Goodwill and Other Intangible Assets - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Finite-Lived Intangible Assets [Line Items]
Goodwill from business combinations	$ 394,028,000
Goodwill impairment	0	$ 0	$ 0
Finite lived intangibles	171,769,000	126,621,000
Impairment of core deposits and intangibles	0	0	0
Amortization expense - other intangible assets	$ 18,400,000	10,000,000	11,300,000
Core deposits
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period, years	8 years
Finite lived intangibles	$ 80,470,000	43,396,000
Other
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period, years	7 years 7 months 8 days
Finite lived intangibles	$ 3,945,000	4,087,000
Loan servicing rights
Finite-Lived Intangible Assets [Line Items]
Finite lived intangibles	87,354,000	79,138,000
Residential mortgage
Finite-Lived Intangible Assets [Line Items]
Aggregate principal balances of mortgage loans serviced	3,200,000,000	$ 2,800,000,000	$ 2,500,000,000
USAmeriBancorp, Inc.
Finite-Lived Intangible Assets [Line Items]
Goodwill from business combinations	$ 5,800,000
USAmeriBancorp, Inc. | Core deposits
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period, years	10 years
Finite lived intangibles				$ 44,600,000
USAmeriBancorp, Inc. | Loan servicing rights
Finite-Lived Intangible Assets [Line Items]
Finite lived intangibles				$ 1,400,000