Goodwill and Other Intangible Assets - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Goodwill from business combinations			$ 395,073,000
Goodwill impairment	$ 0	$ 0	0	$ 0
Gross intangible assets	170,813,000		170,813,000			$ 126,621,000
Amortization of other intangible assets	4,697,000	2,498,000	$ 13,607,000	7,596,000
Core Deposits
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period, years			10 years
Gross intangible assets	80,470,000		$ 80,470,000		$ 44,600,000	43,396,000
Loan servicing rights
Finite-Lived Intangible Assets [Line Items]
Gross intangible assets	86,398,000		$ 86,398,000		$ 1,400,000	79,138,000
Other
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period, years			20 years
Gross intangible assets	3,945,000		$ 3,945,000			$ 4,087,000
Core Deposits and Other
Finite-Lived Intangible Assets [Line Items]
Impairment of core deposits and other intangibles	0	$ 0	$ 0	$ 0
USAmeriBancorp, Inc.
Finite-Lived Intangible Assets [Line Items]
Goodwill from business combinations	$ 6,800,000
USAmeriBancorp, Inc. | Core Deposits
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period, years			10 years