Goodwill and Other Intangible Assets - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Line Items]
Goodwill acquired during the period	$ 173,033,000	$ 111,347,000
Amortization expense of other intangible assets	11,841,000	12,486,000	$ 11,746,000
Impairment charges	$ 0	$ 0	$ 0
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
Anchor Bank (MN) [Member]
Goodwill [Line Items]
Goodwill acquired during the period	$ 173,000,000
Anchor Bank (MN) [Member] | Core Deposit [Member]
Goodwill [Line Items]
Increase (decrease) in intangible assets	$ 27,300,000