Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
Statement of Financial Position [Abstract]
Interest-earning deposits	$ 198,809	$ 122,733
Available-for-sale securities, amortized cost	1,191,455	718,564
Held-to-maturity securities, estimated fair value	0	601,071
Loans receivable, allowance for credit losses	$ 9,226	$ 8,463
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	100,000,000	100,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	1,400,000,000	1,400,000,000
Common stock, shares issued	141,440,030	141,225,516
Common stock, shares outstanding	141,440,030	141,225,516