Fair Value Measurement of Assets and Liabilities - Quantitative Information about Level 3 Inputs (Detail) - Significant Unobservable Inputs (Level 3) - Residential mortgage-backed securities - Discounted cash flow		3 Months Ended
	Mar. 31, 2018	Mar. 31, 2018
Minimum
Fair Value Assets Measured On Recurring Basis
Prepayment rate	4.80%
Default rate	3.30%
Loss severity	45.80%
Maximum
Fair Value Assets Measured On Recurring Basis
Prepayment rate	33.80%
Default rate	47.00%
Loss severity	61.60%
Weighted Average
Fair Value Assets Measured On Recurring Basis
Prepayment rate		19.10%
Default rate		8.30%
Loss severity		56.50%