Certain loans acquired in transfer not accounted for as debt securities outstanding balance1.

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Commercial real estate loans as a percentage of risk-based capital.

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The regulatory guideline limit for commercial real estate loans as a percentage of risk-based capital.

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Maximum number of days past due before loan is placed on nonaccrual status.

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Amount of troubled debt restructured loans that are on non accrual status.

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Amount of allowance for loan losses associated with troubled debt restructured loans.

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Maximum number of days past due before loan is charged off.

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Minimum number of days past due that loan is charged off.

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Loan guarantee percent.

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Amount of loan participations with other financial institutions.

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Amount of loan participations retained.

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Amount of loan participations sold to other financial institutions.

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Number of days contractually past due under the modified terms that a troubled debt restructured loan is considered to be in payment default.

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Loans sold.

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Number of days delinquent without regard to collateral position for small commercial loans.

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Amount of time a borrower of a troubled debt restructuring loan must perform in accordance with the restructured terms before returning to accrual status.

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Current period recognition due to accretion of the excess of a loan's cash flows expected to be collected over the investor's initial investment in the loan for loans accounted for as debt securities, with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

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Reserve for loans impairments recorded after acquisition as a result of current information and events which result in it being probable that estimated cash flows will be less than previously expected or because actual cash flows are less than cash flows previously expected. This relates to loans not accounted for as debt securities, with evidence of deterioration of credit quality since origination that was acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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The increase (decrease) in the allowance for credit losses related to financing receivables.

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Amount of direct write-downs of financing receivables charged against the allowance.

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Reflects the amount of the write-down related to a troubled debt restructuring.

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The net gain (loss) resulting from a sale of loans, including adjustments to record loans classified as held-for-sale at the lower-of-cost-or-market and fair value adjustments to loan held for investment purposes.

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Loans and Allowance for Loan Losses - Additional Information (Detail) - USD ($)	3 Months Ended			6 Months Ended	9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Nov. 01, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Commercial real estate	$ 4,481,554,000		$ 3,370,211,000		$ 4,481,554,000	$ 3,370,211,000	$ 4,354,552,000
Total loans	11,292,659,000		9,398,124,000		$ 11,292,659,000	9,398,124,000	11,118,121,000
Loan placed on nonaccrual when past due, number of days					90 days
Loan participations	752,300,000				$ 752,300,000
Loan participations sold	371,200,000				371,200,000
Loan participations retained	381,100,000				$ 381,100,000
Troubled debt restructuring term					6 months
Minimum number of days for loan charge off to be recorded					120 days
Maximum number of days for loan charge off to be recorded					180 days
Nonaccrual period for loans					90 days
Financing receivable TDR's included with non-accrual loans	29,900,000				$ 29,900,000		34,000,000.0
Financing receivable troubled debt restructurings specific reserves	3,000,000.0				3,000,000.0		5,700,000
Increase (decrease) in allowance for loan losses					0	3,200,000
Allowance for loan losses charge-offs					$ 0	0
Number of days for a loan to be considered to be in payment default					90 days
Outstanding loans including principal, interest, fees and penalties	236,100,000				$ 236,100,000		235,900,000
Accretion of income	1,900,000		$ 5,400,000		8,900,000	$ 12,800,000
Financing receivable allowance for loan losses related to purchased loans	$ 303,000				303,000		166,000
Maximum [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Value of small commercial loans on nonaccrual status or 90 days or more delinquent					$ 250,000
Commercial Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Percentage of risk-based capital	211.00%				211.00%
Regulatory guideline limit	300.00%				300.00%
Total loans	$ 4,481,554,000				$ 4,481,554,000		4,354,552,000
Financing receivable allowance for loan losses related to purchased loans	144,000				144,000
Consumer [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	1,695,625,000				1,695,625,000		1,879,247,000
Financing receivable allowance for loan losses related to purchased loans	$ 151,000				$ 151,000		134,000
Consumer [Member] \| Student Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans							68,200,000
Loan sold				$ 64,900,000
Consumer [Member] \| Student Loan [Member] \| Other Income [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Gain on sale of student loan		$ 2,200,000
Consumer [Member] \| Minimum [Member] \| Student Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan guarantee percentage	97.00%				97.00%
Consumer [Member] \| Maximum [Member] \| Student Loan [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan guarantee percentage	100.00%				100.00%
Anchor Bank (MN) [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Commercial real estate								$ 864,400,000
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Total loans	$ 2,166,203,000				$ 2,166,203,000		$ 2,167,053,000
Credit Concentration Risk [Member] \| Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Concentration Risk Percentage					10.00%
Credit Concentration Risk [Member] \| Non Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Concentration Risk Percentage					10.00%