Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2021	Sep. 30, 2021
Statement of Financial Position [Abstract]
Interest-earning deposits	$ 106,225	$ 24,289
Available-for-sale securities, amortized cost	1,899,027	2,008,456
Loans receivable, allowance for credit losses	$ 17,535	$ 19,823
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	138,842,784	138,832,284
Common stock, shares outstanding	138,842,784	138,832,284