Fair Value Measurements - Assets and Liabilities Measured on Non-Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate owned	$ 959	$ 393
Quoted prices in active markets for identical assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans measured for impairment based on the fair value of the underlying collateral	0	0
Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans measured for impairment based on the fair value of the underlying collateral	0	0
Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans measured for impairment based on the fair value of the underlying collateral	4,367,945	4,301,138
Measured on non-recurring basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate owned	959	393
Loans measured for impairment based on the fair value of the underlying collateral	10,251	14,156
Assets	11,210	14,549
Measured on non-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]
Real estate owned	0	0
Loans measured for impairment based on the fair value of the underlying collateral	0	0
Assets	0	0
Measured on non-recurring basis | Significant other observable inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate owned	0	0
Loans measured for impairment based on the fair value of the underlying collateral	0	0
Assets	0	0
Measured on non-recurring basis | Significant unobservable inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate owned	959	393
Loans measured for impairment based on the fair value of the underlying collateral	10,251	14,156
Assets	$ 11,210	$ 14,549