Securities Available for Sale - Schedule of Amortized Cost and Fair Value of Investments by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016
Amortized Cost
Due in one year or less	$ 5,182	$ 3,706
Due after one year through five years	274,901	265,253
Due after five years through ten years	20,468	18,449
Due after ten years	122	122
Amortized Cost	300,673	287,530
Estimated Fair Value
Due in one year or less	5,173	3,709
Due after one year through five years	275,386	269,242
Due after five years through ten years	19,612	18,413
Due after ten years	122	122
Estimated Fair Value	300,293	291,486
Amortized Cost	1,384,085	1,308,445
Estimated Fair Value	1,371,558	1,317,386
Securities without contractual maturities, Amortized Cost	1,013	1,013
Securities without contractual maturities, Fair Value	1,053	1,062
Mortgage-backed securities
Estimated Fair Value
Amortized Cost	1,082,399	1,019,902
Estimated Fair Value	$ 1,070,212	$ 1,024,838