Fair Value Measurements - Fair Value on Balance Sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Sep. 30, 2017
Financial assets:
Securities available-for-sale, at fair value	$ 852,213	$ 557,176
Securities held-to-maturity	245,477	131,822
Quoted prices in active markets for identical assets (Level 1)
Financial assets:
Cash and cash equivalents	634,381	100,975
Securities available-for-sale, at fair value		28,127
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
Significant other observable inputs (Level 2)
Financial assets:
Cash and cash equivalents	0	0
Securities available-for-sale, at fair value		529,049
Securities held-to-maturity		131,822
Federal Home Loan Bank stock	29,381	35,844
Loans receivable, net	0	0
Derivative assets	669	277
Financial liabilities:
Total deposits	5,382,310	3,880,363
Borrowings	532,051	732,731
Derivative liabilities		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	4,390,761	4,301,138
Derivative assets	0	0
Financial liabilities:
Total deposits	0	0
Borrowings	0	0
Derivative liabilities		0
Carrying Value
Financial assets:
Cash and cash equivalents	634,381	100,975
Securities available-for-sale, at fair value	852,213	557,176
Securities held-to-maturity	254,131	132,939
Federal Home Loan Bank stock	29,381	35,844
Loans receivable, net	4,479,919	4,307,623
Derivative assets	669	277
Financial liabilities:
Total deposits	5,395,253	4,123,428
Borrowings	589,430	733,043
Derivative liabilities		182
Total Fair Value
Financial assets:
Cash and cash equivalents	634,381	100,975
Securities available-for-sale, at fair value	852,213	557,176
Securities held-to-maturity	245,477	131,822
Federal Home Loan Bank stock	29,381	35,844
Loans receivable, net	4,390,761	4,301,138
Derivative assets	669	277
Financial liabilities:
Total deposits	5,382,310	3,880,363
Borrowings	$ 532,051	732,731
Derivative liabilities		$ 182