Fair Value Measurements - Assets and Liabilities Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Sep. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	$ 852,213	$ 557,176
U.S. government and agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	53,897	24,873
Mortgage-backed securities and collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	735,199	473,491
Municipal obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	1,295	1,357
Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	54,012	49,493
Trust preferred securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	4,652	4,708
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	3,158	3,254
Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale		28,127
Derivative assets	0	0
Derivative liabilities		0
Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale		529,049
Derivative assets	669	277
Derivative liabilities		182
Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	0	0
Derivative assets	0	0
Derivative liabilities		0
Measured on recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	852,213	557,176
Derivative assets	669	277
Assets	852,882	557,453
Derivative liabilities		182
Measured on recurring basis | U.S. government and agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	53,897	24,873
Measured on recurring basis | Mortgage-backed securities and collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	735,199	473,491
Measured on recurring basis | Municipal obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	1,295	1,357
Measured on recurring basis | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	54,012	49,493
Measured on recurring basis | Trust preferred securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	4,652	4,708
Measured on recurring basis | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	3,158	3,254
Measured on recurring basis | Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	57,055	28,127
Derivative assets	0	0
Assets	57,055	28,127
Derivative liabilities		0
Measured on recurring basis | Quoted prices in active markets for identical assets (Level 1) | U.S. government and agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	53,897	24,873
Measured on recurring basis | Quoted prices in active markets for identical assets (Level 1) | Mortgage-backed securities and collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	0	0
Measured on recurring basis | Quoted prices in active markets for identical assets (Level 1) | Municipal obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	0	0
Measured on recurring basis | Quoted prices in active markets for identical assets (Level 1) | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	0	0
Measured on recurring basis | Quoted prices in active markets for identical assets (Level 1) | Trust preferred securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	0	0
Measured on recurring basis | Quoted prices in active markets for identical assets (Level 1) | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	3,158	3,254
Measured on recurring basis | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	795,158	529,049
Derivative assets	669	277
Assets	795,827	529,326
Derivative liabilities		182
Measured on recurring basis | Significant other observable inputs (Level 2) | U.S. government and agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	0	0
Measured on recurring basis | Significant other observable inputs (Level 2) | Mortgage-backed securities and collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	735,199	473,491
Measured on recurring basis | Significant other observable inputs (Level 2) | Municipal obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	1,295	1,357
Measured on recurring basis | Significant other observable inputs (Level 2) | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	54,012	49,493
Measured on recurring basis | Significant other observable inputs (Level 2) | Trust preferred securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	4,652	4,708
Measured on recurring basis | Significant other observable inputs (Level 2) | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	0	0
Measured on recurring basis | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	0	0
Derivative assets	0	0
Assets	0	0
Derivative liabilities		0
Measured on recurring basis | Significant unobservable inputs (Level 3) | U.S. government and agency obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	0	0
Measured on recurring basis | Significant unobservable inputs (Level 3) | Mortgage-backed securities and collateralized mortgage obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	0	0
Measured on recurring basis | Significant unobservable inputs (Level 3) | Municipal obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	0	0
Measured on recurring basis | Significant unobservable inputs (Level 3) | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	0	0
Measured on recurring basis | Significant unobservable inputs (Level 3) | Trust preferred securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	0	0
Measured on recurring basis | Significant unobservable inputs (Level 3) | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available-for-sale	$ 0	$ 0