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Borrowings - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
Borrowings - Narrative (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2017
Debt Instrument [Line Items]
Borrowings		$ 1,407,022,000	$ 1,189,180,000
Unused line of credit		250,000,000.0	225,000,000.0
Advances from federal home loan banks		$ 1,900,000,000	$ 1,700,000,000
Weighted average interest rate		2.14%	2.43%
Line of Credit
Debt Instrument [Line Items]
Borrowings		$ 107,800,000	$ 159,600,000
Interest expense, debt	$ 70,000	$ 1,800,000	$ 2,200,000	$ 233,000
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
Interest expense, debt	3,300,000	$ 25,200,000	$ 17,100,000	12,800,000
Weighted average interest rate		2.09%	2.40%
Junior subordinated debt
Debt Instrument [Line Items]
Borrowings		$ 6,932,000	$ 0
Interest expense, debt	1,000,000.0	$ 113,000	$ 3,500,000	4,200,000
Weighted average interest rate		5.09%	0.00%
Subordinated notes
Debt Instrument [Line Items]
Borrowings		$ 16,899,000	$ 0
Interest expense, debt	0	$ 65,000	$ 0	0
Interest rate, effective percentage		5.09%
Weighted average interest rate		6.75%	0.00%
Interest rate, stated percentage		6.75%
Securities sold under agreements to repurchase
Debt Instrument [Line Items]
Interest expense, debt	$ 203,000	$ 0	$ 3,000	$ 1,600,000
Advances from federal home loan banks			246,200,000
Interest rate swaps \| Federal Home Loan Bank advances
Debt Instrument [Line Items]
Notional amount of derivative		$ 410,000,000.0	$ 320,000,000.0
London Interbank Offered Rate (LIBOR) \| Junior subordinated debt
Debt Instrument [Line Items]
Basis spread on variable rate		2.95%