Fair Values of Assets and Liabilities - Valuation Assumption Ranges for Derivative Commitments (Detail) - Derivative Mortgage Loans Commitments [Member]	Sep. 30, 2019
Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected loan close rate	13.00%
Inherent MSR value (basis points per loan)	50.00%
Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected loan close rate	100.00%
Inherent MSR value (basis points per loan)	199.00%
Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected loan close rate	78.00%
Inherent MSR value (basis points per loan)	118.00%