Deposits - Interest Expense on Deposits (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2017
Assets Sold under Agreements to Repurchase [Line Items]
Interest expense on deposits	$ 7,631	$ 61,551	$ 39,523	$ 25,581
Demand Deposits
Assets Sold under Agreements to Repurchase [Line Items]
Interest expense on deposits	2,509	19,922	12,933	8,556
Savings and Clubs
Assets Sold under Agreements to Repurchase [Line Items]
Interest expense on deposits	210	770	993	630
Certificates of Deposit
Assets Sold under Agreements to Repurchase [Line Items]
Interest expense on deposits	$ 4,912	$ 40,859	$ 25,597	$ 16,395