Fair Value Measurement of Assets and Liabilities - Schedule of Quantitative Information about Level 3 Inputs Used to Measure Fair Value of Available for Sale Securities (Detail) - Significant Unobservable Inputs (Level 3) - Discounted Cash Flow - Residential mortgage-backed securities	12 Months Ended
	Dec. 31, 2017
Fair Value Assets Measured On Recurring Basis [Line Items]
Prepayment rate, weighted average of unobservable inputs	19.10%
Default rate, weighted average of unobservable inputs	8.30%
Loss severity, weighted average of unobservable inputs	57.10%
Minimum
Fair Value Assets Measured On Recurring Basis [Line Items]
Prepayment rate, range of unobservable inputs	11.50%
Default rate, range of unobservable inputs	0.90%
Loss severity, range of unobservable inputs	45.00%
Maximum
Fair Value Assets Measured On Recurring Basis [Line Items]
Prepayment rate, range of unobservable inputs	27.30%
Default rate, range of unobservable inputs	38.90%
Loss severity, range of unobservable inputs	62.60%