Securities Available for Sale - Schedule of Amortized Cost and Fair Value of Investments by Contractual Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Amortized  Cost
Due in one year or less	$ 82,615	$ 58,377
Due after one year through five years	35,858	89,836
Due after five years through ten years	11,667	12,110
Due after ten years	0	0
Amortized  Cost	130,140	160,323
Estimated Fair Value
Due in one year or less	83,577	58,343
Due after one year through five years	36,102	90,601
Due after five years through ten years	11,905	12,324
Due after ten years	0	0
Estimated Fair Value	131,584	161,268
Amortized  Cost	1,941,922	1,765,284
Estimated Fair Value	1,990,027	1,783,208
Securities without contractual maturities, Amortized Cost	1,006	1,006
Securities without contractual maturities, Fair Value	1,051	1,037
Mortgage-backed securities
Estimated Fair Value
Amortized  Cost	1,810,776	1,603,955
Estimated Fair Value	$ 1,857,392	$ 1,620,903