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	6 Months Ended
	Jun. 30, 2017
Minimum
Fair Value Assets Measured On Recurring Basis
Prepayment rate	6.20%
Default rate	2.60%
Loss severity	47.20%
Maximum
Fair Value Assets Measured On Recurring Basis
Prepayment rate	31.60%
Default rate	36.70%
Loss severity	66.00%
Weighted Average
Fair Value Assets Measured On Recurring Basis
Prepayment rate	19.80%
Default rate	7.50%
Loss severity	60.40%