Fair Value Measurements - Fair Value on Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017
Financial assets:
Securities available-for-sale, at fair value	$ 710,570	$ 557,176
Securities held-to-maturity	236,125	131,822
Quoted prices in active markets for identical assets (Level 1)
Financial assets:
Cash and cash equivalents	65,498	100,975
Securities available-for-sale, at fair value	42,831	28,128
Securities held-to-maturity	0	0
Federal Home Loan Bank Stock	0	0
Loans receivable, net	0	0
Derivative assets	0	0
Financial liabilities:
Total deposits	0	0
Borrowings	0	0
Derivative liabilities	0	0
Significant other observable inputs (Level 2)
Financial assets:
Cash and cash equivalents	0	0
Securities available-for-sale, at fair value	667,739	529,048
Securities held-to-maturity	236,125	131,822
Federal Home Loan Bank Stock	44,664	35,844
Loans receivable, net	0	0
Derivative assets	490	277
Financial liabilities:
Total deposits	3,959,460	3,880,363
Borrowings	925,032	732,731
Derivative liabilities	203	182
Significant unobservable inputs (Level 3)
Financial assets:
Cash and cash equivalents	0	0
Securities available-for-sale, at fair value	0	0
Securities held-to-maturity	0	0
Federal Home Loan Bank Stock	0	0
Loans receivable, net	4,367,945	4,301,138
Derivative assets	0	0
Financial liabilities:
Total deposits	0	0
Borrowings	0	0
Derivative liabilities	0	0
Carrying Value
Financial assets:
Cash and cash equivalents	65,498	100,975
Securities available-for-sale, at fair value	710,570	557,176
Securities held-to-maturity	239,618	132,939
Federal Home Loan Bank Stock	44,664	35,844
Loans receivable, net	4,400,470	4,307,623
Derivative assets	490	277
Financial liabilities:
Total deposits	4,263,315	4,123,428
Borrowings	929,057	733,043
Derivative liabilities	203	182
Total Fair Value
Financial assets:
Cash and cash equivalents	65,498	100,975
Securities available-for-sale, at fair value	710,570	557,176
Securities held-to-maturity	236,125	131,822
Federal Home Loan Bank Stock	44,664	35,844
Loans receivable, net	4,367,945	4,301,138
Derivative assets	490	277
Financial liabilities:
Total deposits	3,959,460	3,880,363
Borrowings	925,032	732,731
Derivative liabilities	$ 203	$ 182