The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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Goodwill and Other Intangible Assets - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Goodwill [Line Items]
Amortization expense of other intangible assets	$ 3,416,000	$ 2,781,000	$ 7,025,000	$ 5,801,000
Impairment charges			0	$ 0
Anchor Bank (MN) [Member]
Goodwill [Line Items]
Increase (decrease) to goodwill			$ 800,000
Core Deposits and Other Intangible Assets [Member] \| Minimum [Member]
Goodwill [Line Items]
Estimated useful lives of core deposits and customer relationships			5 years
Core Deposits and Other Intangible Assets [Member] \| Maximum [Member]
Goodwill [Line Items]
Estimated useful lives of core deposits and customer relationships			15 years
Core Deposit [Member] \| Anchor Bank (MN) [Member]
Goodwill [Line Items]
Increase (decrease) in core deposit intangibles			$ (700,000)