Borrowings - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Debt Instrument [Line Items]
Borrowings	$ 929,057	$ 1,189,180	$ 929,057
Unused line of credit		225,000
Advances from federal home loan banks	1,600,000	1,700,000	1,600,000
Line of Credit
Debt Instrument [Line Items]
Borrowings	46,000	159,600	46,000
Interest expense, debt	70	2,200		$ 233		$ 42
Federal Home Loan Bank advances
Debt Instrument [Line Items]
Borrowings	822,400	1,029,580	822,400
Interest expense, debt	3,300	17,100		12,800		13,200
Junior subordinated debt
Debt Instrument [Line Items]
Borrowings	50,657	0	50,657
Interest expense, debt	1,000	3,500		$ 4,200		$ 4,200
Deferred issuance costs	890		890
Securities sold under agreements to repurchase
Debt Instrument [Line Items]
Borrowings	10,000	0	10,000
Interest expense, debt	203	3	1,600		$ 2,400
Pledged collateral	12,900	0	12,900
Interest rate products-designated | Federal Home Loan Bank advances
Debt Instrument [Line Items]
Notional amount of derivative	$ 20,000	$ 320,000	$ 20,000