Fair Values of Assets and Liabilities - Valuation Assumption Ranges for Derivative Commitments (Detail) - Derivative Mortgage Loans Commitments [Member]	Sep. 30, 2018
Minimum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected loan close rate	6.00%
Inherent MSR value (basis points per loan)	28.00%
Maximum [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected loan close rate	100.00%
Inherent MSR value (basis points per loan)	208.00%
Weighted Average [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Expected loan close rate	80.00%
Inherent MSR value (basis points per loan)	127.00%