Borrowings - Schedule of borrowed funds (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Borrowings	$ 1,407,022,000	$ 1,189,180,000
Weighted Average Interest Rate	2.14%	2.43%
Overnight lines of credit
Debt Instrument [Line Items]
Borrowings	$ 107,800,000	$ 159,600,000
Weighted Average Interest Rate	1.81%	2.60%
Federal Home Loan Bank advances
Debt Instrument [Line Items]
Borrowings	$ 1,275,391,000	$ 1,029,580,000
Weighted Average Interest Rate	2.09%	2.40%
Subordinated notes
Debt Instrument [Line Items]
Borrowings	$ 16,899,000	$ 0
Weighted Average Interest Rate	6.75%	0.00%
Junior subordinated debt
Debt Instrument [Line Items]
Borrowings	$ 6,932,000	$ 0
Weighted Average Interest Rate	5.09%	0.00%