Fair Values of Assets and Liabilities - Valuation Assumption Ranges for Derivative Commitments (Detail) - Derivative Mortgage Loans Commitments [Member]	Mar. 31, 2019
Minimum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected loan close rate	3.00%
Inherent MSR value (basis points per loan)	17.00%
Maximum [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected loan close rate	100.00%
Inherent MSR value (basis points per loan)	202.00%
Weighted Average [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Expected loan close rate	79.00%
Inherent MSR value (basis points per loan)	127.00%