deferred gain on sale of subsidiary	253			484
	Deferred revenue on contract	481			545
	Other	760			299
	Deferred tax assets	53,046			53,248
	Deferred tax liabilities:
	Fixed assets, principally differences in bases and depreciation	(569		)	(4,227		)
	Investment securities, unrealized gains	(1,351		)	—
	Investment in joint venture partnership, principally due to differences in depreciation of partnership assets	(1,053		)	(700		)
	Prepaid amounts	(1,385		)	(1,247		)
	Government agency stock dividends	(2,407		)	(1,965		)
	Goodwill and core deposit intangibles	(36,732		)	(28,167		)
	Mortgage servicing rights	(4,144		)	(4,337		)
	Other	(531		)	(451		)
	Deferred tax liabilities	(48,172		)	(41,094		)
	Net deferred tax assets	$	4,874		$	12,154
	The Company had a current net income tax payable of $4,693 at December 31, 2014 and income tax receivable of $2,968 at December 31, 2013, which are included in accounts payable and accrued expenses.

StockBased_Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2014
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 2006 Equity Compensation Plan (the “2006 Plan”) and the 2001 Stock Option 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 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 the 2001 Stock Option Plan and all other then existing share-based award plans (collectively, the “Previous Plans”). The Previous Plans continue with respect to awards made prior to May 2006. All shares of common stock available for future grant under the Previous Plans were transferred into the 2006 Plan. At December 31, 2014, there were 783,007 common shares available for future grant under the 2006 Plan. All awards granted subsequent to March 29, 2010 are for shares of Class A common stock. All awards granted prior to March 29, 2010 are for shares of Class B 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.
	Compensation expense related to stock option awards of $905, $1,390 and $1,276 was included in benefits on the Company’s consolidated income statements for the years ended December 31, 2014, 2013 and 2012, respectively. Related income tax benefits recognized for the years ended December 31, 2014, 2013 and 2012 were $346, $531 and $488, respectively.
	The weighted average grant date fair value of options granted was $3.54 and $4.06 during the years ended December 31, 2013 and 2012, respectively. The fair value of each option award is estimated on the date of grant using the Black-Scholes option pricing model. No stock option awards were granted in 2014. The following table presents the weighted-average assumptions used in the option pricing model for the periods indicated:
	Years ended December 31,		2013			2012
	Expected volatility		30.61	%		37.46	%
	Expected dividend yield		3	%		3.35	%
	Risk-free interest rate		0.88	%		1.99	%
	Expected life of options (in years)		5.52			7.85
	Expected dividend yield is based on the Company’s annualized expected dividends per share divided by the average common stock price. Risk-free interest rate is based on the U.S. treasury constant maturity yield for treasury securities with maturities approximating the expected life of the options granted on the date of grant. The expected life of options is based on the Company’s historical exercise and post-vesting termination behaviors. Beginning in 2013, the Company used its own historical volatility of common stock for the expected volatility assumption. Prior to that and subsequent to the Company's initial public offering ("IPO"), which concluded on March 29, 2010, the Company expected the historical volatility of its common stock would not be indicative of future volatility. As such, in 2012 the Company estimated expected volatility based on the share price volatility of a peer group of publicly-traded regional banks of similar size and performance as the Company over the expected life of options.
	The following table summarizes stock option activity under the Company’s active stock option plans for the year ended December 31, 2014:
		Number of			Weighted-Average			Weighted-Average
		Shares			Exercise Price			Remaining
								Contract Life
	Outstanding options, beginning of year	2,523,593			$	17.2
	Granted	—			—
	Exercised	(739,290	)		17.73
	Forfeited	(5,893	)		12.85
	Expired	(30,378	)		16.2
	Outstanding options, end of year	1,748,032			$	17.01		4.43 years
	Outstanding options exercisable, end of year	1,393,010			$	17.15		4.12 years
	The total intrinsic value of fully-vested stock options outstanding as of December 31, 2014 was $14,879. The total intrinsic value of options exercised was $7,363, $7,108 and $1,158 during the years ended December 31, 2014, 2013 and 2012, respectively. The actual tax benefit realized for the tax deduction from option exercises totaled $2,594, $1,963 and $397 for the years ended December 31, 2014, 2013 and 2012, respectively. The Company received cash of $6,299, $9,271 and $1,612 from stock option exercises during the years ended December 31, 2014, 2013 and 2012, respectively. The Company redeemed common stock with aggregate values of $6,829, $8,721 and $2,675 tendered in payment for stock option exercises during the years ended December 31, 2014, 2013 and 2012, respectively.
	Information with respect to the Company’s nonvested stock options as of and for the year ended December 31, 2014 follows:
		Number of			Weighted-Average
		Shares			Grant Date Fair Value
	Nonvested stock options, beginning of year	733,906			$	3.78
	Granted	—
	Vested	(348,506	)		3.94
	Forfeited	(30,378	)		3.7
	Nonvested stock options, end of year	355,022			$	3.62
	As of December 31, 2014, there was $577 of unrecognized compensation cost related to nonvested stock options granted under the Company’s active stock option plans. That cost is expected to be recognized over a weighted-average period of 0.86 years. The total fair value of shares vested during 2014 was $1,363.
	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 the restricted shares. Upon termination of employment, common shares upon which restrictions have not lapsed must be returned to the Company.
	Based on the substantive terms of each award, restricted shares are classified as equity or liability awards. The fair value of equity-classified restricted stock awards is being amortized as compensation expense on a straight-line basis over the period restrictions lapse or performance goals are met. Compensation cost for liability-classified awards is expensed each period from the date of grant to the measurement date based on the fair value of the Company’s common stock at the end of each period. Compensation expense related to restricted stock awards of $2,109, $1,546 and $1,209 was included in benefits on the Company’s consolidated statements of income for the years ended December 31, 2014, 2013 and 2012, respectively. Related income tax benefits recognized for the years ended December 31, 2014, 2013 and 2012 were $807, $591 and $462, respectively.
	The following table presents information regarding the Company’s restricted stock as of December 31, 2014:
		Number of			Weighted-Average
		Shares			Measurement Date
					Fair Value
	Restricted stock, beginning of year	225,525			$	16.08
	Granted	148,278			24.36
	Vested	(96,143	)		15.54
	Forfeited	(27,772	)		16.43
	Canceled	(1,489	)		21.39
	Restricted stock, end of year	248,399			$	21.16
	During 2014, the Company issued 148,278 restricted common shares. The 2014 restricted share awards included 73,938 performance restricted shares of which 24,646 vest in varying percentages upon achievement of defined return on asset performance goals, 24,646 vest in varying percentages upon achievement of defined return on equity performance goals and 24,646 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, 2016. Additionally, 74,340 time-restricted shares were issued during 2014 that vest one-third on each annual anniversary of the grant date through February 15, 2017, contingent on continued employment through the vesting date.
	As of December 31, 2014, there was $3,353 of unrecognized compensation cost related to nonvested restricted stock awards expected to be recognized over a period of 1.60 years.

Employee_Benefit_Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2014
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 authorize contributions to the profit sharing plan that are not to exceed, on an individual basis, the lesser of 100% of compensation or $40 annually. Participants become 100% vested upon the completion of three years of vesting service. Accrued contribution expense for this plan of $1,509, $1,555 and $2,063 in 2014, 2013 and 2012, 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. Accrued contribution expense for this plan of $4,256, $4,067 and $4,034 in 2014, 2013 and 2012, respectively, is included in employee benefits expense in the Company’s consolidated statements of income.
	Postretirement 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.
	The Plan’s unfunded benefit obligation of $3,577 and $4,870 as of December 31, 2014 and 2013, respectively, is included in accounts payable and accrued expenses in the Company’s consolidated balance sheets. Net periodic benefit costs of $400, $571 and $561 for the years ended December 31, 2014, 2013 and 2012, respectively, are included in employee benefits expense in the Company’s consolidated statements of income.
	Weighted average actuarial assumptions used to determine the postretirement benefit obligation at December 31, 2014 , and the net periodic benefit costs for the year then ended, included a discount rate of 3.6% and a 5.5% annual increase in the per capita cost of covered healthcare benefits. Weighted average actuarial assumptions used to determine the postretirement benefit obligation at December 31, 2013, and the net periodic benefit costs for the year then ended, included a discount rate of 4.3% and a 5.0% 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 postretirement benefit obligation. Future benefit payments are expected to be $158, $163, $194, $219, $254 and $1,496 for 2015, 2016, 2017, 2018, 2019, and 2020 through 2024, respectively.
	At December 31, 2014, the Company had accumulated other comprehensive loss related to the Plan of $613, or $413 net of related income tax benefit, comprised of net actuarial losses of $257 and an unamortized transition asset of $356. The Company estimates $35 will be amortized from accumulated other comprehensive loss into net period benefit costs in 2015.

Other_Comprehensive_Income

Other Comprehensive Income	12 Months Ended
	Dec. 31, 2014
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, 2014	Before Tax Amount			Tax Expense (Benefit)			Net of Tax Amount
	Investment securities available-for sale:
	Change in net unrealized gain during period	$	21,147		$	8,321		$	12,826
	Reclassification adjustment for net gains included in net income	(61		)	(24		)	(37		)
	Change in unamortized loss on available-for-sale securities transferred into held-to-maturity	(548		)	(216		)	(332		)
	Defined benefits post-retirement benefit plan:
	Change in net actuarial loss	1,731			681			1,050
	Total other comprehensive income	$	22,269		$	8,762		$	13,507
	Year ended December 31, 2013	Before Tax Amount			Tax Expense (Benefit)			Net of Tax Amount
	Investment securities available-for sale:
	Change in net unrealized loss during period	$	(52,949	)	$	(20,835	)	$	(32,114	)
	Reclassification adjustment for net gains included in net income	(1		)	—			(1		)
	Defined benefits post-retirement benefit plan:
	Change in net actuarial loss	137			54			83
	Total other comprehensive loss	$	(52,813	)	$	(20,781	)	$	(32,032	)
	Year ended December 31, 2012	Before Tax Amount			Tax Expense (Benefit)			Net of Tax Amount
	Investment securities available-for sale:
	Change in net unrealized gain during period	$	(4,648	)	$	(1,829	)	$	(2,819	)
	Reclassification adjustment for net gains included in net income	(348		)	(137		)	(211		)
	Change in unamortized gain on available-for-sale securities transferred into held-to-maturity	56			22			34
	Defined benefits post-retirement benefit plan:
	Change in net actuarial loss	(77		)	(30		)	(47		)
	Total other comprehensive loss	$	(5,017	)	$	(1,974	)	$	(3,043	)
	The components of accumulated other comprehensive income, net of income taxes, are as follows:
	Year ended December 31,	2014				2013
	Net unrealized gain on investment securities available-for-sale	$	(2,121	)		$	(14,578	)
	Net actuarial loss on defined benefit post-retirement benefit plans	(413		)		(1,463		)
	Net accumulated other comprehensive loss	$	(2,534	)		$	(16,041	)

NonCash_Investing_and_Financin

Non-Cash Investing and Financing Activities	12 Months Ended
	Dec. 31, 2014
Supplemental Cash Flow Information [Abstract]
Non-Cash Investing and Financing Activities	NON-CASH INVESTING AND FINANCING ACTIVITIES
	The Company transferred loans of $5,198, $11,545 and $43,541 to other real estate owned in 2014, 2013 and 2012, respectively.
	The Company transferred internally originated mortgage servicing assets of $2,717, $3,581 and $4,563 from loans to mortgage servicing assets in 2014, 2013 and 2012, respectively.
	During 2014, the Company issued 1,378,230 shares of its Class A common stock valued at $35,972 as partial consideration for the acquisition of MWFC.
	The Company transferred real property pending disposal of $1,448 and $566 to other assets in 2013 and 2012, respectively.
	The Company reclassified tax credit investments of $429 from held-to-maturity investment securities to other assets during 2013.
	During 2012, the Company reclassified $50,000 of perpetual preferred stock pending redemption from equity to a liability.

Condensed_Financial_Informatio

Condensed Financial Information (Parent Company Only)	12 Months Ended
	Dec. 31, 2014
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,	2014			2013
	Condensed balance sheets:
	Cash and cash equivalents	$	65,483		$	105,274
	Investment in subsidiaries, at equity:
	Bank subsidiary	936,817			793,892
	Nonbank subsidiaries	1,982			1,980
	Total investment in subsidiaries	938,799			795,872
	Advances to subsidiaries, net	4,337			—
	Other assets	29,335			26,809
	Total assets	$	1,037,954		$	927,955
	Other liabilities	$	26,553		$	17,602
	Advances from subsidiaries, net	—			6,295
	Long-term debt	20,000			20,000
	Subordinated debentures held by subsidiary trusts	82,477			82,477
	Total liabilities	129,030			126,374
	Stockholders’ equity	908,924			801,581
	Total liabilities and stockholders’ equity	$	1,037,954		$	927,955
	Years Ended December 31,	2014			2013			2012
	Condensed statements of income:
	Dividends from subsidiaries	$	58,900		$	71,400		$	40,000
	Other interest income	42			34			92
	Other income, primarily management fees from subsidiaries	12,166			12,809			10,042
	Total income	71,108			84,243			50,134
	Salaries and benefits	15,722			15,914			13,205
	Interest expense	4,002			4,098			6,691
	Acquisition expenses	4,017			—			—
	Other operating expenses, net	7,512			7,546			7,150
	Total expenses	31,253			27,558			27,046
	Earnings before income tax benefit	39,855			56,685			23,088
	Income tax benefit	(6,862		)	(5,703		)	(6,222		)
	Income before undistributed earnings of subsidiaries	46,717			62,388			29,310
	Undistributed earnings of subsidiaries	37,684			23,748			28,914
	Net income	$	84,401		$	86,136		$	58,224
	Years Ended December 31,	2014			2013			2012
	Condensed statements of cash flows:
	Cash flows from operating activities:
	Net income	$	84,401		$	86,136		$	58,224
	Adjustments to reconcile net income to cash provided by operating activities:
	Undistributed earnings of subsidiaries	(37,684		)	(23,748		)	(28,914		)
	Stock-based compensation expense	3,014			2,936			2,485
	Tax benefits from stock-based compensation	2,193			1,898			360
	Excess tax benefits from stock-based compensation	(2,205		)	(2,031		)	(273		)
	Other, net	8,991			(5,804		)	3,327
	Net cash provided by operating activities	58,710			59,387			35,209
	Cash flows from investing activities:
	Capital expenditures, net of sales	—			—			1
	Acquisition of bank holding company, net of cash and cash equivalents received	(37,891		)	—			—
	Net cash provided by (used in) investing activities	$	(37,891	)	$	—		$	1
	Years Ended December 31,	2014			2013			2012
	Condensed statements of cash flows (continued):
	Cash flows from financing activities:
	Net (decrease) increase in advances from nonbank subsidiaries	$	(10,632	)	$	6,992		$	(2,838	)
	Redemption of preferred stock	—			(50,000		)	—
	Repayment of junior subordinated debentures held by subsidiary trusts	(20,439		)	—			(41,238		)
	Proceeds from issuance of common stock, net of stock issuance costs	6,621			9,814			1,911
	Excess tax benefits from stock-based compensation	2,205			2,031			273
	Purchase and retirement of common stock	(9,739		)	(448		)	(263		)
	Dividends paid to common stockholders	(28,626		)	(17,909		)	(26,208		)
	Dividends paid to preferred stockholders	—			—			(3,300		)
	Net cash used in financing activities	(60,610		)	(49,520		)	(71,663		)
	Net change in cash and cash equivalents	(39,791		)	9,867			(36,453		)
	Cash and cash equivalents, beginning of year	105,274			95,407			131,860
Cash and cash equivalents, end of year	$	65,483		$	105,274		$	95,407
Noncash Investing and Financing Activities — During 2012, the Company reclassified $50,000 of perpetual preferred stock pending redemption from equity to a liability.

Fair_Value_Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Fair Value Measurements	FAIR VALUE MEASUREMENTS
	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, 2014	Balance		Quoted Prices in			Significant Other			Significant
				Active Markets for			Observable			Unobservable
				Identical Assets			Inputs			Inputs
				(Level 1)			(Level 2)			(Level 3)
	Investment securities available-for-sale:
	Obligations of U.S. government agencies	$	720,933	$	—		$	720,933		$	—
	U.S. agency mortgage-backed securities & collateralized mortgage obligations	990,666		—			990,666			—
	Private mortgage-backed securities	325		—			325			—
				Fair Value Measurements at Reporting Date Using
	As of December 31, 2013	Balance		Quoted Prices in			Significant Other			Significant
				Active Markets for			Observable			Unobservable
				Identical Assets			Inputs			Inputs
				(Level 1)			(Level 2)			(Level 3)
	Investment securities available-for-sale:
	Obligations of U.S. government agencies	$	763,238	$	—		$	763,238		$	—
	U.S. agency mortgage-backed securities & collateralized mortgage obligations	1,184,053		—			1,184,053			—
	Private mortgage-backed securities	415		—			415			—
	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. The Company obtains fair value measurements for investment securities from an independent pricing service and evaluates mortgage servicing rights for impairment using an independent valuation service. The 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. 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 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.
	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, 2014	Total		Quoted Prices			Significant			Significant		Total
				in Active			Other			Unobservable		Gains (Losses)
				Markets for			Observable			Inputs
				Identical Assets (Level 1)			Inputs			(Level 3)
							(Level 2)
	Impaired loans	$	30,494	$	—		$	—		$	30,494	$	(14,552	)
	Other real estate owned	4,554		—			—			4,554		(12,665		)
	Long-lived assets to be disposed of by sale	1,083		—			—			1,083		(702		)
				Fair Value Measurements at Reporting Date Using
	As of December 31, 2013	Total		Quoted Prices			Significant			Significant		Total
				in Active			Other			Unobservable		Gains (Losses)
				Markets for			Observable			Inputs
				Identical Assets			Inputs			(Level 3)
				(Level 1)			(Level 2)
	Impaired loans	$	57,302	$	—		$	—		$	57,302	$	(23,224	)
	Other real estate owned	8,502		—			—			8,502		(14,441		)
	Long-lived assets to be disposed of by sale	1,186		—			—			1,186		(599		)
	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, 2014, certain impaired loans with a carrying value of $45,046 were reduced by specific valuation allowance allocations of $5,792 and partial loan charge-offs of $8,760 resulting in a reported fair value of $30,494. As of December 31, 2013, certain impaired loans with a carrying value of $80,526 were reduced by specific valuation allowance allocations of $8,929 and partial loan charge-offs of $14,295 resulting in a reported fair value of $57,302.
	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, 2014, long-lived assets to be disposed of by sale with carrying values of $1,785 that were reduced by write-downs of $702 charged to other expense resulting in a reported fair value of $1,083. As of December 31, 2013, the Company had long-lived assets to be disposed of by sale of $1,785 that were reduced by write-downs of $599 charged to other expense resulting in a reported fair value of $1,186.
	In addition, mortgage loans held for sale are required to be measured at the lower of cost or fair value. The fair value of mortgage loans held for sale is based upon binding contracts or quotes or bids from third party investors. As of December 31, 2014 and 2013, all mortgage loans held for sale were recorded at cost.
	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, 2014	Fair		Valuation	Unobservable	Range
		Value		Technique	Inputs	(Weighted Average)
	Impaired loans	$	30,494	Appraisal	Appraisal adjustment	0%	-	51%	-19%
	Other real estate owned	4,554		Appraisal	Appraisal adjustment	0%	-	50%	-15%
	Long-lived assets to be disposed of by sale	1,083		Appraisal	Appraisal adjustment	0%	-	9%	-5%
	As of December 31, 2013	Fair		Valuation	Unobservable	Range
		Value		Technique	Inputs	(Weighted Average)
	Impaired loans	$	57,302	Appraisal	Appraisal adjustment	6%	-	66%	-31%
	Other real estate owned	8,502		Appraisal	Appraisal adjustment	6%	-	55%	-15%
	Long-lived assets to be disposed of by sale	1,186		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 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 floating rate subordinated debenture, floating rate subordinated term loan, fixed rate notes payable, fixed rate subordinated term debt, fixed rate subordinated debentures and capital lease obligation are estimated by discounting future cash flows using current rates for advances with similar characteristics. The carrying value of the preferred stock pending redemption approximates fair value due to the short-term nature of this instrument.
	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, 2014	Carrying Amount		Estimated			Quoted Prices in			Significant Other		Significant
Fair Value				Active Markets for			Observable		Unobservable
				Identical Assets			Inputs		Inputs
				(Level 1)			(Level 2)		(Level 3)
Financial assets:
Cash and cash equivalents	$	798,670	$	798,670		$	798,670		$	—	$	—
Investment securities available-for-sale	1,711,924		1,711,924			—			1,711,924		—
Investment securities held-to-maturity	575,186		584,533			—			584,533		—
Accrued interest receivable	27,063		27,063			—			27,063		—
Mortgage servicing rights, net	14,038		21,434			—			21,434		—
Net loans	4,823,243		4,800,725			—			4,770,231		30,494
Total financial assets	$	7,950,124	$	7,944,349		$	798,670		$	7,115,185	$	30,494
Financial liabilities:
Total deposits, excluding time deposits	$	5,767,992	$	5,767,992		$	5,767,992		$	—	$	—
Time deposits	1,238,220		1,244,324			—			1,244,324		—
Securities sold under repurchase agreements	502,250		502,250			—			502,250		—
Other borrowed funds	9		9			—			9		—
Accrued interest payable	5,833		5,833			—			5,833		—
Long-term debt	38,067		37,781			—			37,781		—
Subordinated debentures held by subsidiary trusts	82,477		75,734			—			75,734		—
Total financial liabilities	$	7,634,848	$	7,633,923		$	5,767,992		$	1,865,931	$	—
						Fair Value Measurements at Reporting Date Using
As of December 31, 2013	Carrying Amount		Estimated			Quoted Prices in			Significant Other		Significant
			Fair Value			Active Markets for			Observable		Unobservable
						Identical Assets			Inputs		Inputs
						(Level 1)			(Level 2)		(Level 3)
Financial assets:
Cash and cash equivalents	$	534,827	$	534,827		$	534,827		$	—	$	—
Investment securities available-for-sale	1,947,706		1,947,706			—			1,947,706		—
Investment securities held-to-maturity	203,837		205,926			—			205,926		—
Accrued interest receivable	26,450		26,450			—			26,450		—
Mortgage servicing rights, net	13,546		25,698			—			25,698		—
Net loans	4,259,514		4,246,539			—			4,189,237		57,302
Total financial assets	$	6,985,880	$	6,987,146		$	534,827		$	6,395,017	$	57,302
Financial liabilities:
Total deposits, excluding time deposits	$	4,943,033	$	4,943,033		$	4,943,033		$	—	$	—
Time deposits	1,190,717		1,196,250			—			1,196,250		—
Securities sold under repurchase agreements	457,437		457,437			—			457,437		—
Other borrowed funds	3		3			—			3		—
Accrued interest payable	4,963		4,963			—			4,963		—
Long-term debt	36,917		34,508			—			34,508		—
Subordinated debentures held by subsidiary trusts	82,477		72,045			—			72,045		—
Total financial liabilities	$	6,715,547	$	6,708,239		$	4,943,033		$	1,765,206	$	—

Related_Party_Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2014
Related Party Transactions [Abstract]
Related Party Transactions	RELATED PARTY TRANSACTIONS
	The Company conducts banking transactions in the ordinary course of business with related parties, including directors, executive officers, shareholders and their associates, on the same terms as those prevailing at the same time for comparable transactions with unrelated persons and that do not involve more than a normal risk of collectibility or present other unfavorable features.
	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 $25,794 and $24,669 at December 31, 2014 and 2013, respectively. During 2014, new loans and advances on existing loans of $9,519 were funded and loan repayments totaled $9,342. 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.
	During 2014, the Company repurchased an aggregate of 37,615 shares of its Class A common stock from two directors and one greater than 10% shareholder of the Company at a weighted average price of $24.96 per share. The repurchases were made in privately negotiated transactions pursuant to a stock repurchase program approved by the Company's Board of Directors in November 2013. For additional information regarding the Company's stock repurchases, see Note 13 - Capital Stock and Dividend Restrictions.
	The Company purchases property, casualty and other insurance through an agency in which a director of the Company, whose term as a director ended in May 2013, has a controlling ownership interest. The Company paid insurance premiums to the agency of $764 and $839 in 2013 and 2012, respectively.
	The Company leases an aircraft from an entity wholly-owned by the chairman of the Company’s Board of Directors. 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 2014, 2013 and 2012, the Company paid total fees and operating expenses of $306, $309 and $262, respectively, for its use of the aircraft. In addition, the Company leases a portion of its hanger and provides pilot services to the related entity. During 2014, 2013 and 2012, the Company received payments from the related entity of $77, $61 and $47, respectively, for hanger use, pilot fees and reimbursement of certain third party operating expenses related to the chairman’s personal use of the aircraft.
	The Company purchases services from an entity in which one greater than 5% shareholder and six directors of the Company, including the chairman and executive vice chairman of the Board of Directors, have an aggregate ownership interest of 18%, and in which one director is the chairman and two 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 2014, 2013 and 2012, the Company paid $255, $224 and $243, respectively, for these services.
	During 2012, the Company entered into a contract with an entity wholly-owned by a director of the Company to provide construction management and advisory services related to the construction of a bank office building. Under the terms of the agreement, the entity will receive payments equal to the lesser of a fixed price of $180 or 4% of the actual construction contract price, with an initial payment of $60 due upon execution of the agreement and the remainder due in two equal installments over a two year period. Under the terms of the contract, the Company paid the entity $60 during each of 2014, 2013 and 2012.
	In conjunction with an acquisition in 2008, the Company assumed certain existing deferred compensatory agreements. Under the terms of one such agreement, the Company is required to make cash payments to a director of the Company for the promotion of growth and development of new business through December 31, 2011. The total amount due under the agreement was fixed prior to the acquisition date at $577, with a portion to be paid over 4 years and the remaining balance due in January 2012. As additional consideration under the agreement, the director provided, among other things, a covenant not to compete. Under the terms of the agreement, the Company made cash payments of $424 during 2012.
	A director of the Company was party to an agreement to guarantee the payment of interest on loans between FIB and an unrelated third party borrower through December 31, 2012. Under the terms of the interest guaranty agreement, the director made interest payments to FIB on behalf of the borrower of $815 in 2012. In addition, the director pledged to FIB collateral for the loans of the unrelated third party borrower. During 2012, the collateral was liquidated and proceeds of $7,998 were applied to the outstanding principal balances of the loans.

Authoritative_Accounting_Guida

Authoritative Accounting Guidance	12 Months Ended
	Dec. 31, 2014
Accounting Changes and Error Corrections [Abstract]
Authoritative Accounting Guidance	AUTHORITATIVE ACCOUNTING GUIDANCE
	ASU 2014-01 “Investments-Equity Method and Joint Ventures (Topic 323): Accounting for Investments in Qualified Affordable Housing Projects.” The amendments in ASU 2014-01 permit reporting entities to make an accounting policy election to account for their investments in qualified affordable housing projects using the proportional amortization method if certain conditions are met. The evaluation of whether conditions have been met to apply the proportional amortization method is conducted at the time of initial investment and subsequently reevaluated if there is a change in the nature of the investment or a change in the relationship with the limited liability entity that could result in the conditions no longer being met. The decision to apply the proportional amortization method of accounting should be applied consistently to all qualifying affordable housing project investments rather than on an individual investment basis. Under the proportional amortization method, an entity amortizes the initial cost of the investment in proportion to the tax credits and other tax benefits received and recognizes the net investment performance in the income statement as a component of income tax expense or benefit. The amendments in ASU 2014-01 are effective for the Company for annual reporting periods and interim reporting periods within those annual periods, beginning after December 15, 2014, with early adoption permitted.
	The Company adopted the amendments in ASU 2014-01 effective January 1, 2014. As of December 31, 2014, the Company had four investments in qualified affordable housing projects with an aggregate carrying value of $5,084 included in other assets on the Company's consolidated balance sheet. The Company elected to account for these investments using the proportional amortization method. During 2014, income tax benefits associated with these projects of $27 were recognized as a component of income tax expense. The Company has commitments to invest additional amounts in these projects of $6,943 in 2015, $37 annually in 2016 through 2021, and $26 annually in 2022 and 2023.
	ASU 2014-04 “Receivables-Troubled Debt Restructurings by Creditors (Subtopic 310-40): Reclassification of Residential Real Estate Collateralized Consumer Mortgage Loans upon Foreclosure.” The amendments in ASU 2014-04 clarify that an in-substance repossession or foreclosures occurs, and a creditor is considered to have received physical possession of residential real estate property collateralizing a consumer mortgage loan, upon either (i) the creditor obtaining legal title to the residential real estate property upon completion of a foreclosure or (ii) the borrower conveying all interest in the residential real estate property to the creditor to satisfy the loan through completion of a deed in lieu of foreclosure or through a similar legal agreement. The amendments in ASU 2014-04 also require interim and annual disclosure of both (i) the amount of foreclosed residential real estate property held by the creditor and (ii) the recorded investment in consumer mortgage loans collateralized by residential real estate property that are in the process of foreclosure. The amendments in ASU 2014-04 are effective for the Company for annual periods, and interim periods within those annual periods, beginning after December 15, 2014 using either a modified retrospective transition method or a prospective transition method. The Company does not expect the amendments in ASU 2014-04 to have a material impact on the Company’s consolidated financial statements, results of operations or liquidity.
	ASU 2014-09 "Revenue from Contracts with Customers." The amendments in ASU 2014-09 introduce 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. The amendments in ASU 2014-09 are effective for the Company for fiscal years beginning after December 15, 2016, including interim periods within that reporting period, and may be adopted retrospectively to each prior reporting period presented or retrospectively with the cumulative effect of initial adoption recognized at the date of initial application. The Company is currently evaluating the new guidance to determine the impact it will have on its consolidated financial statements, results of operations or liquidity.
	ASU 2014-11 "Repurchase-to-Maturity Transactions, Repurchase Financings, and Disclosures." The amendments in ASU 2014-11 expand secured borrowing treatment for certain repurchase agreements. Under the amendments in ASU 2014-11, repurchase-to-maturity transactions and repurchase agreements executed as repurchase financing transactions are required to be accounted for as secured borrowings. ASU 2014-11 requires additional disclosures about certain transactions accounted for as a sale in which the transferor retains substantially all of the exposure to the economic return on the transferred financial assets through an agreement with the same counterparty. ASU 2014-11 also requires disclosure of the types of collateral pledged and liabilities associated with an entity's repurchase agreements, securities lending transactions and repurchase-to-maturity transactions accounted for as secured borrowings. The accounting changes included in the amendments in ASU 2014-11 are effective for the Company on January 1, 2015. The disclosure requirements set forth in the amendment in ASU 2014-11 are effective for the Company for interim and annual periods beginning after December 31, 2014. Adoption of the amendments in ASU 2014-11 will not have a material impact on the Company’s consolidated financial statements, results of operations or liquidity.
	ASU 2014-12 "Accounting for Share-Based Payments When the Terms of an Award Provide That a Performance Target Could be Achieved after the Requisite Service Period." ASU 2014-12 amends Accounting Standards Codification ("ASC") Topic 718, Compensation-Stock Compensation, to clarify that a performance target that affects the vesting of a share-based payment award and that could be achieved after the requisite service period should be treated as a performance condition that affects the vesting of the award. ASU 2014-12 further clarifies that the requisite service period ends when the employees can cease rendering service and still be eligible to vest in the award if the performance target is achieved. The amendments in ASU 2014-12 are effective for annual periods and interim periods within those annual periods beginning after December 15, 2015. The amendments in ASU 2014-12 may be applied prospectively to all awards granted or modified after the effective date or retrospectively to all awards with performance targets that are outstanding as of the beginning of the earliest annual period presented in the financial statements and to all new or modified awards thereafter. The amendments in ASU 2014-12 will not have a material impact on the Company’s consolidated financial statements, results of operations or liquidity.
	ASU 2014-14 "Classification of Certain Government-Guaranteed Mortgage Loans upon Foreclosure." ASU 2014-14 updates ASC Subtopic 310-40, Receivables-Troubled Debt Restructurings by Creditors, to require that a mortgage loan be derecognized and that a separate other receivable be recognized upon foreclosure if (i) the loan has a government guarantee that is not separable from the loan before foreclosure; (ii) at the time of foreclosure, the creditor has the intent to convey the real estate property to the guarantor and make a claim on the guarantee, and the creditor has the ability to recover under that claim; and, (iii) any amount of the claim that is determined on the basis of the fair value of the real estate is fixed at the time of foreclosure. ASU 2014-14 provides that, upon foreclosure, the separate other receivable should be measured based on the amount of the loan balance (principal and interest) expected to be recovered from the guarantor. The amendments in ASU 2014-14 are effective for the Company for annual periods, and interim periods within those annual periods, beginning after December 15, 2014 and may adopted using either a prospective transition method or a modified retrospective transition method. The amendments in ASU 2014-12 will not have a material impact on the Company’s consolidated financial statements, results of operations or liquidity.
	ASU 2014-16 "Determining Whether the Host Contract in a Hybrid Financial Instrument Issued in the Form of a Share is More Akin to Debt or to Equity." The amendments in ASU 2014-16 clarify that an entity should consider all relevant terms and features-including the embedded derivative feature being evaluated for bifurcation-in evaluating the nature of the host contract within a hybrid financial instrument. The amendments further clarify that no single term or feature would necessarily determine the economic characteristics and risk of the host contract. Rather, the nature of the host contract depends upon the economic characteristics and risk of the entire hybrid financial instrument. The amendments in ASU 2014-16 are effective for the Company for annual periods, and interim periods within those annual periods, beginning after December 15, 2015. The effects of initially adopting the amendments in ASU 2014-16 should be applied on a modified retrospective basis to existing hybrid financial instruments issued in the form of a share as of the beginning of the fiscal year for which the amendments are effective. Retrospective application is permitted to all relevant prior periods. The amendments in ASU 2014-16 will not have a material impact on the Company’s consolidated financial statements, results of operations or liquidity.

Subsequent_Events

Subsequent Events	12 Months Ended
	Dec. 31, 2014
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 22, 2015, the Company declared a quarterly dividend to common shareholders of $0.20 per share, paid on February 13, 2015 to shareholders of record as of February 2, 2015. On January 22, 2015, the Company's board of directors approved the repurchase of up to 1,000,000 shares of the Company's outstanding Class A common stock from time to time through open market or privately negotiated transactions, as market and business conditions permit. Share repurchases will be conducted in a manner intended to comply with the safe harbor provisions of Rule 10b-18 promulgated under the Securities Exchange Act of 1934, as amended. Repurchased shares will be returned to authorized but unissued shares of Class A common stock in accordance with Montana law. No other events requiring recognition or disclosure were identified.

Summary_of_Significant_Account1

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
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, 2014, 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 2013 and 2012 to conform to the 2014 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	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
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. As of December 31, 2014 and 2013, the Company had cash of $631,562 and $392,413, respectively, on deposit with the Federal Reserve Bank. In addition, the Company maintained compensating balances with the Federal Reserve Bank of approximately $7,507 and $1,412 as of December 31, 2014 and 2013, respectively, to reduce service charges for check clearing services.
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 (losses). 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.
	The Company invests in securities on behalf of certain officers and directors of the Company who have elected to participate in the Company’s deferred compensation plans. These securities are included in other assets and are carried at their fair value based on quoted market prices. Net realized and unrealized holding gains and losses are included in other non-interest income and employee benefits expense.
Loans and Allowance for Loan Losses	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 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 investment 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.
	Included in loans are certain residential mortgage loans originated for sale. These loans are carried at the lower of aggregate cost or estimated market value. Market value is estimated based on binding contracts or quotes or bids from third party investors. Residential mortgages held for sale were $40,828 and $40,861 as of December 31, 2014 and 2013, respectively. Gains and losses on sales of mortgage loans are determined using the specific identification method and are included in income from the origination and sale of loans.
	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 collectibility 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 analyses 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.
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 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 $24,652 as of December 31, 2014 and $22,401 as of December 31, 2013. Amortization expense related to core deposit intangibles recorded as of December 31, 2014 is expected to total $3,336, $3,099, $1,825, $1,318, 1,118, and 2,587 in 2015, 2016, 2017, 2018, 2019, and thereafter, respectively.
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.
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. Impairment losses of $102, $616 and $70 were recognized in other non-interest expense in 2014, 2013 and 2012, respectively.
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. Write-downs of $224, $3,512 and $6,724 were recorded in 2014, 2013 and 2012, respectively.
Restricted Equity Securities	Restricted Equity Securities. The Company, as a member of the Federal Reserve Bank and the Federal Home Loan Bank (“FHLB”), is required to maintain investments in each of the organization’s capital stock. As of December 31, 2014, restricted equity securities of the Federal Reserve Bank and the Federal Home Loan Bank of $16,187 and $10,662, respectively, were included in other assets at cost. As of December 31, 2013, restricted equity securities of the Federal Reserve Bank and the Federal Home Loan Bank were $13,357 and $7,003, 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 2014, 2013 or 2012.
Derivatives and Hedging Activities	Derivatives and Hedging Activities. During 2014, the Company entered into derivative interest rate swap agreements as part of its interest rate risk management strategy. The interest rate swaps are recognized as assets or liabilities on the Company's balance sheet at fair value. Fair value estimations are obtained from third parties and are based on pricing models. Currently, none of the Company’s derivatives are designated in qualifying hedging relationships. As such, all changes in the fair value of the Company’s derivatives are recognized directly in earnings. As of December 31, 2014, interest rate swap derivative assets of $61 were included in other assets and interest rate swap derivative liabilities of $59 were included in other liabilities on the Company's consolidated balance sheet. During 2014, the Company recorded insignificant amounts of non-interest income related to changes in the fair value of derivatives. The Company does not enter into derivative agreements for trading or speculative purposes.
Income From Fiduciary Activities	Income from Fiduciary Activities. Consistent with industry practice, income for trust services is recognized on the basis of cash received. However, use of this method in lieu of accrual basis accounting does not materially affect reported earnings.
Earnings Per Common Share
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 income statement. With few exceptions, the Company is no longer subject to U.S. federal and state examinations by tax authorities for years before 2011. The Company had no accrued interest or penalties as of December 31, 2014, 2013 or 2012.
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, 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. Advertising expense was $3,734, $3,532, and $3,555 in 2014, 2013 and 2012, respectively.
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. Stock-based compensation expense of $3,014, $2,936 and $2,485 for the years ended December 31, 2014, 2013 and 2012, 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, 2014, 2013 and 2012 were $1,153, $1,122 and $950, respectively. All compensation cost for stock-based awards is expensed at the Parent Company.
Fair Value of Financial Instruments, Policy	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, 2014
Business Combinations [Abstract]
Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed
		As Recorded			Fair Value				As Recorded
	As of July 31, 2014	by MWFC			Adjustments				by the Company
	Assets acquired:
	Cash and cash equivalents	$	74,035		$	—			$	74,035
	Investment securities	104,945			(34		)	-1	104,911
	Loans	378,558			(18,286		)	-2	360,272
	Allowance for loan losses	(11,598		)	11,598			-3	—
	Premises and equipment	35,283			(5,847		)	-4	29,436
	Company-owned life insurance	13,046			—				13,046
	Deferred tax asset, net	6,491			1,135			-5	7,626
	Core deposit intangible	—			11,014			-6	11,014
	Other assets	16,559			(5,300		)	-7	11,259
	Total assets acquired	617,319			(5,720		)		611,599
	Liabilities assumed:
	Deposits	515,538			(159		)	-8	515,379
	Other liabilities	20,501			2,730			-9	23,231
	Subordinated debentures held by subsidiary trusts	20,439			—			-10	20,439
	Total liabilities assumed	556,478			2,571				559,049
	Net assets acquired	$	60,841		$	(8,291	)		52,550
	Consideration paid:
	Cash								38,479
	Class A common stock								35,972
	Total consideration								74,451
	Goodwill								$	21,901
	Explanation of fair value adjustments:
	-1	Write down of the book value of investment securities 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. Except for collateral dependent loans acquired with deteriorated credit quality, the fair value of 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 the 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 MWB allowance for loan losses at acquisition date as the credit risk is accounted for in the fair value adjustment for loans receivable described in (2) above.
	-4	Write down of the book value of premises and equipment to their estimated fair values based upon appraisals obtained from an independent third party appraiser.
	-5	Adjustment represents the net deferred tax assets resulting from fair value adjustments related to acquired assets, assumed liabilities, core deposit intangible assets and other purchase accounting adjustments.
	-6	Adjustment represents the value of the core deposit base assumed in the acquisition based upon a valuation obtained from an independent third party valuation expert.
	-7	Adjustment consists of a reduction in the value of equity method investments, accrued interest receivable and accrued net income taxes receivable, and the write-off of pre-existing goodwill and computer software costs.
	-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.
	-9	Adjustment represents increases in the book values of other liabilities to their estimated fair values at the acquisition date. The adjustment primarily consists of a $2,000 increase to an acquired operating lease obligation and a $473 increase to Federal Home Loan Bank borrowings based upon interest interest rates of similar obligations with similar characteristics on the date of acquisition.
	-10	Recorded value of junior subordinated debentures held by subsidiary trusts approximated fair value as of the acquisition date due to the short-term nature of the instruments. These debentures were redeemed at par value in December 2014.
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities Acquired During Period
	Contractually required principal and interest payments	$	112,882
	Contractual cash flows not expected to be collected ("non-accretable discount")	74,760
	Cash flows expected to be collected	38,122
	Interest component of cash flows expected to be collected ("accretable discount")	5,233
	Fair value of acquired credit-impaired loans	$	32,889
	Information regarding acquired loans not deemed credit-impaired at the acquisition date is as follows:
	Contractually required principal and interest payments	$	445,345
	Contractual cash flows not expected to be collected	(15,090		)
	Fair value at acquisition	327,383
Business Acquisition, Pro Forma Information
	Year ended December 31, (unaudited)	2014			2013
	Interest income	$	280,102		$	280,455
	Non-interest income	114,984			120,005
	Total revenues	$	395,086		$	400,460
	Net income	$	87,129		$	84,264

Investment_Securities_Tables

Investment Securities (Tables)	12 Months Ended
	Dec. 31, 2014
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, 2014	Amortized		Gross			Gross			Estimated
		Cost		Unrealized			Unrealized			Fair
				Gains			Losses			Value
	Available-for-Sale
	Obligations of U.S. government agencies	$	725,408	$	895		$	(5,370	)	$	720,933
	U.S. agency residential mortgage-backed securities &	982,764		11,526			(3,624		)	990,666
	collateralized mortgage obligations
	Private mortgage-backed securities	322		5			(2		)	325
	Total	$	1,708,494	$	12,426		$	(8,996	)	$	1,711,924
	December 31, 2014	Amortized		Gross			Gross			Estimated
		Cost		Unrealized			Unrealized			Fair
				Gains			Losses			Value
	Held-to Maturity
	State, county and municipal securities	$	188,941	$	5,949		$	(386	)	$	194,504
	Corporate securities	32,565		54			(75		)	32,544
	U.S agency residential mortgage-backed securities &	353,176		5,563			(1,758		)	356,981
	collateralized mortgage obligations
	Other investments	504		—			—			504
	Total	$	575,186	$	11,566		$	(2,219	)	$	584,533
	Gross gains of $274 and gross losses of $213 were realized on the disposition of available-for-sale securities in 2014.
	December 31, 2013	Amortized		Gross			Gross			Estimated
		Cost		Unrealized			Unrealized			Fair
				Gains			Losses			Value
	Available-for-Sale
	Obligations of U.S. government agencies	$	774,055	$	1,432		$	(12,249	)	$	763,238
	U.S. agency residential mortgage-backed securities &	1,197,295		11,905			(25,147		)	1,184,053
	collateralized mortgage obligations
	Private mortgage-backed securities	407		9			(1		)	415
	Total	$	1,971,757	$	13,346		$	(37,397	)	$	1,947,706
	December 31, 2013	Amortized		Gross			Gross			Estimated
		Cost		Unrealized			Unrealized			Fair
				Gains			Losses			Value
	Held-to Maturity
	State, county and municipal securities	$	185,818	$	4,043		$	(2,049	)	$	187,812
	Corporate securities	18,019		103			(8		)	$	18,114
	Total	$	203,837	$	4,146		$	(2,057	)	$	205,926
Gross Unrealized Losses and Fair Values of Investment Securities	The following table shows 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, 2014 and 2013.
		Less than 12 Months					12 Months or More						Total
	December 31, 2014	Fair		Gross			Fair			Gross			Fair		Gross
		Value		Unrealized			Value			Unrealized			Value		Unrealized
				Losses						Losses					Losses
	Available-for-Sale
	Obligations of U.S. government agencies	$	135,888	$	(702	)	$	309,283		$	(4,668	)	$	445,171	$	(5,370	)
	U.S. agency residential mortgage-backed	219,214		(887		)	151,380			(2,737		)	370,594		(3,624		)
	securities & collateralized mortgage
	obligations
	Private mortgage-backed securities	—		—			90			(2		)	90		(2		)
	Total	$	355,102	$	(1,589	)	$	460,753		$	(7,407	)	$	815,855	$	(8,996	)
		Less than 12 Months					12 Months or More						Total
	December 31, 2014	Fair		Gross			Fair			Gross			Fair		Gross
		Value		Unrealized			Value			Unrealized			Value		Unrealized
				Losses						Losses					Losses
	Held-to-Maturity
	State, county and municipal securities	$	7,979	$	(13	)	$	20,097		$	(373	)	$	28,076	$	(386	)
	U.S. agency residential mortgage-backed	61,201		(1,758		)	—			—			61,201		(1,758		)
	securities & collateralized mortgage
	obligations
	Corporate securities	14,755		(75		)	—			—			14,755		(75		)
	Total	$	83,935	$	(1,846	)	$	20,097		$	(373	)	$	104,032	$	(2,219	)
		Less than 12 Months					12 Months or More						Total
	December 31, 2013	Fair		Gross			Fair			Gross			Fair		Gross
		Value		Unrealized			Value			Unrealized			Value		Unrealized
				Losses						Losses					Losses
	Available-for-Sale
	Obligations of U.S. government agencies	$	458,385	$	(10,355	)	$	59,362		$	(1,894	)	$	517,747	$	(12,249	)
	U.S. agency residential mortgage-backed	634,199		(17,273		)	166,930			(7,874		)	801,129		(25,147		)
	securities & collateralized mortgage
	obligations
	Private mortgage-backed securities	—		—			104			(1		)	104		(1		)
	Total	$	1,092,584	$	(27,628	)	$	226,396		$	(9,769	)	$	1,318,980	$	(37,397	)
		Less than 12 Months					12 Months or More						Total
	December 31, 2013	Fair		Gross			Fair			Gross			Fair		Gross
		Value		Unrealized			Value			Unrealized			Value		Unrealized
				Losses						Losses					Losses
	Held-to-Maturity
	State, county and municipal securities	$	37,550	$	(1,319	)	$	14,296		$	(730	)	$	51,846	$	(2,049	)
	Corporate securities	7,294		(8		)	—			—			7,294		(8		)
	Total	$	44,844	$	(1,327	)	$	14,296		$	(730	)	$	59,140	$	(2,057	)
Maturities of Investment Securities	All other investment securities maturities are shown at contractual maturity dates.
		Available-for-Sale						Held-to-Maturity
	December 31, 2014	Amortized		Estimated				Amortized			Estimated
		Cost		Fair Value				Cost			Fair Value
	Within one year	$	252,677	$	254,560			$	85,751		$	86,983
	After one year but within five years	1,218,143		1,219,799				283,000			286,783
	After five years but within ten years	188,520		188,015				150,601			153,287
	After ten years	49,154		49,550				55,834			57,480
	Total	$	1,708,494	$	1,711,924			$	575,186		$	584,533

Loans_Tables

Loans (Tables)	12 Months Ended
	Dec. 31, 2014
Receivables [Abstract]
Schedule of Loans by Class	The following table presents loans by class as of the dates indicated:
	December 31,	2014				2013
	Real estate loans:
	Commercial	$	1,639,422			$	1,449,174
	Construction:
	Land acquisition & development	220,443				205,911
	Residential	96,580				76,488
	Commercial	101,246				69,236
	Total construction loans	418,269				351,635
	Residential	999,903				867,912
	Agricultural	167,659				173,534
	Total real estate loans	3,225,253				2,842,255
	Consumer:
	Indirect consumer	552,863				476,012
	Other consumer	144,141				133,039
	Credit card	65,467				62,536
	Total consumer loans	762,471				671,587
	Commercial	740,073				676,544
	Agricultural	124,859				111,872
	Other, including overdrafts	3,959				1,734
	Loans held for investment	4,856,615				4,303,992
	Mortgage loans held for sale	40,828				40,861
	Total loans	$	4,897,443			$	4,344,853
Deteriorated Loans Transferred in
	December 31,	2014			2013
	Outstanding balance	$	41,910		$	—
	Carrying value
	Loans on accrual status	31,870			—
	Loans on non-accrual status	—			—
	Total carrying value	$	31,870		$	—
	The following table summarizes changes in the accretable yield for loans acquired credit impaired for year ended December 31, 2014 and 2013:
	Year Ended December 31,	2014			2013
	Beginning balance	$	—		$	—
	Acquisition	5,233			—
	Accretion income	(735		)	—
	Reductions due to exit events	(201		)	—
	Reclassifications from (to) nonaccretable differences	1,484			—
	Ending balance	$	5,781		$	—
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, 2014	Past Due			Past Due			Past Due			Past Due			Loans			Loans			Loans
	Real estate
	Commercial	$	4,692		$	1,609		$	331		$	6,632		$	1,605,421		$	27,369		$	1,639,422
	Construction:
	Land acquisition & development	839			383			—			1,222			210,969			8,252			220,443
	Residential	—			475			—			475			95,833			272			96,580
	Commercial	100			—			—			100			98,582			2,564			101,246
	Total construction loans	939			858			—			1,797			405,384			11,088			418,269
	Residential	6,969			645			1,762			9,376			987,735			2,792			999,903
	Agricultural	1,624			236			—			1,860			158,957			6,842			167,659
	Total real estate loans	14,224			3,348			2,093			19,665			3,157,497			48,091			3,225,253
	Consumer:
	Indirect consumer	3,235			482			6			3,723			548,757			383			552,863
	Other consumer	988			140			32			1,160			142,432			549			144,141
	Credit card	369			284			315			968			64,484			15			65,467
	Total consumer loans	4,592			906			353			5,851			755,673			947			762,471
	Commercial	3,659			994			147			4,800			722,575			12,698			740,073
	Agricultural	1,125			—			—			1,125			123,288			446			124,859
	Other, including overdrafts	—			—			—			—			3,959			—			3,959
	Loans held for investment	23,600			5,248			2,593			31,441			4,762,992			62,182			4,856,615
	Mortgage loans originated for sale	—			—			—			—			40,828			—			40,828
	Total loans	$	23,600		$	5,248		$	2,593		$	31,441		$	4,803,820		$	62,182		$	4,897,443
											Total Loans
		30 - 59			60 - 89			> 90			30 or More
		Days			Days			Days			Days			Current			Non-accrual			Total
	As of December 31, 2013	Past Due			Past Due			Past Due			Past Due			Loans			Loans			Loans
	Real estate
	Commercial	$	5,924		$	2,472		$	22		$	8,418		$	1,391,823		$	48,933		$	1,449,174
	Construction:
	Land acquisition & development	1,062			468			38			1,568			188,074			16,269			205,911
	Residential	933			250			—			1,183			73,933			1,372			76,488
	Commercial	584			—			—			584			68,427			225			69,236
	Total construction loans	2,579			718			38			3,335			330,434			17,866			351,635
	Residential	3,630			206			1,162			4,998			856,800			6,114			867,912
	Agricultural	328			646			—			974			163,986			8,574			173,534
	Total real estate loans	12,461			4,042			1,222			17,725			2,743,043			81,487			2,842,255
	Consumer:
	Indirect consumer	3,303			430			9			3,742			471,906			364			476,012
	Other consumer	925			130			1			1,056			131,508			475			133,039
	Credit card	364			187			515			1,066			61,451			19			62,536
	Total consumer loans	4,592			747			525			5,864			664,865			858			671,587
	Commercial	2,791			1,186			563			4,540			660,035			11,969			676,544
	Agricultural	453			672			—			1,125			110,622			125			111,872
	Other, including overdrafts	—			—			—			—			1,734			—			1,734
	Loans held for investment	20,297			6,647			2,310			29,254			4,180,299			94,439			4,303,992
	Mortgage loans originated for sale	—			—			—			—			40,861			—			40,861
	Total loans	$	20,297		$	6,647		$	2,310		$	29,254		$	4,221,160		$	94,439		$	4,344,853
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, 2014
		Unpaid			Recorded			Recorded			Total			Related
		Total			Investment			Investment			Recorded			Allowance
		Principal			With No			With			Investment
		Balance			Allowance			Allowance
	Real estate:
	Commercial	$	41,603		$	28,143		$	11,246		$	39,389		$	1,608
	Construction:
	Land acquisition & development	12,511			7,262			1,615			8,877			574
	Residential	459			272			—			272			—
	Commercial	2,729			253			2,442			2,695			904
	Total construction loans	15,699			7,787			4,057			11,844			1,478
	Residential	2,959			2,452			341			2,793			143
	Agricultural	8,844			6,444			2,305			8,749			732
	Total real estate loans	69,105			44,826			17,949			62,775			3,961
	Commercial	16,904			11,882			2,644			14,526			1,190
	Agricultural	1,231			342			837			1,179			641
	Total	$	87,240		$	57,050		$	21,430		$	78,480		$	5,792
		December 31, 2013
		Unpaid			Recorded			Recorded			Total			Related
		Total			Investment			Investment			Recorded			Allowance
		Principal			With No			With			Investment
		Balance			Allowance			Allowance
	Real estate:
	Commercial	$	64,780		$	29,216		$	33,937		$	63,153		$	5,210
	Construction:
	Land acquisition & development	23,906			9,901			7,226			17,127			1,434
	Residential	1,816			1,095			277			1,372			26
	Commercial	397			279			84			363			85
	Total construction loans	26,119			11,275			7,587			18,862			1,545
	Residential	9,448			5,081			967			6,048			249
	Agricultural	8,895			6,429			2,370			8,799			335
	Total real estate loans	109,242			52,001			44,861			96,862			7,339
	Commercial	15,448			10,684			2,901			13,585			1,504
	Agricultural	177			39			86			125			86
	Total	$	124,867		$	62,724		$	47,848		$	110,572		$	8,929
		December 31, 2012
		Unpaid			Recorded			Recorded			Total			Related
		Total			Investment			Investment			Recorded			Allowance
		Principal			With No			With			Investment
		Balance			Allowance			Allowance
	Real estate:
	Commercial	$	84,300		$	39,049		$	34,774		$	73,823		$	4,112
	Construction:
	Land acquisition & development	28,558			15,891			7,173			23,064			1,457
	Residential	3,018			1,976			710			2,686			251
	Commercial	10,447			7,785			340			8,125			69
	Total construction loans	42,023			25,652			8,223			33,875			1,777
	Residential	13,271			6,152			4,495			10,647			1,677
	Agricultural	5,559			1,834			3,227			5,061			784
	Total real estate loans	145,153			72,687			50,719			123,406			8,350
	Commercial	12,770			9,036			3,206			12,242			1,919
	Agricultural	589			509			28			537			28
	Total	$	158,512		$	82,232		$	53,953		$	136,185		$	10,297
Schedule of Average Recorded Investment In and Income Recognized on Impaired Loans	The following tables present the average recorded investment in and income recognized on impaired loans for the periods indicated:
		Year Ended December 31,
		2014							2013							2012
		Average Recorded Investment			Income Recognized				Average Recorded Investment			Income Recognized				Average Recorded Investment			Income Recognized
	Real estate:
	Commercial	$	54,701		$	876			$	66,330		$	1,092			$	78,670		$	1,339
	Construction:
	Land acquisition & development	13,056			43				19,523			487				44,457			110
	Residential	822			—				1,893			—				8,431			4
	Commercial	1,529			8				3,936			4				16,401			—
	Total construction loans	15,407			51				25,352			491				69,289			114
	Residential	4,537			5				8,104			17				13,703			26
	Agricultural	8,774			78				8,230			8				6,936			41
	Total real estate loans	83,419			1,010				108,016			1,608				168,598			1,520
	Commercial	13,789			50				15,047			68				15,741			84
	Agricultural	447			23				313			16				942			27
	Total	$	97,655		$	1,083			$	123,376		$	1,692			$	185,281		$	1,631
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, 2014					Interest only period			Extension of terms or maturity			Interest rate adjustment			Other
	Real estate:
	Commercial		13			$	4,753		$	672		$	84		$	1,047		$	6,556
	Residential		1			—			—			—			15			15
	Total real estate loans		14			4,753			672			84			1,062			6,571
	Consumer		1			—			113			—			—			113
	Commercial		5			476			—			—			30			506
	Total		20			$	5,229		$	785		$	84		$	1,092		$	7,190
			Number of Notes			Type of Concession												Principal Balance at Restructure Date
	Year ended December 31, 2013					Interest only period			Extension of terms or maturity			Interest rate adjustment			Other
	Real estate:
	Commercial		19			$	543		$	1,378		$	11,420		$	2,310		$	15,651
	Construction:
	Land acquisition & development		8			528			7,308			1,952			—			9,788
	Residential		3			—			408			411			—			819
	Total construction loans		11			528			7,716			2,363			—			10,607
	Residential		5			—			708			—			79			787
	Agriculture		1			—			—			188			—			188
	Total real estate loans		36			1,071			9,802			13,971			2,389			27,233
	Consumer		1			—			—			27			—			27
	Commercial		6			613			178			265			87			1,143
	Total		43			$	1,684		$	9,980		$	14,263		$	2,476		$	28,403
			Number of Notes			Type of Concession												Principal Balance at Restructure Date
	Year ended December 31, 2012					Interest only period			Extension of terms or maturity			Interest rate adjustment			Other
	Real estate:
	Commercial		16			$	—		$	959		$	4,504		$	8,611		$	14,074
	Construction:
	Commercial		1			—			—			—			3,155			3,155
	Land acquisition & development		5			—			1,000			1,757			623			3,380
	Residential		2			—			280			233			—			513
	Total construction loans		8			—			1,280			1,990			3,778			7,048
	Residential		2			568			25			—			—			593
	Agriculture		1			—			154			—			—			154
	Total real estate loans		27			568			2,418			6,494			12,389			21,869
	Consumer		1			—			69			—			—			69
	Commercial		10			387			217			—			218			822
	Total		38			$	955		$	2,704		$	6,494		$	12,607		$	22,760
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, 2014	Other Assets			Substandard			Doubtful			Total
		Especially									Criticized
		Mentioned									Loans
	Real estate:
	Commercial	$	84,533		$	83,448		$	15,246		$	183,227
	Construction:
	Land acquisition & development	11,826			15,016			2,507			29,349
	Residential	2,029			2,666			—			4,695
	Commercial	39			253			2,442			2,734
	Total construction loans	13,894			17,935			4,949			36,778
	Residential	10,473			10,848			1,121			22,442
	Agricultural	10,122			12,328			612			23,062
	Total real estate loans	119,022			124,559			21,928			265,509
	Consumer:
	Indirect consumer	916			1,590			121			2,627
	Other consumer	553			1,085			432			2,070
	Credit card	—			348			1,263			1,611
	Total consumer loans	1,469			3,023			1,816			6,308
	Commercial	25,766			32,433			10,273			68,472
	Agricultural	7,827			3,660			837			12,324
	Total	$	154,084		$	163,675		$	34,854		$	352,613
	As of December 31, 2013	Other Assets			Substandard			Doubtful			Total
		Especially									Criticized
		Mentioned									Loans
	Real estate:
	Commercial	$	79,747		$	86,426		$	24,840		$	191,013
	Construction:
	Land acquisition & development	13,211			19,677			7,329			40,217
	Residential	1,859			1,649			277			3,785
	Commercial	—			409			84			493
	Total construction loans	15,070			21,735			7,690			44,495
	Residential	7,500			7,188			4,184			18,872
	Agricultural	13,597			10,245			2,370			26,212
	Total real estate loans	115,914			125,594			39,084			280,592
	Consumer:
	Indirect consumer	875			1,524			115			2,514
	Other consumer	573			969			268			1,810
	Credit card	—			392			2,010			2,402
	Total consumer loans	1,448			2,885			2,393			6,726
	Commercial	33,318			23,833			3,745			60,896
	Agricultural	8,401			1,788			86			10,275
	Total	$	159,081		$	154,100		$	45,308		$	358,489

Allowance_for_Loan_Losses_Tabl

Allowance for Loan Losses (Tables)	12 Months Ended
	Dec. 31, 2014
Receivables [Abstract]
Schedule of Allowance for Loan Losses by Portfolio Segment	The following table presents a summary of changes in the allowance for loan losses by portfolio segment:
	Year ended December 31, 2014	Real Estate			Consumer			Commercial			Agriculture			Other		Total
	Allowance for loan losses:
	Beginning balance	$	63,923		$	6,193		$	14,747		$	476		$	—	$	85,339
	Provision charged (credited) to operating	(10,348		)	1,382			1,809			535			—		(6,622		)
	expense
	Less loans charged-off	(3,014		)	(4,887		)	(6,030		)	(64		)	—		(13,995		)
	Add back recoveries of loans previously charged-off	3,323			2,347			3,781			27			—		9,478
	Ending balance	$	53,884		$	5,035		$	14,307		$	974		$	—	$	74,200
	Individually evaluated for impairment	$	3,961		$	—		$	1,190		$	641		$	—	$	5,792
	Collectively evaluated for impairment	49,923			5,035			13,117			333			—		68,408
	Ending balance	$	53,884		$	5,035		$	14,307		$	974		$	—	$	74,200
	Total loans:
	Individually evaluated for impairment	$	62,775		$	—		$	14,526		$	1,179		$	—	$	78,480
	Collectively evaluated for impairment	3,203,306			762,471			725,547			123,680			3,959		4,818,963
	Total loans	$	3,266,081		$	762,471		$	740,073		$	124,859		$	3,959	$	4,897,443
	Year ended December 31, 2013	Real Estate			Consumer			Commercial			Agriculture			Other		Total
	Allowance for loan losses:
	Beginning balance	$	75,782		$	7,141		$	17,085		$	503		$	—	$	100,511
	Provision charged (credited) to operating	(7,722		)	1,605			41			(49		)	—		(6,125		)
	expense
	Less loans charged-off	(10,224		)	(4,612		)	(5,672		)	(5		)	—		(20,513		)
	Add back recoveries of loans previously charged-off	6,087			2,059			3,293			27			—		11,466
	Ending balance	$	63,923		$	6,193		$	14,747		$	476		$	—	$	85,339
	Individually evaluated for impairment	$	7,339		$	—		$	1,504		$	86		$	—	$	8,929
	Collectively evaluated for impairment	56,584			6,193			13,243			390			—		76,410
	Ending balance	$	63,923		$	6,193		$	14,747		$	476		$	—	$	85,339
	Total loans:
	Individually evaluated for impairment	$	96,862		$	—		$	13,585		$	125		$	—	$	110,572
	Collectively evaluated for impairment	2,786,254			671,587			662,959			111,747			1,734		4,234,281
	Total loans	$	2,883,116		$	671,587		$	676,544		$	111,872		$	1,734	$	4,344,853
	Year ended December 31, 2012	Real Estate			Consumer			Commercial			Agriculture			Other		Total
	Allowance for loan losses:
	Beginning balance	$	87,396		$	8,594		$	15,325		$	1,266		$	—	$	112,581
	Provision charged (credited) to operating	28,651			1,922			10,845			(668		)	—		40,750
	expense
	Less loans charged-off	(43,506		)	(5,320		)	(11,990		)	(120		)	—		(60,936		)
	Add back recoveries of loans previously charged-off	3,241			1,945			2,905			25			—		8,116
	Ending balance	$	75,782		$	7,141		$	17,085		$	503		$	—	$	100,511
	Individually evaluated for impairment	$	8,350		$	—		$	1,919		$	28		$	—	$	10,297
	Collectively evaluated for impairment	67,432			7,141			15,166			475			—		90,214
	Ending balance	$	75,782		$	7,141		$	17,085		$	503		$	—	$	100,511
	Total loans:
	Individually evaluated for impairment	$	123,406		$	—		$	12,242		$	537		$	—	$	136,185
	Collectively evaluated for impairment	2,660,420			636,794			676,511			113,090			912		4,087,727
	Total loans	$	2,783,826		$	636,794		$	688,753		$	113,627		$	912	$	4,223,912

Premises_and_Equipment_Tables

Premises and Equipment (Tables)	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Premises and Equipment and Related Accumulated Depreciation	Premises and equipment and related accumulated depreciation are as follows:
	December 31,	2014			2013
	Land	$	40,620		$	37,581
	Buildings and improvements	209,328			196,950
	Furniture and equipment	74,959			68,870
		324,907			303,401
	Less accumulated depreciation	(129,695		)	(123,711		)
	Premises and equipment, net	$	195,212		$	179,690

Company_Owned_Life_Insurance_T

Company - Owned Life Insurance (Tables)	12 Months Ended
	Dec. 31, 2014
Investments, All Other Investments [Abstract]
Schedule of Life Insurance Corporate or Bank Owned	Company-owned life insurance consists of the following:
	December 31,	2014		2013
	Key executive, principal shareholder	$	3,798	$	3,660
	Key executive split dollar	4,707		4,628
	Group life	145,316		113,887
	Total	$	153,821	$	122,175

Other_Real_Estate_Owned_Tables

Other Real Estate Owned (Tables)	12 Months Ended
	Dec. 31, 2014
Repossessed Assets [Abstract]
Other Real Estate Owned Roll Forward	Information with respect to the Company’s other real estate owned follows:
	Year Ended December 31,	2014			2013			2012
	Balance at beginning of year	$	15,504		$	32,571		$	37,452
	Acquisitions	3,608			—			—
	Additions	5,198			11,545			43,541
	Capitalized improvements	—			65			75
	Valuation adjustments	(224		)	(3,512		)	(6,724		)
	Dispositions	(10,532		)	(25,165		)	(41,773		)
	Balance at end of year	$	13,554		$	15,504		$	32,571

Mortgage_Servicing_Rights_Tabl

Mortgage Servicing Rights (Tables)	12 Months Ended
	Dec. 31, 2014
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,	2014			2013			2012
	Balance at beginning of year	$	14,018		$	13,224		$	13,450
	Sales of mortgage servicing rights	—			—			(735		)
	Originations of mortgage servicing rights	2,717			3,581			4,563
	Amortization expense	(2,361		)	(2,787		)	(3,501		)
	Write-off of permanent impairment	—			—			(553		)
	Balance at end of year	14,374			14,018			13,224
	Less valuation reserve	(336		)	(472		)	(571		)
	Balance at end of year	$	14,038		$	13,546		$	12,653
	Principal balance of serviced loans underlying mortgage servicing rights	$	2,615,311		$	2,416,621		$	2,146,351
	Mortgage servicing rights as a percentage of serviced loans	0.54		%	0.56		%	0.59		%

Deposits_Tables

Deposits (Tables)	12 Months Ended
	Dec. 31, 2014
Deposits [Abstract]
Schedule of Deposits, By Type	Deposits are summarized as follows:
	December 31,	2014		2013
	Non-interest bearing demand	$	1,791,364	$	1,491,683
	Interest bearing:
	Demand	2,133,273		1,848,806
	Savings	1,843,355		1,602,544
	Time, $100 and over	520,125		492,051
	Time, other	718,095		698,666
	Total interest bearing	5,214,848		4,642,067
	Total deposits	$	7,006,212	$	6,133,750
Schedule of Maturities of Time Deposits	Maturities of time deposits at December 31, 2014 are as follows:
		Time, $100		Total Time
		and Over
	2015	$	337,404	$	803,986
	2016	90,509		209,192
	2017	58,387		127,367
	2018	20,971		62,084
	2019	12,854		35,577
	Thereafter	—		14
	Total	$	520,125	$	1,238,220

LongTerm_Debt_and_Other_Borrow1

Long-Term Debt and Other Borrowed Funds (Tables)	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Summary of Long-Term Debt	A summary of long-term debt follows:
	December 31,	2014				2013
	Parent Company:
	6.81% subordinated term loan maturing January 9, 2018, principal due at maturity, interest payable quarterly	$	20,000			$	20,000
	Subsidiaries:
	Variable rate subordinated term loan maturing February 28, 2018, principal due at maturity, interest payable quarterly (rate of 2.24% at December 31, 2014)	15,000				15,000
	4.86% note payable to FHLB, maturing October 31, 2015	225				225
	8.00% capital lease obligation with term ending October 25, 2029	1,643				1,692
	6.24% note payable maturing September 2032, principal due at maturity, interest payable monthly	1,199				—
	Total long-term debt	$	38,067			$	36,917
Schedule of Maturities of Long-term Debt
	Maturities of long-term debt at December 31, 2014 are as follows:
	2015			$	285
	2016			65
	2017			71
	2018			35,077
	2019			83
	Thereafter			2,486
	Total			$	38,067

Subordinated_Debentures_Held_b1

Subordinated Debentures Held by Subsidiary Trusts (Tables)	12 Months Ended
	Dec. 31, 2014
Subordinated Borrowings [Abstract]
Schedule of Subordinated Borrowings	A summary of Subordinated Debenture issuances follows:
			Principal Amount Outstanding
			as of December 31,
	Issuance	Maturity Date	2014		2013
	October 2007	January 1, 2038	$	10,310	$	10,310
	November 2007	December 15, 2037	15,464		15,464
	December 2007	December 15, 2037	20,619		20,619
	December 2007	April 1, 2038	15,464		15,464
	January 2008	April 1, 2038	10,310		10,310
	January 2008	April 1, 2038	10,310		10,310
	Total subordinated debentures held by subsidiary trusts		$	82,477	$	82,477

Earnings_per_Share_Tables

Earnings per Share (Tables)	12 Months Ended
	Dec. 31, 2014
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,	2014		2013		2012
	Net income	$	84,401	$	86,136	$	58,224
	Less preferred stock dividends	—		—		3,300
	Net income available to common shareholders, basic and diluted	$	84,401	$	86,136	$	54,924
	Weighted average common shares outstanding for basic earnings per share computation	44,615,060		43,566,681		42,965,987
	Dilutive effects of stock-based compensation	595,501		477,921		126,991
	Weighted average common shares outstanding for diluted earnings per common share computation	45,210,561		44,044,602		43,092,978
	Basic earnings per common share	$	1.89	$	1.98	$	1.28
	Diluted earnings per common share	1.87		1.96		1.27

Regulatory_Capital_Tables

Regulatory Capital (Tables)	12 Months Ended
	Dec. 31, 2014
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, 2014 and 2013 are presented in the following table:
		Actual				Adequately Capitalized				Well Capitalized
		Amount		Ratio		Amount		Ratio		Amount		Ratio
	December 31, 2014
	Total risk-based capital:
	Consolidated	$	897,769	16.2	%	$	444,685	8	%	NA		NA
	FIB	832,907		15.1		442,468		8		$	553,085	10	%
	Tier 1 risk-based capital:
	Consolidated	807,229		14.5		222,343		4		NA		NA
	FIB	754,708		13.7		221,234		4		$	331,851	6
	Leverage capital ratio:
	Consolidated	807,229		9.6		335,897		4		NA		NA
	FIB	754,708		9.2		330,006		4		$	412,507	5
		Actual				Adequately Capitalized				Well Capitalized
		Amount		Ratio		Amount		Ratio		Amount		Ratio
	December 31, 2013
	Total risk-based capital:
	Consolidated	$	829,443	16.7	%	$	396,210	8	%	NA		NA
	FIB	723,955		14.7		394,038		8		$	492,548	10	%
	Tier 1 risk-based capital:
	Consolidated	739,246		14.9		198,105		4		NA		NA
	FIB	650,093		13.2		197,019		4		$	295,529	6
	Leverage capital ratio:
	Consolidated	739,246		10.1		293,414		4		NA		NA
	FIB	650,093		8.9		292,199		4		$	365,248	5
	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 revise 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 are effective for the Company on January 1, 2015, subject to a phase-in period for certain provisions.

Commitments_and_Contingencies_

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2014
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, 2014, are as follows:
		Third		Related		Total
		Parties		Partnership
	For the year ending December 31:
	2015	$	2,132	$	676	$	2,808
	2016	2,236		676		2,912
	2017	1,728		499		2,227
	2018	1,537		188		1,725
	2019	1,487		—		1,487
	Thereafter	9,941		—		9,941
	Total	$	19,061	$	2,039	$	21,100

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Schedule of Income Tax Expense	Income tax expense consists of the following:
	Year ended December 31,	2014			2013			2012
	Current:
	Federal	$	34,941		$	30,757		$	18,458
	State	4,928			4,533			2,818
	Total current	39,869			35,290			21,276
	Deferred:
	Federal	4,870			10,056			7,697
	State	475			1,220			1,065
	Total deferred	5,345			11,276			8,762
	Total income tax expense	$	45,214		$	46,566		$	30,038
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 percent in 2014, 2013 and 2012 to income before income taxes as a result of the following:
	Year ended December 31,	2014			2013			2012
	Tax expense at the statutory tax rate	$	45,365		$	46,446		$	30,892
	Increase (decrease) in tax resulting from:
	Tax-exempt income	(4,255		)	(3,620		)	(3,498		)
	State income tax, net of federal income tax benefit	3,541			3,741			2,524
	Other, net	563			(1		)	120
	Tax expense at effective tax rate	$	45,214		$	46,566		$	30,038
Schedule of Deferred Tax Assets and Liabilities	The tax effects of temporary differences between the financial statement carrying amounts and tax bases of assets and liabilities that give rise to significant portions of the net deferred tax asset (liability) relate to the following:
	December 31,	2014			2013
	Deferred tax assets:
	Loans, principally due to allowance for loan losses	$	28,506		$	30,903
	Lease Costs	719			—
	Loss Contingencies	1,785			—
	Loan Discount	7,022			—
	Investment securities, unrealized losses	2,728			9,473
	Employee benefits	8,256			6,813
	Other real estate owned write-downs and carrying costs	2,536			4,731
	Deferred gain on sale of subsidiary	253			484
	Deferred revenue on contract	481			545
	Other	760			299
	Deferred tax assets	53,046			53,248
	Deferred tax liabilities:
	Fixed assets, principally differences in bases and depreciation	(569		)	(4,227		)
	Investment securities, unrealized gains	(1,351		)	—
	Investment in joint venture partnership, principally due to differences in depreciation of partnership assets	(1,053		)	(700		)
	Prepaid amounts	(1,385		)	(1,247		)
	Government agency stock dividends	(2,407		)	(1,965		)
	Goodwill and core deposit intangibles	(36,732		)	(28,167		)
	Mortgage servicing rights	(4,144		)	(4,337		)
	Other	(531		)	(451		)
	Deferred tax liabilities	(48,172		)	(41,094		)
	Net deferred tax assets	$	4,874		$	12,154

StockBased_Compensation_Tables

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Weighted-Average Assumptions used in Option Pricing Model	The following table presents the weighted-average assumptions used in the option pricing model for the periods indicated:
	Years ended December 31,		2013			2012
	Expected volatility		30.61	%		37.46	%
	Expected dividend yield		3	%		3.35	%
	Risk-free interest rate		0.88	%		1.99	%
	Expected life of options (in years)		5.52			7.85
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, 2014:
		Number of			Weighted-Average			Weighted-Average
		Shares			Exercise Price			Remaining
								Contract Life
	Outstanding options, beginning of year	2,523,593			$	17.2
	Granted	—			—
	Exercised	(739,290	)		17.73
	Forfeited	(5,893	)		12.85
	Expired	(30,378	)		16.2
	Outstanding options, end of year	1,748,032			$	17.01		4.43 years
	Outstanding options exercisable, end of year	1,393,010			$	17.15		4.12 years
	Information with respect to the Company’s nonvested stock options as of and for the year ended December 31, 2014 follows:
		Number of			Weighted-Average
		Shares			Grant Date Fair Value
	Nonvested stock options, beginning of year	733,906			$	3.78
	Granted	—
	Vested	(348,506	)		3.94
	Forfeited	(30,378	)		3.7
	Nonvested stock options, end of year	355,022			$	3.62
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, 2014:
		Number of			Weighted-Average
		Shares			Measurement Date
					Fair Value
	Restricted stock, beginning of year	225,525			$	16.08
	Granted	148,278			24.36
	Vested	(96,143	)		15.54
	Forfeited	(27,772	)		16.43
	Canceled	(1,489	)		21.39
	Restricted stock, end of year	248,399			$	21.16

Other_Comprehensive_Income_Tab

Other Comprehensive Income (Tables)	12 Months Ended
	Dec. 31, 2014
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, 2014	Before Tax Amount			Tax Expense (Benefit)			Net of Tax Amount
	Investment securities available-for sale:
	Change in net unrealized gain during period	$	21,147		$	8,321		$	12,826
	Reclassification adjustment for net gains included in net income	(61		)	(24		)	(37		)
	Change in unamortized loss on available-for-sale securities transferred into held-to-maturity	(548		)	(216		)	(332		)
	Defined benefits post-retirement benefit plan:
	Change in net actuarial loss	1,731			681			1,050
	Total other comprehensive income	$	22,269		$	8,762		$	13,507
	Year ended December 31, 2013	Before Tax Amount			Tax Expense (Benefit)			Net of Tax Amount
	Investment securities available-for sale:
	Change in net unrealized loss during period	$	(52,949	)	$	(20,835	)	$	(32,114	)
	Reclassification adjustment for net gains included in net income	(1		)	—			(1		)
	Defined benefits post-retirement benefit plan:
	Change in net actuarial loss	137			54			83
	Total other comprehensive loss	$	(52,813	)	$	(20,781	)	$	(32,032	)
	Year ended December 31, 2012	Before Tax Amount			Tax Expense (Benefit)			Net of Tax Amount
	Investment securities available-for sale:
	Change in net unrealized gain during period	$	(4,648	)	$	(1,829	)	$	(2,819	)
	Reclassification adjustment for net gains included in net income	(348		)	(137		)	(211		)
	Change in unamortized gain on available-for-sale securities transferred into held-to-maturity	56			22			34
	Defined benefits post-retirement benefit plan:
	Change in net actuarial loss	(77		)	(30		)	(47		)
	Total other comprehensive loss	$	(5,017	)	$	(1,974	)	$	(3,043	)
Schedule of Accumulated Other Comprehensive Income (Loss)	The components of accumulated other comprehensive income, net of income taxes, are as follows:
	Year ended December 31,	2014				2013
	Net unrealized gain on investment securities available-for-sale	$	(2,121	)		$	(14,578	)
	Net actuarial loss on defined benefit post-retirement benefit plans	(413		)		(1,463		)
	Net accumulated other comprehensive loss	$	(2,534	)		$	(16,041	)

Condensed_Financial_Informatio1

Condensed Financial Information (Parent Company Only) (Tables)	12 Months Ended
	Dec. 31, 2014
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,	2014			2013
	Condensed balance sheets:
	Cash and cash equivalents	$	65,483		$	105,274
	Investment in subsidiaries, at equity:
	Bank subsidiary	936,817			793,892
	Nonbank subsidiaries	1,982			1,980
	Total investment in subsidiaries	938,799			795,872
	Advances to subsidiaries, net	4,337			—
	Other assets	29,335			26,809
	Total assets	$	1,037,954		$	927,955
	Other liabilities	$	26,553		$	17,602
	Advances from subsidiaries, net	—			6,295
	Long-term debt	20,000			20,000
	Subordinated debentures held by subsidiary trusts	82,477			82,477
	Total liabilities	129,030			126,374
	Stockholders’ equity	908,924			801,581
	Total liabilities and stockholders’ equity	$	1,037,954		$	927,955
	Years Ended December 31,	2014			2013			2012
	Condensed statements of income:
	Dividends from subsidiaries	$	58,900		$	71,400		$	40,000
	Other interest income	42			34			92
	Other income, primarily management fees from subsidiaries	12,166			12,809			10,042
	Total income	71,108			84,243			50,134
	Salaries and benefits	15,722			15,914			13,205
	Interest expense	4,002			4,098			6,691
	Acquisition expenses	4,017			—			—
	Other operating expenses, net	7,512			7,546			7,150
	Total expenses	31,253			27,558			27,046
	Earnings before income tax benefit	39,855			56,685			23,088
	Income tax benefit	(6,862		)	(5,703		)	(6,222		)
	Income before undistributed earnings of subsidiaries	46,717			62,388			29,310
	Undistributed earnings of subsidiaries	37,684			23,748			28,914
	Net income	$	84,401		$	86,136		$	58,224
	Years Ended December 31,	2014			2013			2012
	Condensed statements of cash flows:
	Cash flows from operating activities:
	Net income	$	84,401		$	86,136		$	58,224
	Adjustments to reconcile net income to cash provided by operating activities:
	Undistributed earnings of subsidiaries	(37,684		)	(23,748		)	(28,914		)
	Stock-based compensation expense	3,014			2,936			2,485
	Tax benefits from stock-based compensation	2,193			1,898			360
	Excess tax benefits from stock-based compensation	(2,205		)	(2,031		)	(273		)
	Other, net	8,991			(5,804		)	3,327
	Net cash provided by operating activities	58,710			59,387			35,209
	Cash flows from investing activities:
	Capital expenditures, net of sales	—			—			1
	Acquisition of bank holding company, net of cash and cash equivalents received	(37,891		)	—			—
	Net cash provided by (used in) investing activities	$	(37,891	)	$	—		$	1
	Years Ended December 31,	2014			2013			2012
	Condensed statements of cash flows (continued):
	Cash flows from financing activities:
	Net (decrease) increase in advances from nonbank subsidiaries	$	(10,632	)	$	6,992		$	(2,838	)
	Redemption of preferred stock	—			(50,000		)	—
	Repayment of junior subordinated debentures held by subsidiary trusts	(20,439		)	—			(41,238		)
	Proceeds from issuance of common stock, net of stock issuance costs	6,621			9,814			1,911
	Excess tax benefits from stock-based compensation	2,205			2,031			273
	Purchase and retirement of common stock	(9,739		)	(448		)	(263		)
	Dividends paid to common stockholders	(28,626		)	(17,909		)	(26,208		)
	Dividends paid to preferred stockholders	—			—			(3,300		)
	Net cash used in financing activities	(60,610		)	(49,520		)	(71,663		)
	Net change in cash and cash equivalents	(39,791		)	9,867			(36,453		)
	Cash and cash equivalents, beginning of year	105,274			95,407			131,860
	Cash and cash equivalents, end of year	$	65,483		$	105,274		$	95,407

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2014
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, 2014	Balance		Quoted Prices in			Significant Other			Significant
				Active Markets for			Observable			Unobservable
				Identical Assets			Inputs			Inputs
				(Level 1)			(Level 2)			(Level 3)
	Investment securities available-for-sale:
	Obligations of U.S. government agencies	$	720,933	$	—		$	720,933		$	—
	U.S. agency mortgage-backed securities & collateralized mortgage obligations	990,666		—			990,666			—
	Private mortgage-backed securities	325		—			325			—
				Fair Value Measurements at Reporting Date Using
	As of December 31, 2013	Balance		Quoted Prices in			Significant Other			Significant
				Active Markets for			Observable			Unobservable
				Identical Assets			Inputs			Inputs
				(Level 1)			(Level 2)			(Level 3)
	Investment securities available-for-sale:
	Obligations of U.S. government agencies	$	763,238	$	—		$	763,238		$	—
	U.S. agency mortgage-backed securities & collateralized mortgage obligations	1,184,053		—			1,184,053			—
	Private mortgage-backed securities	415		—			415			—
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, 2014	Total		Quoted Prices			Significant			Significant		Total
				in Active			Other			Unobservable		Gains (Losses)
				Markets for			Observable			Inputs
				Identical Assets (Level 1)			Inputs			(Level 3)
							(Level 2)
	Impaired loans	$	30,494	$	—		$	—		$	30,494	$	(14,552	)
	Other real estate owned	4,554		—			—			4,554		(12,665		)
	Long-lived assets to be disposed of by sale	1,083		—			—			1,083		(702		)
				Fair Value Measurements at Reporting Date Using
	As of December 31, 2013	Total		Quoted Prices			Significant			Significant		Total
				in Active			Other			Unobservable		Gains (Losses)
				Markets for			Observable			Inputs
				Identical Assets			Inputs			(Level 3)
				(Level 1)			(Level 2)
	Impaired loans	$	57,302	$	—		$	—		$	57,302	$	(23,224	)
	Other real estate owned	8,502		—			—			8,502		(14,441		)
	Long-lived assets to be disposed of by sale	1,186		—			—			1,186		(599		)
Fair Value Inputs, Assets, Quantitative Information
	As of December 31, 2014	Fair		Valuation	Unobservable	Range
		Value		Technique	Inputs	(Weighted Average)
	Impaired loans	$	30,494	Appraisal	Appraisal adjustment	0%	-	51%	-19%
	Other real estate owned	4,554		Appraisal	Appraisal adjustment	0%	-	50%	-15%
	Long-lived assets to be disposed of by sale	1,083		Appraisal	Appraisal adjustment	0%	-	9%	-5%
	As of December 31, 2013	Fair		Valuation	Unobservable	Range
		Value		Technique	Inputs	(Weighted Average)
	Impaired loans	$	57,302	Appraisal	Appraisal adjustment	6%	-	66%	-31%
	Other real estate owned	8,502		Appraisal	Appraisal adjustment	6%	-	55%	-15%
	Long-lived assets to be disposed of by sale	1,186		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, 2014	Carrying Amount		Estimated			Quoted Prices in			Significant Other		Significant
				Fair Value			Active Markets for			Observable		Unobservable
							Identical Assets			Inputs		Inputs
							(Level 1)			(Level 2)		(Level 3)
	Financial assets:
	Cash and cash equivalents	$	798,670	$	798,670		$	798,670		$	—	$	—
	Investment securities available-for-sale	1,711,924		1,711,924			—			1,711,924		—
	Investment securities held-to-maturity	575,186		584,533			—			584,533		—
	Accrued interest receivable	27,063		27,063			—			27,063		—
	Mortgage servicing rights, net	14,038		21,434			—			21,434		—
	Net loans	4,823,243		4,800,725			—			4,770,231		30,494
	Total financial assets	$	7,950,124	$	7,944,349		$	798,670		$	7,115,185	$	30,494
	Financial liabilities:
	Total deposits, excluding time deposits	$	5,767,992	$	5,767,992		$	5,767,992		$	—	$	—
	Time deposits	1,238,220		1,244,324			—			1,244,324		—
	Securities sold under repurchase agreements	502,250		502,250			—			502,250		—
	Other borrowed funds	9		9			—			9		—
	Accrued interest payable	5,833		5,833			—			5,833		—
	Long-term debt	38,067		37,781			—			37,781		—
	Subordinated debentures held by subsidiary trusts	82,477		75,734			—			75,734		—
	Total financial liabilities	$	7,634,848	$	7,633,923		$	5,767,992		$	1,865,931	$	—
							Fair Value Measurements at Reporting Date Using
	As of December 31, 2013	Carrying Amount		Estimated			Quoted Prices in			Significant Other		Significant
				Fair Value			Active Markets for			Observable		Unobservable
							Identical Assets			Inputs		Inputs
							(Level 1)			(Level 2)		(Level 3)
	Financial assets:
	Cash and cash equivalents	$	534,827	$	534,827		$	534,827		$	—	$	—
	Investment securities available-for-sale	1,947,706		1,947,706			—			1,947,706		—
	Investment securities held-to-maturity	203,837		205,926			—			205,926		—
	Accrued interest receivable	26,450		26,450			—			26,450		—
	Mortgage servicing rights, net	13,546		25,698			—			25,698		—
	Net loans	4,259,514		4,246,539			—			4,189,237		57,302
	Total financial assets	$	6,985,880	$	6,987,146		$	534,827		$	6,395,017	$	57,302
	Financial liabilities:
	Total deposits, excluding time deposits	$	4,943,033	$	4,943,033		$	4,943,033		$	—	$	—
	Time deposits	1,190,717		1,196,250			—			1,196,250		—
	Securities sold under repurchase agreements	457,437		457,437			—			457,437		—
	Other borrowed funds	3		3			—			3		—
	Accrued interest payable	4,963		4,963			—			4,963		—
	Long-term debt	36,917		34,508			—			34,508		—
	Subordinated debentures held by subsidiary trusts	82,477		72,045			—			72,045		—
	Total financial liabilities	$	6,715,547	$	6,708,239		$	4,943,033		$	1,765,206	$	—

Summary_of_Significant_Account2

Summary of Significant Accounting Policies (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash and Cash Equivalents [Abstract]
Cash on deposit with Federal Reserve Bank	$631,562	$392,413
Compensating balances with Federal Reserve Bank	7,507	1,412
Loans and Leases Receivable Disclosure [Abstract]
Mortgage loans held for sale	40,828	40,861
Impairment losses	102	616	70
Write-downs of OREO	224	3,512	6,724
Federal Home Loan Bank Stock and Federal Reserve Bank Stock [Abstract]
Federal Reserve Bank stock	16,187	13,357
Federal Home Loan Bank stock	10,662	7,003
Advertising expense	3,734	3,532	3,555
Share-based Compensation [Abstract]
Stock compensation expense	3,014	2,936	2,485
Tax benefits from stock-based compensation	$1,153	$1,122	$950

Summary_of_Significant_Account3

Summary of Significant Accounting Policies - Premises and Equipment (Details)	12 Months Ended
	Dec. 31, 2014
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

Summary_of_Significant_Account4

Summary of Significant Accounting Policies - Core Deposit Intangibles (Details) (Core Deposits Intangibles, USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Core Deposits Intangibles
Intangible Assets, Net (Excluding Goodwill) [Abstract]
Accumulated core deposit intangibles amortization	$24,652	$22,401
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
2015	3,336
2016	3,099
2017	1,825
2018	1,318
2019	1,118
Thereafter	$2,587

Summary_of_Significant_Account5

Summary of Significant Accounting Policies Derivatives and Hedging (Details) (Interest Rate Swap [Member], USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
Interest Rate Swap [Member]
Derivative [Line Items]
Derivative Asset	$61
Derivative Liability	$59

Acquisitions_Business_Combinat

Acquisitions Business Combinations (Details) (USD $)	12 Months Ended			3 Months Ended	0 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2014	Jul. 31, 2014	Feb. 10, 2014
Business Acquisition [Line Items]
Acquisition Expense	$4,017	$0	$0
Mountain West Bank
Business Acquisition [Line Items]
Number of Subsidiary Banks				1
Number of Shared Markets				5
Mountain West Bank
Business Acquisition [Line Items]
Business Acquisition Conversion Rate						$7.13
Payments to Acquire Businesses, Gross					38,479
Business Combination, Consideration Transferred					$74,451
Business Acquisition, Effective Date of Acquisition					31-Jul-14
Mountain West Bank | Class A Common Stock
Business Acquisition [Line Items]
Business Acquisition Conversion Share Rate						0.2552
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	1,378,230				1,378,230
Business Acquisition, Share Price					$26.10

Acquisitions_Schedule_of_Recog

Acquisitions Schedule of Recognized Identifiable Assets Acquired and Liabilities Assumed (Details) (USD $)	0 Months Ended
In Thousands, unless otherwise specified	Jul. 31, 2014		Dec. 31, 2014	Dec. 31, 2013
Business Acquisition [Line Items]
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned			$35,972
Goodwill			205,574	183,673
Class A Common Stock
Business Acquisition [Line Items]
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned	35,972
Mountain West Financial Corp Acquisition, As Recorded by MWFC
Business Acquisition [Line Items]
Cash and Cash Equivalents	74,035
Investment Securities	104,945
Loans	378,558
Allowance for Loan Losses	-11,598
Premises and Equipment	35,283
Company Owned LIfe Insurance	13,046
Deferred Tax Assets, Net	6,491
Core Deposit Intangibles	0
Other Assets	16,559
Total Assets Acquired	617,319
Deposits	515,538
Other Liabilities	20,501
Subordinated Debentures Held by Subsidiary Trusts	20,439
Total Liabilities Assumed	556,478
Net Assets Acquired	60,841
Mountain West Bank Acquisition, Fair Value Adjustment
Business Acquisition [Line Items]
Cash and Cash Equivalents	0
Investment Securities	-34	[1]
Loans	-18,286	[2]
Allowance for Loan Losses	11,598	[3]
Premises and Equipment	-5,847	[4]
Company Owned LIfe Insurance	0
Deferred Tax Assets, Net	1,135	[5]
Core Deposit Intangibles	11,014	[6]
Other Assets	-5,300	[7]
Total Assets Acquired	-5,720
Deposits	-159	[8]
Other Liabilities	2,730	[9]
Subordinated Debentures Held by Subsidiary Trusts	0	[10]
Total Liabilities Assumed	2,571
Net Assets Acquired	-8,291
Mountain West Bank Acquisition, As Recorded by the Company
Business Acquisition [Line Items]
Cash and Cash Equivalents	74,035
Investment Securities	104,911
Loans	360,272
Allowance for Loan Losses	0
Premises and Equipment	29,436
Company Owned LIfe Insurance	13,046
Deferred Tax Assets, Net	7,626
Core Deposit Intangibles	11,014
Other Assets	11,259
Total Assets Acquired	611,599
Deposits	515,379
Other Liabilities	23,231
Subordinated Debentures Held by Subsidiary Trusts	20,439
Total Liabilities Assumed	559,049
Net Assets Acquired	52,550
Payments to Acquire Businesses, Gross	38,479
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned	35,972
Business Combination, Consideration Transferred	74,451
Goodwill	$21,901
[1]	(1) Write down of the book value of investment securities to their estimated fair values on the date of acquisition based upon quotes obtained from an independent third party pricing service.
[2]	(2) Write down of the book value of loans to their estimated fair values. Except for collateral dependent loans acquired with deteriorated credit quality, the fair value of 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 the 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]	(3) Adjustment to remove the MWB allowance for loan losses at acquisition date as the credit risk is accounted for in the fair value adjustment for loans receivable described in (2) above.
[4]	(4) Write down of the book value of premises and equipment to their estimated fair values based upon appraisals obtained from an independent third party appraiser.
[5]	(5) Adjustment represents the net deferred tax assets resulting from fair value adjustments related to acquired assets, assumed liabilities, core deposit intangible assets and other purchase accounting adjustments.
[6]	(6) Adjustment represents the value of the core deposit base assumed in the acquisition based upon a valuation obtained from an independent third party valuation expert.
[7]	(7) Adjustment consists of a reduction in the value of equity method investments and accrued interest receivable and the write-off of federal and state income taxes receivable, pre-existing goodwill and computer software costs.
[8]	(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.
[9]	(9) Adjustment represents increases in the book values of other liabilities to their estimated fair values at the acquisition date. The adjustment primarily consists of a $2,000 increase to an acquired capital lease obligation and a $473 increase to Federal Home Loan Bank borrowings based upon interest interest rates of similar obligations with similar characteristics on the date of acquisition.
[10]	(10) Recorded value of junior subordinated debentures held by subsidiary trusts approximated fair value as of the acquisition date due to the short-term nature of the instruments. These debentures were redeemed at par value in December 2014.

Acquisitions_Finite_Lived_Inta

Acquisitions Finite Lived Intangible Assets Acquired as Part of a Business Combination (Details) (Core Deposits Intangibles, USD $)	0 Months Ended	3 Months Ended
In Thousands, unless otherwise specified	Jul. 31, 2014	Sep. 30, 2014
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	10 years
Mountain West Bank
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived Intangible Assets Acquired		$11,014

Acquisitions_Deteriorated_Loan

Acquisitions Deteriorated Loans Transferred In (Details) (USD $)	Jul. 31, 2014
In Thousands, unless otherwise specified
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities Acquired During Period [Line Items]
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, Contractually Required Payments Receivable at Acquisition	$112,882
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Non-Accretable Yield	74,760
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, Cash Flows Expected to be Collected at Acquisition	38,122
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, Internet Component Expected to be Collected	5,233
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Acquired During Period, at Acquisition, at Fair Value	$32,889

Acquisitions_Loans_Transferred

Acquisitions Loans Transferred In (Details) (USD $)	Jul. 31, 2014
In Thousands, unless otherwise specified
Loans Transferred In [Abstract]
Certain Loans Acquired in Transfer Without Evidence of Credit Deterioration, Acquired During Period, Contractually Required Payments Receivable at Acquisition	$445,345
Certain Loans Acquired in Transfer Without Evidence of Credit Deterioration, Acquired During Period, Contractual Cash Flows Not Expected to be Collected	-15,090
Certain Loans Acquired in Transfer Without Evidence of Credit Deterioration, Acquired During Period, at Acquisition, at Fair Value	$327,383

Acquisitions_Business_Acquisit

Acquisitions Business Acquisition, Pro Forma Information (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Business Acquisition [Line Items]
Expenses Related to Acquisition Excluded From Pro Forma Net Income	$5,052
Business Acquisition, Pro Forma Interest Income	280,102	280,455
Business Acquisition, Pro Forma Interest Expense	114,984	120,005
Business Acquisition, Pro Forma Revenue	395,086	400,460
Business Acquisition, Pro Forma Net Income (Loss)	$87,129	$84,264

Investment_Securities_Amortize

Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Available-for-Sale:
Available-for-Sale, Amortized Cost	$1,708,494	$1,971,757
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	12,426	13,346
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	-8,996	-37,397
Available-for-Sale, Estimated Fair Value	1,711,924	1,947,706
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	575,186	203,837
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	11,566	4,146
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	-2,219	-2,057
Held-to-Maturity Estimated Fair Value	584,533	205,926
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Amortized Cost	725,408	774,055
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	895	1,432
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	-5,370	-12,249
Available-for-Sale, Estimated Fair Value	720,933	763,238
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Amortized Cost	982,764	1,197,295
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	11,526	11,905
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	-3,624	-25,147
Available-for-Sale, Estimated Fair Value	990,666	1,184,053
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	353,176
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	5,563
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	-1,758
Held-to-Maturity Estimated Fair Value	356,981
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Amortized Cost	322	407
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	5	9
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	-2	-1
Available-for-Sale, Estimated Fair Value	325	415
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	188,941	185,818
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	5,949	4,043
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	-386	-2,049
Held-to-Maturity Estimated Fair Value	194,504	187,812
Corporate securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	32,565	18,019
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	54	103
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	-75	-8
Held-to-Maturity Estimated Fair Value	32,544	18,114
Other Investments
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	504
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain	0
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss	0
Held-to-Maturity Estimated Fair Value	$504

Investment_Securities_Realized

Investment Securities - Realized Gains (Losses) on Investments (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Investments, Debt and Equity Securities [Abstract]
Gross realized gains	$274	$49	$351
Gross realized losses	213	48	3
Political Subdivisions
Schedule of Available-for-sale Securities
Available-for-sale Securities, Amortized Cost Basis	131,845
Political Subdivisions | Montana, Wyoming, and South Dakota
Schedule of Available-for-sale Securities
Available-for-sale Securities, Amortized Cost Basis	$68,927

Investment_Securities_Transfer

Investment Securities Transfer from AFS to HTM (Details) (USD $)	0 Months Ended
In Thousands, unless otherwise specified	Jun. 27, 2014
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 3 months 17 days
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 to Held-to-Maturity, Amortized Costs	396,640
Available-for-Sale Securities Transferred to Held-to-Maturity, Fair Values	388,808
Available-for-Sale Securities Transferred to Held-to-Maturity, Unrealized Net Gain (Loss)	7,832

Investment_Securities_Gross_Un

Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified	securities	securities
Available-for-Sale:
Available-for-Sale, less than 12 months fair value	$355,102	$1,092,584
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	-1,589	-27,628
Available-for-Sale, 12 months or more fair value	460,753	226,396
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	-7,407	-9,769
Available-for-Sale Securities, Fair Value	815,855	1,318,980
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	-8,996	-37,397
Held-to-Maturity:
Held-to-maturity, less than 12 months fair value	83,935	44,844
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	-1,846	-1,327
Held-to-maturity, 12 months or more fair value	20,097	14,296
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	-373	-730
Held-to-maturity Securities, Fair Value	104,032	59,140
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	-2,219	-2,057
Available-for-Sale and Held-to-Maturity:
Investment securities in an unrealized loss position (number of securities)	154	229
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, less than 12 months fair value	135,888	458,385
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	-702	-10,355
Available-for-Sale, 12 months or more fair value	309,283	59,362
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	-4,668	-1,894
Available-for-Sale Securities, Fair Value	445,171	517,747
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	-5,370	-12,249
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, less than 12 months fair value	219,214	634,199
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	-887	-17,273
Available-for-Sale, 12 months or more fair value	151,380	166,930
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	-2,737	-7,874
Available-for-Sale Securities, Fair Value	370,594	801,129
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	-3,624	-25,147
Held-to-Maturity:
Held-to-maturity, less than 12 months fair value	61,201
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	-1,758
Held-to-maturity, 12 months or more fair value	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Held-to-maturity Securities, Fair Value	61,201
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	-1,758
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, less than 12 months fair value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Available-for-Sale, 12 months or more fair value	90	104
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	-2	-1
Available-for-Sale Securities, Fair Value	90	104
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	-2	-1
State, county and municipal securities
Held-to-Maturity:
Held-to-maturity, less than 12 months fair value	7,979	37,550
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	-13	-1,319
Held-to-maturity, 12 months or more fair value	20,097	14,296
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	-373	-730
Held-to-maturity Securities, Fair Value	28,076	51,846
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	-386	-2,049
Corporate securities
Held-to-Maturity:
Held-to-maturity, less than 12 months fair value	14,755	7,294
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	-75	-8
Held-to-maturity, 12 months or more fair value	0	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Held-to-maturity Securities, Fair Value	14,755	7,294
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	($75)	($8)

Investment_Securities_Maturiti

Investment Securities - Maturities of Investment Securities (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Rolling Maturity
Available-for-sale amortized cost, within one year	$252,677
Available-for-sale amortized cost, after one year but within five years	1,218,143
Available-for-sale amortized cost, after five years but within ten years	188,520
Available-for-sale amortized cost, after ten years	49,154
Available-for-Sale, Amortized Cost	1,708,494	1,971,757
Available-for-sale Securities, Debt Maturities, Fair Value, Rolling Maturity
Available-for-sale estimated fair value, within one year	254,560
Available-for-sale estimated fair value, after one year but within five years	1,219,799
Available-for-sale estimated fair value, after ten years	188,015
Available-for-ale estimated fair value, after five years but within ten years	49,550
Available-for-Sale, Estimated Fair Value	1,711,924	1,947,706
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount
Held-to-maturity amortized cost, within one year	85,751
Held-to-maturity amoritzed cost, after one year but within five years	283,000
Held-to-maturity amortized cost, after five years but within ten years	150,601
Held-to-maturity amortized cost, after ten years	55,834
Held-to-Maturity, Amortized Cost	575,186	203,837
Held-to-maturity Securities, Debt Maturities, Fair Value, Rolling Maturity
Held-to-maturity estimated fair value, within one year	86,983
Held-to-maturity estimated fair value, after one year but within five years	286,783
Held-to-maturity estimated fair value, after five years but within ten years	153,287
Held-to-maturity estimated fair value, after ten years	57,480
Held-to-Maturity Estimated Fair Value	584,533	205,926
Available-for-sale Securities and Held-to-maturity Securities
Securities sold under agreements to repurchase	1,351,787	1,288,750
Pledged assets separately reported, securities pledged for repurchase agreements, at fair value	1,356,341	1,278,663
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	176,023
Investment securities primarily classified as available-for-sale, fair value, after one year but within five years	175,892
Callable structured notes
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	29,995
Investment securities primarily classified as available-for-sale, fair value, after one year but within five years	30,012
Mortgage Backed Securities, Variable Rate
Available-for-sale Securities and Held-to-maturity Securities
Available-for-sale securities, amortized cost basis	$40,666	$46,315

Loans_Schedule_of_Loans_by_Cla

Loans - Schedule of Loans by Class (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Loans and Leases Receivable
Loans held for investment	$4,856,615	$4,303,992
Mortgage loans held for sale	40,828	40,861
Total loans	4,897,443	4,344,853
Commercial real estate
Loans and Leases Receivable
Loans held for investment	1,639,422	1,449,174
Land acquisition & development construction real estate
Loans and Leases Receivable
Loans held for investment	220,443	205,911
Residential construction real estate
Loans and Leases Receivable
Loans held for investment	96,580	76,488
Commercial construction real estate
Loans and Leases Receivable
Loans held for investment	101,246	69,236
Total construction loans
Loans and Leases Receivable
Loans held for investment	418,269	351,635
Residential real estate
Loans and Leases Receivable
Loans held for investment	999,903	867,912
Loans receivable, maturity	15 years
Agricultural real estate
Loans and Leases Receivable
Loans held for investment	167,659	173,534
Total real estate loans
Loans and Leases Receivable
Loans held for investment	3,225,253	2,842,255
Indirect consumer
Loans and Leases Receivable
Loans held for investment	552,863	476,012
Other consumer
Loans and Leases Receivable
Loans held for investment	144,141	133,039
Credit card consumer
Loans and Leases Receivable
Loans held for investment	65,467	62,536
Total consumer loans
Loans and Leases Receivable
Loans held for investment	762,471	671,587
Commercial
Loans and Leases Receivable
Loans held for investment	740,073	676,544
Agricultural
Loans and Leases Receivable
Loans held for investment	124,859	111,872
Other, including overdrafts
Loans and Leases Receivable
Loans held for investment	3,959	1,734
Home equity line of credit
Loans and Leases Receivable
Loans held for investment	$298,692	$272,415
Loans receivable, loan to value limit	80.00%
Commercial and agricultural real estate
Loans and Leases Receivable
Loans receivable, maturity	5 years

Loans_Schedule_of_Loans_Acquir

Loans Schedule of Loans Acquired from Business Combinations (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Deteriorated Loans Transferred in [Abstract]
Outstanding Balance	$41,910	$0
Loans on Accrual Status	31,870	0
Loans on Non-Accrual Status	0	0
Total Carrying Value	$31,870	$0

Loans_Certain_Loans_Acquired_i

Loans Certain Loans Acquired in Transfer Not Accounted for as Debt Securities (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Rollforward [Abstract]
Beginning Balance	$0	$0
Acquisition	5,233	0
Accretion Income	-735	0
Reductions Due to Exit Events	-201	0
Reclassifications from (to) nonaccretable differences	1,484	0
Ending Balance	$5,781	$0

Loans_Schedule_of_Recorded_Inv

Loans - Schedule of Recorded Investment in Past Due Loans by Class (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Loans and Leases Receivable Recorded Investment, Past Due
Loans held for investment	$4,856,615	$4,303,992
Mortgage loans held for sale	40,828	40,861
Total loans	4,897,443	4,344,853
Interest Income on Non-Accrual Loans if Accrued	3,970	4,630	8,537
Commercial real estate
Loans and Leases Receivable Recorded Investment, Past Due
30 to 59 Days Past Due	4,692	5,924
60 to 89 Days Past Due	1,609	2,472
Greater than 90 Days Past Due	331	22
Total Loans 30 or More Days Past Due	6,632	8,418
Current Loans	1,605,421	1,391,823
Non-accrual Loans	27,369	48,933
Loans held for investment	1,639,422	1,449,174
Land acquisition & development construction real estate
Loans and Leases Receivable Recorded Investment, Past Due
30 to 59 Days Past Due	839	1,062
60 to 89 Days Past Due	383	468
Greater than 90 Days Past Due	0	38
Total Loans 30 or More Days Past Due	1,222	1,568
Current Loans	210,969	188,074
Non-accrual Loans	8,252	16,269
Loans held for investment	220,443	205,911
Residential construction real estate
Loans and Leases Receivable Recorded Investment, Past Due
30 to 59 Days Past Due	0	933
60 to 89 Days Past Due	475	250
Greater than 90 Days Past Due	0	0
Total Loans 30 or More Days Past Due	475	1,183
Current Loans	95,833	73,933
Non-accrual Loans	272	1,372
Loans held for investment	96,580	76,488
Commercial construction real estate
Loans and Leases Receivable Recorded Investment, Past Due
30 to 59 Days Past Due	100	584
60 to 89 Days Past Due	0	0
Greater than 90 Days Past Due	0	0
Total Loans 30 or More Days Past Due	100	584
Current Loans	98,582	68,427
Non-accrual Loans	2,564	225
Loans held for investment	101,246	69,236
Total construction loans
Loans and Leases Receivable Recorded Investment, Past Due
30 to 59 Days Past Due	939	2,579
60 to 89 Days Past Due	858	718
Greater than 90 Days Past Due	0	38
Total Loans 30 or More Days Past Due	1,797	3,335
Current Loans	405,384	330,434
Non-accrual Loans	11,088	17,866
Loans held for investment	418,269	351,635
Residential real estate
Loans and Leases Receivable Recorded Investment, Past Due
30 to 59 Days Past Due	6,969	3,630
60 to 89 Days Past Due	645	206
Greater than 90 Days Past Due	1,762	1,162
Total Loans 30 or More Days Past Due	9,376	4,998
Current Loans	987,735	856,800
Non-accrual Loans	2,792	6,114
Loans held for investment	999,903	867,912
Agricultural real estate
Loans and Leases Receivable Recorded Investment, Past Due
30 to 59 Days Past Due	1,624	328
60 to 89 Days Past Due	236	646
Greater than 90 Days Past Due	0	0
Total Loans 30 or More Days Past Due	1,860	974
Current Loans	158,957	163,986
Non-accrual Loans	6,842	8,574
Loans held for investment	167,659	173,534
Total real estate loans
Loans and Leases Receivable Recorded Investment, Past Due
30 to 59 Days Past Due	14,224	12,461
60 to 89 Days Past Due	3,348	4,042
Greater than 90 Days Past Due	2,093	1,222
Total Loans 30 or More Days Past Due	19,665	17,725
Current Loans	3,157,497	2,743,043
Non-accrual Loans	48,091	81,487
Loans held for investment	3,225,253	2,842,255
Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due
30 to 59 Days Past Due	3,235	3,303
60 to 89 Days Past Due	482	430
Greater than 90 Days Past Due	6	9
Total Loans 30 or More Days Past Due	3,723	3,742
Current Loans	548,757	471,906
Non-accrual Loans	383	364
Loans held for investment	552,863	476,012
Other consumer
Loans and Leases Receivable Recorded Investment, Past Due
30 to 59 Days Past Due	988	925
60 to 89 Days Past Due	140	130
Greater than 90 Days Past Due	32	1
Total Loans 30 or More Days Past Due	1,160	1,056
Current Loans	142,432	131,508
Non-accrual Loans	549	475
Loans held for investment	144,141	133,039
Credit card consumer
Loans and Leases Receivable Recorded Investment, Past Due
30 to 59 Days Past Due	369	364
60 to 89 Days Past Due	284	187
Greater than 90 Days Past Due	315	515
Total Loans 30 or More Days Past Due	968	1,066
Current Loans	64,484	61,451
Non-accrual Loans	15	19
Loans held for investment	65,467	62,536
Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due
30 to 59 Days Past Due	4,592	4,592
60 to 89 Days Past Due	906	747
Greater than 90 Days Past Due	353	525
Total Loans 30 or More Days Past Due	5,851	5,864
Current Loans	755,673	664,865
Non-accrual Loans	947	858
Loans held for investment	762,471	671,587
Commercial
Loans and Leases Receivable Recorded Investment, Past Due
30 to 59 Days Past Due	3,659	2,791
60 to 89 Days Past Due	994	1,186
Greater than 90 Days Past Due	147	563
Total Loans 30 or More Days Past Due	4,800	4,540
Current Loans	722,575	660,035
Non-accrual Loans	12,698	11,969
Loans held for investment	740,073	676,544
Agricultural
Loans and Leases Receivable Recorded Investment, Past Due
30 to 59 Days Past Due	1,125	453
60 to 89 Days Past Due	0	672
Greater than 90 Days Past Due	0	0
Total Loans 30 or More Days Past Due	1,125	1,125
Current Loans	123,288	110,622
Non-accrual Loans	446	125
Loans held for investment	124,859	111,872
Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due
30 to 59 Days Past Due	0	0
60 to 89 Days Past Due	0	0
Greater than 90 Days Past Due	0	0
Total Loans 30 or More Days Past Due	0	0
Current Loans	3,959	1,734
Non-accrual Loans	0	0
Loans held for investment	3,959	1,734
Loans held for investment
Loans and Leases Receivable Recorded Investment, Past Due
30 to 59 Days Past Due	23,600	20,297
60 to 89 Days Past Due	5,248	6,647
Greater than 90 Days Past Due	2,593	2,310
Total Loans 30 or More Days Past Due	31,441	29,254
Current Loans	4,762,992	4,180,299
Non-accrual Loans	62,182	94,439
Loans held for investment	4,856,615	4,303,992
Mortgage loans originated for sale
Loans and Leases Receivable Recorded Investment, Past Due
30 to 59 Days Past Due	0	0
60 to 89 Days Past Due	0	0
Greater than 90 Days Past Due	0	0
Total Loans 30 or More Days Past Due	0	0
Current Loans	40,828	40,861
Non-accrual Loans	0	0
Mortgage loans held for sale	40,828	40,861
Total loans
Loans and Leases Receivable Recorded Investment, Past Due
30 to 59 Days Past Due	23,600	20,297
60 to 89 Days Past Due	5,248	6,647
Greater than 90 Days Past Due	2,593	2,310
Total Loans 30 or More Days Past Due	31,441	29,254
Current Loans	4,803,820	4,221,160
Non-accrual Loans	62,182	94,439
Total loans	$4,897,443	$4,344,853

Loans_Schedule_of_Recorded_Inv1

Loans - Schedule of Recorded Investment in Impaired Loans (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
In Thousands, unless otherwise specified
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	$87,240	$124,867	$158,512
Recorded Investment With No Allowance	57,050	62,724	82,232
Recorded Investment With Allowance	21,430	47,848	53,953
Total Recorded Investment	78,480	110,572	136,185
Related allowance	5,792	8,929	10,297
Commercial real estate
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	41,603	64,780	84,300
Recorded Investment With No Allowance	28,143	29,216	39,049
Recorded Investment With Allowance	11,246	33,937	34,774
Total Recorded Investment	39,389	63,153	73,823
Related allowance	1,608	5,210	4,112
Land acquisition & development construction real estate
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	12,511	23,906	28,558
Recorded Investment With No Allowance	7,262	9,901	15,891
Recorded Investment With Allowance	1,615	7,226	7,173
Total Recorded Investment	8,877	17,127	23,064
Related allowance	574	1,434	1,457
Residential construction real estate
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	459	1,816	3,018
Recorded Investment With No Allowance	272	1,095	1,976
Recorded Investment With Allowance	0	277	710
Total Recorded Investment	272	1,372	2,686
Related allowance	0	26	251
Commercial construction real estate
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	2,729	397	10,447
Recorded Investment With No Allowance	253	279	7,785
Recorded Investment With Allowance	2,442	84	340
Total Recorded Investment	2,695	363	8,125
Related allowance	904	85	69
Total construction loans
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	15,699	26,119	42,023
Recorded Investment With No Allowance	7,787	11,275	25,652
Recorded Investment With Allowance	4,057	7,587	8,223
Total Recorded Investment	11,844	18,862	33,875
Related allowance	1,478	1,545	1,777
Residential real estate
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	2,959	9,448	13,271
Recorded Investment With No Allowance	2,452	5,081	6,152
Recorded Investment With Allowance	341	967	4,495
Total Recorded Investment	2,793	6,048	10,647
Related allowance	143	249	1,677
Agricultural real estate
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	8,844	8,895	5,559
Recorded Investment With No Allowance	6,444	6,429	1,834
Recorded Investment With Allowance	2,305	2,370	3,227
Total Recorded Investment	8,749	8,799	5,061
Related allowance	732	335	784
Total real estate loans
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	69,105	109,242	145,153
Recorded Investment With No Allowance	44,826	52,001	72,687
Recorded Investment With Allowance	17,949	44,861	50,719
Total Recorded Investment	62,775	96,862	123,406
Related allowance	3,961	7,339	8,350
Commercial
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	16,904	15,448	12,770
Recorded Investment With No Allowance	11,882	10,684	9,036
Recorded Investment With Allowance	2,644	2,901	3,206
Total Recorded Investment	14,526	13,585	12,242
Related allowance	1,190	1,504	1,919
Agricultural
Impaired Financing Receivable, Unpaid Principal Balance
Unpaid Total Principal Balance	1,231	177	589
Recorded Investment With No Allowance	342	39	509
Recorded Investment With Allowance	837	86	28
Total Recorded Investment	1,179	125	537
Related allowance	$641	$86	$28

Loans_Schedule_of_Average_Reco

Loans - Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans
Average Recorded Investment	$97,655	$123,376	$185,281
Income Recognized	1,083	1,692	1,631
Impaired Financing Receivable, Average Recorded Investment and Cash Basis Method Interest Income
Interest income on impaired loans if interest had been accrued	4,951	5,786	8,463
Commercial real estate
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans
Average Recorded Investment	54,701	66,330	78,670
Income Recognized	876	1,092	1,339
Land acquisition & development construction real estate
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans
Average Recorded Investment	13,056	19,523	44,457
Income Recognized	43	487	110
Residential construction real estate
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans
Average Recorded Investment	822	1,893	8,431
Income Recognized	0	0	4
Commercial construction real estate
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans
Average Recorded Investment	1,529	3,936	16,401
Income Recognized	8	4	0
Total construction loans
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans
Average Recorded Investment	15,407	25,352	69,289
Income Recognized	51	491	114
Residential real estate
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans
Average Recorded Investment	4,537	8,104	13,703
Income Recognized	5	17	26
Agricultural real estate
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans
Average Recorded Investment	8,774	8,230	6,936
Income Recognized	78	8	41
Total real estate loans
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans
Average Recorded Investment	83,419	108,016	168,598
Income Recognized	1,010	1,608	1,520
Commercial
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans
Average Recorded Investment	13,789	15,047	15,741
Income Recognized	50	68	84
Agricultural
Schedule of Average Recorded Investment in and Income Recognized on Impaired Loans
Average Recorded Investment	447	313	942
Income Recognized	$23	$16	$27

Loans_Schedule_of_Loans_Renego

Loans - Schedule of Loans Renegotiated in Troubled Debt Restructurings (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
	notes	notes	notes
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivables, Impaired, Troubled Debt Restructuring, Write-down	$0	$0	$0
Loans renegotiated in troubled debt restructurings	44,227	59,792
Loans renegotiated in troubled debt restructurings, non-accrual loans	23,275	38,011
Loans tenegotiated in troubled debt restructurings, accrual loans	20,952	21,781
Financing receivable, modifications, subsequent default, number of contracts, during period	20	43	38
Financing Receivable, Modifications, Pre-Modification Recorded Investment	7,190	28,403	22,760
Interest only period
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment	5,229	1,684	955
Extension of terms or maturity
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment	785	9,980	2,704
Interest rate adjustment
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment	84	14,263	6,494
Other
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment	1,092	2,476	12,607
Commercial real estate
Loans and Leases Receivable, Troubled Debt Restructuring
Financing receivable, modifications, subsequent default, number of contracts, during period	13	19	16
Financing Receivable, Modifications, Pre-Modification Recorded Investment	6,556	15,651	14,074
Commercial real estate | Interest only period
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment	4,753	543	0
Commercial real estate | Extension of terms or maturity
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment	672	1,378	959
Commercial real estate | Interest rate adjustment
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment	84	11,420	4,504
Commercial real estate | Other
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment	1,047	2,310	8,611
Commercial construction real estate
Loans and Leases Receivable, Troubled Debt Restructuring
Financing receivable, modifications, subsequent default, number of contracts, during period			1
Financing Receivable, Modifications, Pre-Modification Recorded Investment			3,155
Commercial construction real estate | Interest only period
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment			0
Commercial construction real estate | Extension of terms or maturity
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment			0
Commercial construction real estate | Interest rate adjustment
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment			0
Commercial construction real estate | Other
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment			3,155
Land acquisition & development construction real estate
Loans and Leases Receivable, Troubled Debt Restructuring
Financing receivable, modifications, subsequent default, number of contracts, during period		8	5
Financing Receivable, Modifications, Pre-Modification Recorded Investment		9,788	3,380
Land acquisition & development construction real estate | Interest only period
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment		528	0
Land acquisition & development construction real estate | Extension of terms or maturity
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment		7,308	1,000
Land acquisition & development construction real estate | Interest rate adjustment
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment		1,952	1,757
Land acquisition & development construction real estate | Other
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment		0	623
Residential construction real estate
Loans and Leases Receivable, Troubled Debt Restructuring
Financing receivable, modifications, subsequent default, number of contracts, during period		3	2
Financing Receivable, Modifications, Pre-Modification Recorded Investment		819	513
Residential construction real estate | Interest only period
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment		0	0
Residential construction real estate | Extension of terms or maturity
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment		408	280
Residential construction real estate | Interest rate adjustment
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment		411	233
Residential construction real estate | Other
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment		0	0
Total construction loans
Loans and Leases Receivable, Troubled Debt Restructuring
Financing receivable, modifications, subsequent default, number of contracts, during period		11	8
Financing Receivable, Modifications, Pre-Modification Recorded Investment		10,607	7,048
Total construction loans | Interest only period
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment		528	0
Total construction loans | Extension of terms or maturity
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment		7,716	1,280
Total construction loans | Interest rate adjustment
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment		2,363	1,990
Total construction loans | Other
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment		0	3,778
Residential real estate
Loans and Leases Receivable, Troubled Debt Restructuring
Financing receivable, modifications, subsequent default, number of contracts, during period	1	5	2
Financing Receivable, Modifications, Pre-Modification Recorded Investment, Restructured During Period	15
Financing Receivable, Modifications, Pre-Modification Recorded Investment		787	593
Residential real estate | Interest only period
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment, Restructured During Period	0
Financing Receivable, Modifications, Pre-Modification Recorded Investment		0	568
Residential real estate | Extension of terms or maturity
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment, Restructured During Period	0
Financing Receivable, Modifications, Pre-Modification Recorded Investment		708	25
Residential real estate | Interest rate adjustment
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment, Restructured During Period	0
Financing Receivable, Modifications, Pre-Modification Recorded Investment		0	0
Residential real estate | Other
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment, Restructured During Period	15
Financing Receivable, Modifications, Pre-Modification Recorded Investment		79	0
Agricultural real estate
Loans and Leases Receivable, Troubled Debt Restructuring
Financing receivable, modifications, subsequent default, number of contracts, during period		1	1
Financing Receivable, Modifications, Pre-Modification Recorded Investment		188	154
Agricultural real estate | Interest only period
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment		0	0
Agricultural real estate | Extension of terms or maturity
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment		0	154
Agricultural real estate | Interest rate adjustment
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment		188	0
Agricultural real estate | Other
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment		0	0
Total real estate loans
Loans and Leases Receivable, Troubled Debt Restructuring
Financing receivable, modifications, subsequent default, number of contracts, during period	14	36	27
Financing Receivable, Modifications, Pre-Modification Recorded Investment	6,571	27,233	21,869
Total real estate loans | Interest only period
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment	4,753	1,071	568
Total real estate loans | Extension of terms or maturity
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment	672	9,802	2,418
Total real estate loans | Interest rate adjustment
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment	84	13,971	6,494
Total real estate loans | Other
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment	1,062	2,389	12,389
Total consumer loans
Loans and Leases Receivable, Troubled Debt Restructuring
Financing receivable, modifications, subsequent default, number of contracts, during period	1	1	1
Financing Receivable, Modifications, Pre-Modification Recorded Investment	113	27	69
Total consumer loans | Interest only period
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment	0	0	0
Total consumer loans | Extension of terms or maturity
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment	113	0	69
Total consumer loans | Interest rate adjustment
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment	0	27	0
Total consumer loans | Other
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment	0	0	0
Commercial
Loans and Leases Receivable, Troubled Debt Restructuring
Financing receivable, modifications, subsequent default, number of contracts, during period	5	6	10
Financing Receivable, Modifications, Pre-Modification Recorded Investment	506	1,143	822
Commercial | Interest only period
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment	476	613	387
Commercial | Extension of terms or maturity
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment	0	178	217
Commercial | Interest rate adjustment
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment	0	265	0
Commercial | Other
Loans and Leases Receivable, Troubled Debt Restructuring
Financing Receivable, Modifications, Pre-Modification Recorded Investment	$30	$87	$218

Loans_Schedule_of_Recorded_Inv2

Loans - Schedule of Recorded Investment in Criticized Loans by Class and Credit Quality Indicator (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	$4,856,615	$4,303,992
Commercial real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,639,422	1,449,174
Land acquisition & development construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	220,443	205,911
Residential construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	96,580	76,488
Commercial construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	101,246	69,236
Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	418,269	351,635
Residential real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	999,903	867,912
Agricultural real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	167,659	173,534
Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3,225,253	2,842,255
Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	552,863	476,012
Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	144,141	133,039
Credit card consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	65,467	62,536
Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	762,471	671,587
Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	740,073	676,544
Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	124,859	111,872
Special Mention [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	154,084	159,081
Special Mention [Member] | Commercial real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	84,533	79,747
Special Mention [Member] | Land acquisition & development construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	11,826	13,211
Special Mention [Member] | Residential construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2,029	1,859
Special Mention [Member] | Commercial construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	39	0
Special Mention [Member] | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	13,894	15,070
Special Mention [Member] | Residential real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	10,473	7,500
Special Mention [Member] | Agricultural real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	10,122	13,597
Special Mention [Member] | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	119,022	115,914
Special Mention [Member] | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	916	875
Special Mention [Member] | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	553	573
Special Mention [Member] | Credit card consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0	0
Special Mention [Member] | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,469	1,448
Special Mention [Member] | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	25,766	33,318
Special Mention [Member] | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	7,827	8,401
Substandard [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	163,675	154,100
Substandard [Member] | Commercial real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	83,448	86,426
Substandard [Member] | Land acquisition & development construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	15,016	19,677
Substandard [Member] | Residential construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2,666	1,649
Substandard [Member] | Commercial construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	253	409
Substandard [Member] | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	17,935	21,735
Substandard [Member] | Residential real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	10,848	7,188
Substandard [Member] | Agricultural real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	12,328	10,245
Substandard [Member] | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	124,559	125,594
Substandard [Member] | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,590	1,524
Substandard [Member] | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,085	969
Substandard [Member] | Credit card consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	348	392
Substandard [Member] | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3,023	2,885
Substandard [Member] | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	32,433	23,833
Substandard [Member] | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	3,660	1,788
Doubtful [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	34,854	45,308
Doubtful [Member] | Commercial real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	15,246	24,840
Doubtful [Member] | Land acquisition & development construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2,507	7,329
Doubtful [Member] | Residential construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	0	277
Doubtful [Member] | Commercial construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2,442	84
Doubtful [Member] | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	4,949	7,690
Doubtful [Member] | Residential real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,121	4,184
Doubtful [Member] | Agricultural real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	612	2,370
Doubtful [Member] | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	21,928	39,084
Doubtful [Member] | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	121	115
Doubtful [Member] | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	432	268
Doubtful [Member] | Credit card consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,263	2,010
Doubtful [Member] | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,816	2,393
Doubtful [Member] | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	10,273	3,745
Doubtful [Member] | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	837	86
Total Criticized Loans [Member]
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	352,613	358,489
Total Criticized Loans [Member] | Commercial real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	183,227	191,013
Total Criticized Loans [Member] | Land acquisition & development construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	29,349	40,217
Total Criticized Loans [Member] | Residential construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	4,695	3,785
Total Criticized Loans [Member] | Commercial construction real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2,734	493
Total Criticized Loans [Member] | Total construction loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	36,778	44,495
Total Criticized Loans [Member] | Residential real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	22,442	18,872
Total Criticized Loans [Member] | Agricultural real estate
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	23,062	26,212
Total Criticized Loans [Member] | Total real estate loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	265,509	280,592
Total Criticized Loans [Member] | Indirect consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2,627	2,514
Total Criticized Loans [Member] | Other consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	2,070	1,810
Total Criticized Loans [Member] | Credit card consumer
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	1,611	2,402
Total Criticized Loans [Member] | Total consumer loans
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	6,308	6,726
Total Criticized Loans [Member] | Commercial
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	68,472	60,896
Total Criticized Loans [Member] | Agricultural
Loans and Leases Receivable Recorded Investment, Criticized Loans
Loans held for investment	$12,324	$10,275

Schedule_of_Allowance_for_Loan

Schedule of Allowance for Loan Losses by Portfolio Segment (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financing Receivable, Allowance for Credit Losses
Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Allowance for Loan Losses	$287
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	85,339	100,511	112,581
Provision charged to operating expense	-6,622	-6,125	40,750
Less loans charged-off	-13,995	-20,513	-60,936
Add back recoveries of loans previously charged-off	9,478	11,466	8,116
Ending balance	74,200	85,339	100,511
Allowance for loan losses:
Loans individually evaluated for impairment	5,792	8,929	10,297
Loans collectively evaluated for impairment	68,408	76,410	90,214
Ending balance	74,200	85,339	100,511
Total loans:
Individually evaluated for impairment	78,480	110,572	136,185
Individually evaluated for impairment	4,818,963	4,234,281	4,087,727
Total Loans	4,897,443	4,344,853	4,223,912
Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	63,923	75,782	87,396
Provision charged to operating expense	-10,348	-7,722	28,651
Less loans charged-off	-3,014	-10,224	-43,506
Add back recoveries of loans previously charged-off	3,323	6,087	3,241
Ending balance	53,884	63,923	75,782
Allowance for loan losses:
Loans individually evaluated for impairment	3,961	7,339	8,350
Loans collectively evaluated for impairment	49,923	56,584	67,432
Ending balance	53,884	63,923	75,782
Total loans:
Individually evaluated for impairment	62,775	96,862	123,406
Individually evaluated for impairment	3,203,306	2,786,254	2,660,420
Total Loans	3,266,081	2,883,116	2,783,826
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	6,193	7,141	8,594
Provision charged to operating expense	1,382	1,605	1,922
Less loans charged-off	-4,887	-4,612	-5,320
Add back recoveries of loans previously charged-off	2,347	2,059	1,945
Ending balance	5,035	6,193	7,141
Allowance for loan losses:
Loans individually evaluated for impairment	0	0	0
Loans collectively evaluated for impairment	5,035	6,193	7,141
Ending balance	5,035	6,193	7,141
Total loans:
Individually evaluated for impairment	0	0	0
Individually evaluated for impairment	762,471	671,587	636,794
Total Loans	762,471	671,587	636,794
Commercial
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	14,747	17,085	15,325
Provision charged to operating expense	1,809	41	10,845
Less loans charged-off	-6,030	-5,672	-11,990
Add back recoveries of loans previously charged-off	3,781	3,293	2,905
Ending balance	14,307	14,747	17,085
Allowance for loan losses:
Loans individually evaluated for impairment	1,190	1,504	1,919
Loans collectively evaluated for impairment	13,117	13,243	15,166
Ending balance	14,307	14,747	17,085
Total loans:
Individually evaluated for impairment	14,526	13,585	12,242
Individually evaluated for impairment	725,547	662,959	676,511
Total Loans	740,073	676,544	688,753
Agriculture
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	476	503	1,266
Provision charged to operating expense	535	-49	-668
Less loans charged-off	-64	-5	-120
Add back recoveries of loans previously charged-off	27	27	25
Ending balance	974	476	503
Allowance for loan losses:
Loans individually evaluated for impairment	641	86	28
Loans collectively evaluated for impairment	333	390	475
Ending balance	974	476	503
Total loans:
Individually evaluated for impairment	1,179	125	537
Individually evaluated for impairment	123,680	111,747	113,090
Total Loans	124,859	111,872	113,627
Other
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance	0	0	0
Provision charged 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
Allowance for loan losses:
Loans individually evaluated for impairment	0	0	0
Loans collectively evaluated for impairment	0	0	0
Ending balance	0	0	0
Total loans:
Individually evaluated for impairment	0	0	0
Individually evaluated for impairment	3,959	1,734	912
Total Loans	$3,959	$1,734	$912

Premises_and_Equipment_Details

Premises and Equipment (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Property, Plant and Equipment
Premises and equipment, gross	$324,907	$303,401
Less accumulated depreciation	-129,695	-123,711
Premises and equipment, net	195,212	179,690
Land
Property, Plant and Equipment
Premises and equipment, gross	40,620	37,581
Buildings and improvements
Property, Plant and Equipment
Premises and equipment, gross	209,328	196,950
Furniture and equipment
Property, Plant and Equipment
Premises and equipment, gross	$74,959	$68,870

Company_Owned_Life_Insurance_D

Company - Owned Life Insurance (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Schedule of Company - Owned Life Insurance, By Type
Company owned life insurance	$153,821	$122,175
Key executive, principal shareholder
Schedule of Company - Owned Life Insurance, By Type
Cash surrender value of life insurance	3,798	3,660
Key executive split dollar
Schedule of Company - Owned Life Insurance, By Type
Cash surrender value of life insurance	4,707	4,628
Group life
Schedule of Company - Owned Life Insurance, By Type
Cash surrender value of life insurance	145,316
FIB | Group life
Schedule of Company - Owned Life Insurance, By Type
Cash surrender value of life insurance	132,111	113,887
Mountain West Bank | Group life
Schedule of Company - Owned Life Insurance, By Type
Cash surrender value of life insurance	$13,205

Other_Real_Estate_Owned_Detail

Other Real Estate Owned (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Real Estate Acquired Through Foreclosure [Roll Forward]
Balance at beginning of year	$15,504	$32,571	$37,452
Acquisitions	3,608	0	0
Additions	5,198	11,545	43,541
Capitalized improvements	0	65	75
Valuation adjustments	-224	-3,512	-6,724
Dispositions	-10,532	-25,165	-41,773
Balance at end of year	13,554	15,504	32,571
Valuation adjustments	-224	-3,512	-6,724
Write-down due to change in appraised value	93	1,083	702
Write-down due to change in management estimates	$131	$2,429	$6,022

Mortgage_Servicing_Rights_At_A

Mortgage Servicing Rights - At Amortized Cost (Details) (Mortgage loans, USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Mortgage loans
Servicing Asset at Amortized Value, Balance [Roll Forward]
Servicing asset at amortized cost, before valuation reserve, beginning	$14,018	$13,224	$13,450
Sales of mortgage servicing rights	0	0	-735
Originations of mortgage servicing rights	2,717	3,581	4,563
Amorization expense	-2,361	-2,787	-3,501
Write-off of permanent impairment	0	0	-553
Servicing asset at amortized cost, before valuation reserve, ending	14,374	14,018	13,224
Less valuation reserve	-336	-472	-571
Servicing asset at fair value, ending	14,038	13,546	12,653
Principal balance of serviced loans underlying mortgage servicing rights	2,615,311	2,416,621	2,146,351
Mortgage servicing rights as a percentage of serviced loans	0.0054	0.0056	0.0059
Disclosure of Transfer of Securitizations or Asset-backed Financing Financial Assets Accounted for as Sale
Carrying value of sold mortgage servicing rights		735
Losses on sale of mortgage servicing rights recorded as other expense		($19)

Mortgage_Servicing_Rights_At_F

Mortgage Servicing Rights - At Fair Value (Details) (Mortgage loans, USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]
Mortagage servicing rights, fair value	$21,434
Servicing asset at fair value, assumptions used to estimate fair value, weighted average life	6 years 10 months 24 days
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%
Servicing assets at fair value, assumptions used to estimate fair value, prepayment speed	0.60%
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	21.00%
Servicing assets at fair value, assumptions used to estimate fair value, prepayment speed	2.40%

Mortgage_Servicing_Rights_Valu

Mortgage Servicing Rights - Valuation of Impairment (Details) (Mortgage loans, USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Mortgage loans
Valuation Allowance for Impairment of Recognized Servicing Assets
Impairment reversals	$136	$99	$771
Permanent impairment charged against carrying value	0	0	553
Carrying value of sold mortgage servicing rights		735
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale, Gain (Loss) on Sale		$19

Deposits_Details

Deposits (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Deposits [Abstract]
Interest-bearing Domestic Deposit, Brokered	$0	$0
Deposits, by Type [Abstract]
Non-interest bearing demand	1,791,364	1,491,683
Interest bearing:
Demand	2,133,273	1,848,806
Savings	1,843,355	1,602,544
Time, $100 and over	520,125	492,051
Time, other	718,095	698,666
Total interest bearing	5,214,848	4,642,067
Total deposits	7,006,212	6,133,750
Time deposits obtained through Certificate of Deposit Account Registry Service (CDARS)	40,491	51,526
Contractual Maturities, Time Deposits, $100,000 or More [Abstract]
2015, Time, $100 and Over	337,404
2016, Time, $100 and Over	90,509
2017, Time, $100 and Over	58,387
2018, Time, $100 and Over	20,971
2019, Time, $100 and Over	12,854
Thereafter, Time, $100 and Over	0
Time Deposits, $100,000 or More	520,125	492,051
Time Deposits, Fiscal Year Maturity [Abstract]
2015, Time	803,986
2016, Time	209,192
2017, Time	127,367
2018, Time	62,084
2019, Time	35,577
Thereafter, Time	14
Time Deposits	1,238,220
Interest expense, time deposits, $100,000 or more	4,003	4,880	6,951
Time Deposits, $250,000 or More	228,410	207,504
FDIC Deposit Insurance Limit	$250

LongTerm_Debt_and_Other_Borrow2

Long-Term Debt and Other Borrowed Funds (Details) (USD $)	1 Months Ended
In Thousands, unless otherwise specified	Feb. 29, 2008	Dec. 31, 2014	Dec. 31, 2013	Jan. 10, 2008	Feb. 28, 2008
Debt Instrument
Long-term debt and capital lease obligations		$38,067	$36,917
Fiscal Year Maturities of Long-term Debt
2015		285
2016		65
2017		71
2018		35,077
2019		83
Thereafter		2,486
Long-term debt and capital lease obligations		38,067	36,917
Other borrowings		9	3
Parent Company
Debt Instrument
Long-term debt		20,000	20,000
Subordinated Debt | Subordinated Term Loan, 6.81% | Parent Company
Debt Instrument
Long-term debt		20,000	20,000
Fiscal Year Maturities of Long-term Debt
Debt instrument, face amount				20,000
Debt instrument, interest rate, stated percentage		6.81%		6.81%
Subordinated Debt | Subordinated Term Loan, Variable Rate | Subsidiaries
Debt Instrument
Long-term debt		15,000	15,000
Fiscal Year Maturities of Long-term Debt
Debt instrument, face amount					15,000
Debt instrument, interest rate at period end		2.24%
Subordinated Debt | Subordinated Term Loan, Variable Rate | Subsidiaries | LIBOR
Fiscal Year Maturities of Long-term Debt
Debt instrument, basis spread on variable rate	2.00%
Subordinated Debt | Note Payable, 6.24% | Parent Company
Fiscal Year Maturities of Long-term Debt
Debt instrument, interest rate, stated percentage				6.24%
Notes Payable to Banks | Notes Payable to FHLB | Subsidiaries
Debt Instrument
Long-term debt		225	225
Fiscal Year Maturities of Long-term Debt
FHLB, advances, weighted average interest rate		4.86%
Capital Lease Obligations | Capital Lease Obligation, 8.00% | Subsidiaries
Debt Instrument
Capital Lease Obligations		1,643	1,692
Fiscal Year Maturities of Long-term Debt
Debt instrument, interest rate, stated percentage		8.00%
Federal Home Loan Bank | Notes Payable to FHLB | Subsidiaries
Fiscal Year Maturities of Long-term Debt
FHLB, advances, maximum amount available		701,768
Long-term FHLB advances			225
Federal Funds Purchased
Fiscal Year Maturities of Long-term Debt
Line of credit facility, maximum borrowing capacity		115,000
Federal Reserve Bank Advances
Fiscal Year Maturities of Long-term Debt
Line of credit facility, maximum borrowing capacity		364,205
Notes Payable, Other Payables [Member] | Note Payable, 6.24% | Subsidiaries
Fiscal Year Maturities of Long-term Debt
Debt instrument, face amount		$1,199

Subordinated_Debentures_Held_b2

Subordinated Debentures Held by Subsidiary Trusts - Financial Insruments Subject to Mandatory Redemption (Details) (Trusts, USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
Trust Preferred Securities Subject to Mandatory Redemption
Financial Instruments Subject to Mandatory Redemption by Settlement Terms
Financial Instruments Subject to Mandatory Redemption, Settlement Terms, Share Value, Amount	$80,000
Common Stock Subject to Mandatory Redemption
Financial Instruments Subject to Mandatory Redemption by Settlement Terms
Financial Instruments Subject to Mandatory Redemption, Settlement Terms, Share Value, Amount	$2,477

Subordinated_Debentures_Held_b3

Subordinated Debentures Held by Subsidiary Trusts - Subordinated Debentures (Details) (USD $)	12 Months Ended	1 Months Ended				0 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Oct. 31, 2007	Nov. 30, 2007	Dec. 31, 2007	Jan. 31, 2008	Dec. 26, 2014	Dec. 15, 2014	Dec. 31, 2013
	trusts
Subordinated Borrowing
Number of company sponsored wholly-owned business 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	10,310	10,310						10,310
Debt instrument, interest rate at period end	2.49%
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	15,464		15,464					15,464
Debt instrument, interest rate at period end	2.99%
First Interstate Statutory Trust I | Junior Subordinated Deferrable Interest Debentures Issued by FIST I | Debt Instrument, Redemption, At Issuance through Year Five
Subordinated Borrowing
Debt instrument, interest rate, stated percentage			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	20,619			20,619				20,619
Debt instrument, interest rate at period end	2.64%
First Interstate Statutory Trust III | Junior Subordinated Deferrable Interest Debentures Issued by FIST III | Debt Instrument, Redemption, At Issuance through Year Five
Subordinated Borrowing
Debt instrument, interest rate, stated percentage				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	15,464							15,464
Debt instrument, interest rate at period end	2.94%
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	10,310				10,310			10,310
Debt instrument, interest rate at period end	2.99%
First Interstate Statutory Trust V | Junior Subordinated Deferrable Interest Debentures Issued by FIST V | Debt Instrument, Redemption, At Issuance through Year Five
Subordinated Borrowing
Debt instrument, interest rate, stated percentage					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	10,310				10,310			10,310
Debt instrument, interest rate at period end	2.99%
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%
Mountain West Trusts [Member] | Junior Subordinated Deferrable Interest Debenture Issues by MWFC [Member]
Subordinated Borrowing
Redemption price						6,006	14,433
Liquidation amount per security						1	1
Mandatory redemption of trust preferred securities						5,825	14,000
Capitalization of subsidiaries						181	433
Mountain West Trusts [Member] | Junior Subordinated Deferrable Interest Debenture Issues by MWFC [Member] | LIBOR
Subordinated Borrowing
Debt instrument, basis spread on variable rate						3.10%	1.85%
Trust Preferred Securities Subject to Mandatory Redemption | Mountain West Trusts [Member]
Subordinated Borrowing
Financial Instruments Subject to Mandatory Redemption, Settlement Terms, Share Value, Amount	19,825
Common Stock Subject to Mandatory Redemption | Mountain West Trusts [Member]
Subordinated Borrowing
Financial Instruments Subject to Mandatory Redemption, Settlement Terms, Share Value, Amount	$614

Capital_Stock_and_Dividend_Res1

Capital Stock and Dividend Restrictions (Details) (USD $)	12 Months Ended			0 Months Ended	9 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jul. 31, 2014	Sep. 30, 2014	Jan. 24, 2014
Preferred Stock
Nonvoting, noncumulative preferred stock, shares outstanding	0	0
Common Stock, Recapitalization
Common stock, shares authorized	200,000,000
Common Stock
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned	$35,972,000
Payments for Repurchase of Common Stock	9,739,000	448,000	263,000
Series A Preferred Stock
Preferred Stock
Nonvoting, noncumulative preferred stock, shares outstanding	5,000,000
Preferred stock, value, issued	50,000,000
Preferred stock, dividend rate, per-dollar-amount	$675
Preferred stock, aggregate redemption price	50,150
Preferred stock, redemption price per share	$10,000
Class B Common Stock
Common Stock, Recapitalization
Common stock, shares authorized	100,000,000
Common Stock, Voting Rights	5
Common Stock
Common stock, shares, outstanding	23,859,483	24,287,045
Class A Common Stock
Class of Stock
Stock Repurchased and Retired During Period, Weighted Average Price	$25.07
Common Stock, Recapitalization
Common stock, shares authorized	100,000,000
Common Stock, Voting Rights	1
Common Stock
Common stock, shares, outstanding	21,928,932	19,868,018
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned				35,972,000
Stock Repurchased and Retired During Period, Shares	362,121
Payments for Repurchase of Common Stock	9,078,000
Amendment to 2006 Equity Compensation Plan [Domain] | Class A Common Stock
Class of Stock
Additional Shares Authorized						1,500,000
Mountain West Bank | Class A Common Stock
Common Stock
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	1,378,230			1,378,230
Board of Directors [Member]
Common Stock
Share-based Compensation Arrangement by Share-based Payment Award, Aggregate Value of Shares Issued		$543,000			$620,000
Board of Directors [Member] | Class A Common Stock
Common Stock
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period		26,096			24,581

Earnings_per_Common_Share_Comp

Earnings per Common Share Computation of Basic and Diluted Earnings per Share (Details) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net Income (Loss) Available to Common Stockholders, Basic [Abstract]
Net income	$84,401	$86,136	$58,224
Preferred stock dividends	0	0	3,300
Net income available to common shareholders	$84,401	$86,136	$54,924
Weighted Average Number of Shares Outstanding, Basic and Diluted [Abstract]
Weighted average common shares outstanding for basic earnings per share computation	44,615,060	43,566,681	42,965,987
Dilutive effect of stock-based compensation	595,501	477,921	126,991
Weighted average common shares outstanding for diluted earnings per common share computation	45,210,561	44,044,602	43,092,978
Earnings Per Share, Basic and Diluted [Abstract]
Basic earnings per common share (in dollars per share)	$1.89	$1.98	$1.28
Diluted earnings per common share (in dollars per share)	$1.87	$1.96	$1.27
Stock Options
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)	5,000	21,372	2,427,823
Unvested Restricted Stock
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)	88,797	37,734	41,240

Regulatory_Capital_Schedule_of

Regulatory Capital Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Total risk-based capital:
Actual Amount	$897,769	$829,443
Actual Ratio	16.20%	16.70%
Adequately Capitalized Amount	444,685	396,210
Adequately Capitalized Ratio	8.00%	8.00%
Tier 1 risk-based capital:
Actual Amount	807,229	739,246
Actual Ratio	14.50%	14.90%
Adequately Capitalized Amount	222,343	198,105
Adequately Capitalized Ratio	4.00%	4.00%
Leverage capital ratio:
Tier One Leverage Capital	807,229	739,246
Actual Ratio	9.60%	10.10%
Adequately Capitalized Amount	335,897	293,414
Adequately Capitalized Ratio	4.00%	4.00%
FIB
Total risk-based capital:
Actual Amount	832,907	723,955
Actual Ratio	15.10%	14.70%
Adequately Capitalized Amount	442,468	394,038
Adequately Capitalized Ratio	8.00%	8.00%
Well Capitalized Amount	553,085	492,548
Well Capitalized Ratio	10.00%	10.00%
Tier 1 risk-based capital:
Actual Amount	754,708	650,093
Actual Ratio	13.70%	13.20%
Adequately Capitalized Amount	221,234	197,019
Adequately Capitalized Ratio	4.00%	4.00%
Well Capitalized Amount	331,851	295,529
Well Capitalized Ratio	6.00%	6.00%
Leverage capital ratio:
Tier One Leverage Capital	754,708	650,093
Actual Ratio	9.20%	8.90%
Adequately Capitalized Amount	330,006	292,199
Adequately Capitalized Ratio	4.00%	4.00%
Well Capitalized Amount	$412,507	$365,248
Well Capitalized Ratio	5.00%	5.00%

Commitments_and_Contingencies_1

Commitments and Contingencies - Unrecorded Purchase Commitments (Details) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
Construction Contracts
Unrecorded Unconditional Purchase Obligation
Commitments to Purchase	$4,047
Residential Mortgage Backed Securities [Member]
Unrecorded Unconditional Purchase Obligation
Commitments to Purchase	$16,840

Commitments_and_Contingencies_2

Commitments and Contingencies - Operating Lease Commitments (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Future Minimum Rental Commitments
Operating leases, rent expense	$1,190	$1,403	$1,423
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2015	2,808
2016	2,912
2017	2,227
2018	1,725
2019	1,487
Thereafter	9,941
Operating leases, future minimum payments due	21,100
Third Parties
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2015	2,132
2016	2,236
2017	1,728
2018	1,537
2019	1,487
Thereafter	9,941
Operating leases, future minimum payments due	19,061
Related Partnership
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2015	676
2016	676
2017	499
2018	188
2019	0
Thereafter	0
Operating leases, future minimum payments due	$2,039
Ownership percentage in partnership	50.00%

Commitments_and_Contingencies_3

Commitments and Contingencies - Residential Mortgage Loans Sold with Recourse Obligations (Details) (Obligation to repurchase loans sold, USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Obligation to repurchase loans sold
Loss Contingencies
Loss contingency, estimate of possible loss	$4,486	$5,871

Commitments_and_Contingencies_4

Commitments and Contingencies Legal Proceedings (Details) (Pending Litigation [Member], Kelly Logging Inc. v. First Interstate Bank [Member], USD $)	0 Months Ended	1 Months Ended
In Thousands, unless otherwise specified	Oct. 01, 2014	Aug. 31, 2014	Dec. 31, 2014
Pending Litigation [Member] | Kelly Logging Inc. v. First Interstate Bank [Member]
Loss Contingencies
Loss Contingency, Damages Awarded, Value	$17,047	$17,047
Loss Contingency, Compensatory Damages Awarded, Value		287
Loss Contingency, Punitive Damages Awarded, Value		16,760
Loss Contingency Accrual			$4,000

Financial_Instruments_with_Off1

Financial Instruments with Off-Balance Sheet Risk (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Entity Information
Credit Extension Commitments	$1,415,119	$1,238,269
Credit Extension Commitments, Beyond One Year	363,609	304,789
Standby Letters of Credit
Entity Information
Credit Extension Commitments	58,950	53,508
Unused Credit Card Lines
Entity Information
Credit Extension Commitments	$476,752	$401,021

Income_Taxes_Schedule_of_Incom

Income Taxes - Schedule of Income Tax Exepnse (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current:
Federal	$34,941	$30,757	$18,458
State	4,928	4,533	2,818
Total current	39,869	35,290	21,276
Deferred:
Federal	4,870	10,056	7,697
State	475	1,220	1,065
Total deferred	5,345	11,276	8,762
Total income tax expense	$45,214	$46,566	$30,038

Income_Taxes_Reconciliation_of

Income Taxes - Reconciliation of Tax Expense at Effective Tax Rate (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Effective income tax rate reconciliation, at federal statutory income tax rate	35.00%	35.00%	35.00%
Income Tax Expense, Continuing Operations Income Tax Reconciliation [Abstract]
Tax expense at the statutory tax rate	$45,365	$46,446	$30,892
Increase (decrease) in tax resulting from:
Tax-exempt income	-4,255	-3,620	-3,498
State income tax, net of federal income tax benefit	3,541	3,741	2,524
Other, net	563	-1	120
Total income tax expense	$45,214	$46,566	$30,038

Income_Taxes_Deferred_Tax_Asse

Income Taxes - Deferred Tax Assets and Liabilities (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Deferred tax assets:
Loans, principally due to allowance for loan losses	$28,506	$30,903
Deferred Costs, Leasing, Gross	719	0
Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals, Contingencies	1,785	0
Deferred Discounts, Finance Charges and Interest Included in Receivables	7,022	0
Deferred Tax Assets, Unrealized Losses on Trading Securities	2,728	9,473
Employee benefits	8,256	6,813
Other real estate owned write-downs and carrying costs	2,536	4,731
Deferred gain on sale of subsidiary	253	484
Deferred revenue on contract	481	545
Other	760	299
Deferred tax assets	53,046	53,248
Deferred tax liabilities:
Fixed assets, principally differences in bases and depreciation	-569	-4,227
Deferred Tax Liabilities, Unrealized Gains on Available For Sale Securities	-1,351	0
Investment in joint venture partnership, principally due to differences in depreciation of partnership assets	-1,053	-700
Prepaid amounts	-1,385	-1,247
Government agency stock dividends	-2,407	-1,965
Goodwill and core deposit intangibles	-36,732	-28,167
Mortgage servicing rights	-4,144	-4,337
Other	-531	-451
Deferred tax liabilities	-48,172	-41,094
Net deferred tax assets	4,874	12,154
Income Taxes Payable, Current	4,693
Income Taxes Receivable, Current		$2,968

StockBased_Compensation_Weight

Stock-Based Compensation - Weighted-Average Assumptions Used in Option Pricing Model (Details) (Stock Option, USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Stock Option
Schedule of Share-Based Compensation Award, Options, Fair Value Assumption Inputs
Weighted average grant date fair value of options granted (in dollars per share)	$3.54	$4.06
Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions and Methodology [Abstract]
Expected volatility	30.61%	37.46%
Expected dividend yield	3.00%	3.35%
Risk-free interest rate	0.88%	1.99%
Expected life of options (in years)	5 years 6 months 6 days	7 years 10 months 6 days

StockBased_Compensation_Stock_

Stock-Based Compensation - Stock Options Activity Rollforward (Details) (USD $)	12 Months Ended
	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding options, beginning of year, number of shares	2,523,593
Granted, number of shares	0
Exercised, number of shares	-739,290
Forfeited, number of shares	-5,893
Expired, number of shares	-30,378
Outstanding options, end of year, number of shares	1,748,032
Outstanding options exercisable, end of year, number of shares	1,393,010
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	$17.20
Granted, weighted average exercise price	$0
Exercised, weighted average exercise price	$17.73
Forfeited, weighted average exercise price	$12.85
Expired, weighted average exercise price	$16.20
Outstanding options, end of year, weighted average exercise price	$17.01
Outstanding options exercisable, end of year, weighted average exercise price	$17.15
Outstanding options, end of year weighted average remaining contractual term	4 years 5 months 4 days
Outstanding options exercisable, end of year, weighted average remaining contractual term	4 years 1 month 13 days

StockBased_Compensation_NonVes

Stock-Based Compensation - Non-Vested Stock Options Rollforward (Details) (USD $)	12 Months Ended
	Dec. 31, 2014
Share-Based Compensation Arrangement by Share Based Payment Award, Options, Nonvested, Number of Shares [Roll Forward]
Nonvested stock options, beginning of year, number of shares	733,906
Nonvested stock options, granted, number of shares	0
Nonvested stock options, vested, number of shares	-348,506
Nonvested stock options, forfeited, number of shares	-30,378
Nonvested stock options, end of year, number of shares	355,022
Share-Based Compensation Arrangement by Share Based Payment Award, Options, Nonvested, Weighted-Average Grant Date Fair Value [Roll Forward]
Nonvested stock options, beginning of period, weighted-average grant date fair value	$3.78
Nonvested stock options, granted, weighted-average grant date fair value
Nonvested stock options, vested, weighted-average grant date fair value	$3.94
Nonvested stock options, forfeited, weighted-average grant date fair value	$3.70
Nonvested stock options, end of period, weighted-average grant date fair value	$3.62

StockBased_Compensation_Restri

Stock-Based Compensation - Restricted Stock Activity Rollforward (Details) (Restricted Stock, USD $)	12 Months Ended
	Dec. 31, 2014
Restricted Stock
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	225,525
Granted, number of shares	148,278
Vested, number of shares	-96,143
Forfeited, number of shares	-27,772
Canceled, number of shares	-1,489
Restricted stock, end of year, number of shares	248,399
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	$16.08
Granted, weighted average grant date fair value	$24.36
Vested, weighted average grant date fair value	$15.54
Forfeited, weighted average grant date fair value	$16.43
Cancelled, weighted average grant date fair value	$21.39
Restricted stock, end of year, weighted-average measurement date fair value	$21.16

StockBased_Compensation_Narrat

Stock-Based Compensation - Narrative (Details) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award
Stock compensation expense	$3,014	$2,936	$2,485
Tax benefits from stock-based compensation	1,153	1,122	950
Stock Option
Share-based Compensation Arrangement by Share-based Payment Award
Stock compensation expense	905	1,390	1,276
Tax benefits from stock-based compensation	346	531	488
Intrinsic value of fully-vested stock options outstanding	14,879
Total intrinsic value of options exercised	7,363	7,108	1,158
Tax benefit realized from stock option exercises	2,594	1,963	397
Proceeds from stock options exercised	6,299	9,271	1,612
Common stock redeemed for payment of stock option exercises, aggregate value	6,829	8,721	2,675
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized	577
Unrecognized compensation cost related to nonvested restricted stock to be recognized over weighted average period (in years)	0 years 10 months 9 days
Share-Based CompensationaArrangement By Share Based Payment Award, Options, Vested In Period, Fair Value	1,363
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award
Stock compensation expense	2,109	1,546	1,209
Tax benefits from stock-based compensation	807	591	462
Unrecognized compensation cost related to nonvested restricted stock to be recognized over weighted average period (in years)	1 year 7 months 6 days
Stock issued during period, shares, restricted stock award	148,278
Unrecognized compensation cost related to nonvested restricted stock awards	$3,353
Restricted Stock | Upon Achievement of Performance Goals
Share-based Compensation Arrangement by Share-based Payment Award
Stock issued during period, shares, restricted stock award	73,938
Restricted Stock | Upon Achievement of Defined Return on Asset Performance Goals
Share-based Compensation Arrangement by Share-based Payment Award
Stock issued during period, shares, restricted stock award	24,646
Restricted Stock | Upon Achievement of Defined Return on Equity Performance Goals
Share-based Compensation Arrangement by Share-based Payment Award
Stock issued during period, shares, restricted stock award	24,646
Restricted Stock | Upon Achievement of Defined Total Return to Shareholder Goals
Share-based Compensation Arrangement by Share-based Payment Award
Stock issued during period, shares, restricted stock award	24,646
Restricted Stock | Upon One-Third Annual Anniversary
Share-based Compensation Arrangement by Share-based Payment Award
Stock issued during period, shares, restricted stock award	74,340
2006 Plan | Stock Option
Share-based Compensation Arrangement by Share-based Payment Award
Shares available for grant	783,007
2006 Plan | Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award
Restricted stock award, vesting period	3 years

Employee_Benefit_Plans_Profit_

Employee Benefit Plans - Profit Sharing and Savings Plans (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
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, lesser of percent of compensation	100.00%
Eligibility requirement, lesser of annual contribution amount	$40,000
Eligibility requirement, requisite service period	3 years
Compensation expense	1,509,000	1,555,000	2,063,000
Savings Plan, Employee Elected
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits
Compensation expense	4,256,000	4,067,000	4,034,000
Maximum annual contribution per employee, amount per dollar of employee contribution	$1.25
Maximum annual contribution per employee, percent	4.00%

Employee_Benefit_Plans_Postret

Employee Benefit Plans - Postretirement Healthcare Plan (Details) (Postretirement Healthcare Plan, USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Postretirement Healthcare Plan
Defined Benefit Plan Disclosure
Minimum age requirement	55 years
Requisite service period	15 years
Unfunded benefit obligation	$3,577	$4,870
Net periodic benefit cost	400	571	561
Assumptions used calculating net periodic benefit cost, discount rate	3.60%	4.30%
Assumptions used calculating net periodic benefit cost, rate of compensation increase	5.50%	5.00%
Defined Benefit Plan, Expected Future Benefit Payments, Fiscal Year Maturity [Abstract]
2015	158
2016	163
2017	194
2018	219
2019	254
2020 through 2024	1,496
Accumulated other comprehensive income (loss), before tax, related to the Plan	613
Accumulated other comprehensive income (loss), net of tax	413
Net acturarial loss, net of tax	257
Unamortized transition asset, net of tax	356
Estimated amount amortized from accumulated other comprehensive loss into net periodic benefit costs	$35

Other_Comprehensive_Income_Com

Other Comprehensive Income - Comprehensive Income (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Change in net unrealized Gain during period:
Change in net unrealized gain during period, before tax amount	$21,147	($52,949)	($4,648)
Change in net unrealized gain during period, tax expense (benefit)	8,321	-20,835	-1,829
Change in net unrealized gain during period, net of tax amount	12,826	-32,114	-2,819
Investment securities available-for-sale:
Reclassification adjustment for net gains included in net income, before tax amount	-61	-1	-348
Reclassification adjustment for net, gains included in net income, tax expense (benefit)	-24	0	-137
Reclassification adjustment for net gains included in net income, net of tax amount	-37	-1	-211
Unamortized Premium on Available-For-Sale Securities Transferred to Held-To-Maturity
Unamortized premium on available-to-sale securities transferred into held-for-maturity, before tax amount	-548		56
Unamortized premium on available-to-sale securities transferred into held-for-maturity, tax expense (benefit)	-216		22
Unamortized premium on available-to-sale securities transferred into held-for-maturity, net of tax amount	-332		34
Change in net actuarial loss:
Change in net actuarial loss, before tax amount	1,731	137	-77
Change in net actuarial loss, tax expense (benefit)	681	54	-30
Change in net actuarial loss, net of tax amount	1,050	83	-47
Other comprehensive income (Loss)
Other comprehensive income (loss), before tax	22,269	-52,813	-5,017
Total other comprehensive income, tax expense (benefit)	8,762	-20,781	-1,974
Other comprehensive income, net of tax	$13,507	($32,032)	($3,043)

Other_Comprehensive_Income_Acc

Other Comprehensive Income - Accumulated Other Comprehensive Income (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Net accumulated other comprehensive income:
Net unrealized gain on investment securities available-for-sale	($2,121)	($14,578)
Net actuarial loss on defined benefit post-retirement benefit plans	-413	-1,463
Net accumulated other comprehensive income	($2,534)	($16,041)

NonCash_Investing_and_Financin1

Non-Cash Investing and Financing Activities (Details) (USD $)	12 Months Ended			0 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jul. 31, 2014
Class of Stock
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned	$35,972
Transferred loans to OREO	5,198	11,545	43,541
Transferred loans to mortgage servicing assets	2,717	3,581	4,563
Transfer of premises and equipment pending disposal to other assets, book value		1,448	566
Transfer from Investments		429
Transfer of preferred stock pending redemption from equity to liability			50,000
Class A Common Stock
Class of Stock
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned				35,972
Mountain West Bank | Class A Common Stock
Class of Stock
Business Acquisition, Equity Interest Issued or Issuable, Number of Shares	1,378,230			1,378,230

Condensed_Financial_Informatio2

Condensed Financial Information (Parent Company Only) - Condensed Balance Sheets (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
In Thousands, unless otherwise specified
Condensed balance sheets:
Cash and cash equivalents	$798,670	$534,827
Investment in subsidiaries, at equity:
Other assets	73,495	61,056
Total assets	8,609,936	7,564,651
Subordinated debentures held by subsidiary trusts	82,477	82,477
Total liabilities	7,701,012	6,763,070
Stockholders’ equity	908,924	801,581	751,186	771,020
Total liabilities and stockholders’ equity	8,609,936	7,564,651
Parent Company
Condensed balance sheets:
Cash and cash equivalents	65,483	105,274
Investment in subsidiaries, at equity:
Equity method investments	938,799	795,872
Advances from subsidiaries, net	4,337	0
Other assets	29,335	26,809
Total assets	1,037,954	927,955
Other liabilities	26,553	17,602
Advances to subsidiaries, net	0	6,295
Long-term debt	20,000	20,000
Subordinated debentures held by subsidiary trusts	82,477	82,477
Total liabilities	129,030	126,374
Stockholders’ equity	908,924	801,581
Total liabilities and stockholders’ equity	1,037,954	927,955
Bank subsidiary
Investment in subsidiaries, at equity:
Equity method investments	936,817	793,892
Nonbank subsidiaries
Investment in subsidiaries, at equity:
Equity method investments	$1,982	$1,980

Condensed_Financial_Informatio3

Condensed Financial Information (Parent Company Only) - Condensed Statements of Income (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions
Salaries and benefits	$96,513	$94,002	$89,833
Interest expense	18,606	20,695	30,114
Acquisition Expense	4,017	0	0
Earnings before income tax benefit	129,615	132,702	88,262
Income tax benefit	45,214	46,566	30,038
Net income	84,401	86,136	58,224
Parent Company
Condensed Financial Statements, Captions
Dividends from subsidiaries	58,900	71,400	40,000
Other interest income	42	34	92
Other income, primarily management fees from subsidiaries	12,166	12,809	10,042
Total income	71,108	84,243	50,134
Salaries and benefits	15,722	15,914	13,205
Interest expense	4,002	4,098	6,691
Acquisition Expense	4,017
Other operating expenses, net	7,512	7,546	7,150
Total expenses	31,253	27,558	27,046
Earnings before income tax benefit	39,855	56,685	23,088
Income tax benefit	-6,862	-5,703	-6,222
Income before undistributed earnings of subsidiaries	46,717	62,388	29,310
Undistributed earnings of subsidiaries	37,684	23,748	28,914
Net income	$84,401	$86,136	$58,224

Condensed_Financial_Informatio4

Condensed Financial Information (Parent Company Only) - Condensed Statements of Cash Flows (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities:
Net income	$84,401	$86,136	$58,224
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	16,855	16,245	17,112
Write-down of equipment pending disposal	102	616	70
Deferred income tax expense (benefit)	5,345	11,276	8,762
Stock-based compensation expense	3,014	2,936	2,485
Tax benefits from stock-based compensation	2,193	1,898	360
Excess tax benefits from stock-based compensation	-2,205	-2,031	-273
Net cash provided by operating activities	121,388	150,225	135,822
Cash flows from investing activities:
Payments to Acquire Businesses, Net of Cash Acquired	35,556	0	0
Net cash used in investing activities	-195,842	-204,937	-141,079
Cash flows from financing activities:
Redemption of preferred stock	-48	-243	-40
Repayment of junior subordinated debentures held by subsidiary trusts	-20,439	0	-41,238
Proceeds from issuance of common stock, net of stock issuance costs	6,919	9,814	1,911
Excess tax benefits from stock-based compensation	2,205	2,031	273
Purchase and retirement of common stock	-9,739	-448	-263
Dividends paid to common stockholders	-28,626	-17,909	-26,208
Dividends paid to preferred stockholders	0	0	-3,300
Net cash used in financing activities	338,297	-211,793	334,142
Net change in cash and cash equivalents	263,843	-266,505	328,885
Cash and cash equivalents at beginning of year	534,827	801,332	472,447
Cash and cash equivalents at end of year	798,670	534,827	801,332
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Transfer of preferred stock pending redemption from equity to liability			50,000
Parent Company
Cash flows from operating activities:
Net income	84,401	86,136	58,224
Adjustments to reconcile net income to cash provided by operating activities:
Undistributed earnings of subsidiaries	-37,684	-23,748	-28,914
Stock-based compensation expense	3,014	2,936	2,485
Tax benefits from stock-based compensation	2,193	1,898	360
Excess tax benefits from stock-based compensation	-2,205	-2,031	-273
Other, net	8,991	-5,804	3,327
Net cash provided by operating activities	58,710	59,387	35,209
Cash flows from investing activities:
Capital expenditures, net of sales	0	0	1
Payments to Acquire Businesses, Net of Cash Acquired	-37,891	0	0
Net cash used in investing activities	-37,891	0	1
Cash flows from financing activities:
Net (decrease) increase in advances from nonbank subsidiaries	-10,632	6,992	-2,838
Redemption of preferred stock	0	-50,000	0
Repayment of junior subordinated debentures held by subsidiary trusts	-20,439	0	-41,238
Proceeds from issuance of common stock, net of stock issuance costs	6,621	9,814	1,911
Excess tax benefits from stock-based compensation	2,205	2,031	273
Purchase and retirement of common stock	-9,739	-448	-263
Dividends paid to common stockholders	-28,626	-17,909	-26,208
Dividends paid to preferred stockholders	0	0	-3,300
Net cash used in financing activities	-60,610	-49,520	-71,663
Net change in cash and cash equivalents	-39,791	9,867	-36,453
Cash and cash equivalents at beginning of year	105,274	95,407	131,860
Cash and cash equivalents at end of year	65,483	105,274	95,407
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Transfer of preferred stock pending redemption from equity to liability	$50,000

Fair_Value_Measurements_Financ

Fair Value Measurements - Financial Assets and Financial Liabilities Measured at Fair Value on a Recurring Basis (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
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
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
Investment securities available-for-sale	720,933	763,238
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
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
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
Investment securities available-for-sale	990,666	1,184,053
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
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
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
Investment securities available-for-sale	325	415
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
Investment securities available-for-sale	0	0
Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investment securities available-for-sale	1,711,924	1,947,706
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investment securities available-for-sale	0	0
Estimated Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investment securities available-for-sale	1,711,924	1,947,706
Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Investment securities available-for-sale	0	0
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
Investment securities available-for-sale	720,933	763,238
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
Investment securities available-for-sale	990,666	1,184,053
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
Investment securities available-for-sale	$325	$415

Fair_Value_Measurements_Assets

Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Non-Recurring Basis (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Market Approach Valuation Technique [Member] | Loan and Leases Receivable, Impaired Loans [Member] | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair Value Inputs, Discount Rate	0.00%	-6.00%
Market Approach Valuation Technique [Member] | Loan and Leases Receivable, Impaired Loans [Member] | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair Value Inputs, Discount Rate	-51.00%	-66.00%
Market Approach Valuation Technique [Member] | Loan and Leases Receivable, Impaired Loans [Member] | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair Value Inputs, Discount Rate	-19.00%	-31.00%
Market Approach Valuation Technique [Member] | Real Estate Acquired in Satisfaction of Debt [Member] | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair Value Inputs, Discount Rate	0.00%	-6.00%
Market Approach Valuation Technique [Member] | Real Estate Acquired in Satisfaction of Debt [Member] | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair Value Inputs, Discount Rate	-50.00%	-55.00%
Market Approach Valuation Technique [Member] | Real Estate Acquired in Satisfaction of Debt [Member] | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair Value Inputs, Discount Rate	-15.00%	-15.00%
Market Approach Valuation Technique [Member] | Assets Held-for-sale [Member] | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair Value Inputs, Discount Rate	0.00%	0.00%
Market Approach Valuation Technique [Member] | Assets Held-for-sale [Member] | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair Value Inputs, Discount Rate	-9.00%	-9.00%
Market Approach Valuation Technique [Member] | Assets Held-for-sale [Member] | Weighted Average [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Fair Value Inputs, Discount Rate	-5.00%	-6.00%
Estimated Fair Value | Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Loans Receivable, Impaired, Fair Value Disclosure	30,494	57,302
Real Estate Acquired Through Foreclosure, Fair Value Disclosure	4,554	8,502
Long-lived assets to be disposed, fair value disclosure	1,083	1,186
Estimate of Fair Value, Fair Value Disclosure, Gain (Loss) | Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Loans Receivable, Impaired, Fair Value Disclosure	-14,552	-23,224
Real Estate Acquired Through Foreclosure, Fair Value Disclosure	-12,665	-14,441
Long-lived assets to be disposed, fair value disclosure	-702	-599
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Loans Receivable, Impaired, Fair Value Disclosure	30,494	57,302
Real Estate Acquired Through Foreclosure, Fair Value Disclosure	4,554	8,502
Long-lived assets to be disposed, fair value disclosure	1,083	1,186
Significant Unobservable Inputs (Level 3) | Estimated Fair Value | Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Loans Receivable, Impaired, Fair Value Disclosure	30,494	57,302
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Loans Receivable, Impaired, Fair Value Disclosure	0	0
Real Estate Acquired Through Foreclosure, Fair Value Disclosure	0	0
Long-lived assets to be disposed, fair value disclosure	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Loans Receivable, Impaired, Fair Value Disclosure	0	0
Real Estate Acquired Through Foreclosure, Fair Value Disclosure	0	0
Long-lived assets to be disposed, fair value disclosure	0	0

Fair_Value_Measurements_Estima

Fair Value Measurements - Estimated Fair Values of Financial Instruments by Level of Valuation Inputs (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Financial assets:
Held-to-Maturity Estimated Fair Value	$584,533	$205,926
Carrying Amount
Financial assets:
Cash and cash equivalents	798,670	534,827
Investment securities available-for-sale	1,711,924	1,947,706
Held-to-Maturity Estimated Fair Value	575,186	203,837
Accrued interest receivable	27,063	26,450
Mortgage servicing rights	14,038	13,546
Net loans	4,823,243	4,259,514
Total financial assets	7,950,124	6,985,880
Financial liabilities:
Total deposits, excluding time deposits	5,767,992	4,943,033
Time deposits	1,238,220	1,190,717
Securities sold under repurchase agreements	502,250	457,437
Other borrowed funds	9	3
Accrued interest payable	5,833	4,963
Long-term debt	38,067	36,917
Subordinated debentures held by subsidiary trusts	82,477	82,477
Total financial liabilities	7,634,848	6,715,547
Estimated Fair Value
Financial assets:
Cash and cash equivalents	798,670	534,827
Investment securities available-for-sale	1,711,924	1,947,706
Held-to-Maturity Estimated Fair Value	584,533	205,926
Accrued interest receivable	27,063	26,450
Mortgage servicing rights	21,434	25,698
Net loans	4,800,725	4,246,539
Total financial assets	7,944,349	6,987,146
Financial liabilities:
Total deposits, excluding time deposits	5,767,992	4,943,033
Time deposits	1,244,324	1,196,250
Securities sold under repurchase agreements	502,250	457,437
Other borrowed funds	9	3
Accrued interest payable	5,833	4,963
Long-term debt	37,781	34,508
Subordinated debentures held by subsidiary trusts	75,734	72,045
Total financial liabilities	7,633,923	6,708,239
Quoted Prices in Active Markets for Identical Assets (Level 1) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	798,670	534,827
Investment securities available-for-sale	0	0
Held-to-Maturity Estimated Fair Value	0	0
Accrued interest receivable	0	0
Mortgage servicing rights	0	0
Net loans	0	0
Total financial assets	798,670	534,827
Financial liabilities:
Total deposits, excluding time deposits	5,767,992	4,943,033
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
Total financial liabilities	5,767,992	4,943,033
Significant Other Observable Inputs (Level 2) | Estimated Fair Value
Financial assets:
Cash and cash equivalents	0	0
Investment securities available-for-sale	1,711,924	1,947,706
Held-to-Maturity Estimated Fair Value	584,533	205,926
Accrued interest receivable	27,063	26,450
Mortgage servicing rights	21,434	25,698
Net loans	4,770,231	4,189,237
Total financial assets	7,115,185	6,395,017
Financial liabilities:
Total deposits, excluding time deposits	0	0
Time deposits	1,244,324	1,196,250
Securities sold under repurchase agreements	502,250	457,437
Other borrowed funds	9	3
Accrued interest payable	5,833	4,963
Long-term debt	37,781	34,508
Subordinated debentures held by subsidiary trusts	75,734	72,045
Total financial liabilities	1,865,931	1,765,206
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 Estimated Fair Value	0	0
Accrued interest receivable	0	0
Mortgage servicing rights	0	0
Net loans	30,494	57,302
Total financial assets	30,494	57,302
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
Total financial liabilities	$0	$0

Fair_Value_Measurements_Narrat

Fair Value Measurements - Narrative (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Related allowance	$5,792	$8,929	$10,297
Impairment losses	102	616	70
Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Long-lived assets to be disposed of, carrying value	1,785	1,785
Impairment losses	702	599
Long-lived assets to be disposed of, fair value	1,083	1,186
Fair Value, Measurements, Nonrecurring | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Loans Receivable, Impaired, Fair Value Disclosure	30,494	57,302
Related allowance	5,792	8,929
Impaired financing receivable, related allowance, partial loan charge-offs	8,760	14,295
Fair Value, Measurements, Nonrecurring | Carrying Amount | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Loans Receivable, Impaired, Fair Value Disclosure	45,046	80,526
Fair Value, Measurements, Nonrecurring | Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Loans Receivable, Impaired, Fair Value Disclosure	30,494	57,302
Fair Value, Measurements, Nonrecurring | Estimated Fair Value | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis
Loans Receivable, Impaired, Fair Value Disclosure	$30,494	$57,302

Related_Party_Transactions_Det

Related Party Transactions (Details) (USD $)	12 Months Ended
In Thousands, except Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Director | Deferred Compensation Agreement
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Related party transaction, expenses from transactions with related party		$424
Related party transaction, liability from acquisition		577
Related party transaction, liability from acquisition, payout period	4 years
Director | Interest Guaranty Agreement, Interest Payments on Loans
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Revenue from related parties			815
Director | Interest Guaranty Agreement, Proceeds From Liquidated Collateral
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Related party transactions, other revenues			7,998
Shareholder
Related Party Transaction
Ownership percentage threshhold for related party	5.00%
Executive Officers, Directors, Shareholders Greater than Five Percent, and Related Entities and Individuals to Such Persons [Member]
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Loans and leases receivable, related parties	25,794	24,669
Loans and leases receivable, related parties, additions	9,519
Loans and leases receivable, related parties, collections	9,342
Director, Controlling Ownership Interest in Entity | Insurance Premiums
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Related party transaction, selling, general and administrative expenses from transactions with related party		764	839
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	306	309	262
Related party transactions, other revenues	77	61	47
1 Shareholder Greater than 5% and 6 Directors
Related Party Transaction
Ownership percentage threshhold for related party	5.00%
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Ownership Percentage, Related Party	18.00%
Entity Majority Owned by Shareholders and Directors
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Related party relationship, number of significant shareholders	1
Related party relationship, number of directors	6
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	255	224	243
Director, Wholly-Owned Entity | Construction Management and Adivsory Services
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Related party transaction, expenses from transactions with related party	60	60	60
Related party transaction, contractual agreement, amount	180
Related party transaction, contractual agreement, percentage of contract price	4.00%
Related party transaction, contractual agreement, initial payment	$60
Class A Common Stock
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Stock Repurchased and Retired During Period, Shares	362,121
Stock Repurchased and Retired During Period, Weighted Average Price	$25.07
Class A Common Stock | Director
Loans and Leases Receivable, Related Parties Disclosure [Abstract]
Stock Repurchased and Retired During Period, Shares	37,615
Stock Repurchased and Retired During Period, Weighted Average Price	$24.96

Authoritative_Accounting_Guida1

Authoritative Accounting Guidance New Accounting Pronouncements and Changes in Accounting Principles (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
	Investments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of Investments in Other Assets, Noncurrent	4
Current Income Tax Expense (Benefit)	$39,869	$35,290	$21,276
Accounting Standards Update 2014-01 [Member] | Investment in Affordable Housing Projects [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
2015	6,943
2016	37
2017	37
2018	37
2019	37
2020	37
2021	37
2022	0
2023	26
Accounting Standards Update 2014-01 [Member] | Investment in Affordable Housing Projects [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other Assets, Miscellaneous	5,084
Current Income Tax Expense (Benefit)	($27)

Subsequent_Events_Subsequent_E

Subsequent Events Subsequent Events (Details) (Common Class A Stoc, USD $)	Jan. 22, 2015
Subsequent Event [Member]
Subsequent Event [Line Items]
Dividends Payable, Amount Per Share	$0.20
January 2015 Board Approved Stock Repurchased Program [Member]
Subsequent Event [Line Items]
Additional Shares Authorized	1,000,000