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, 2017
Minimum
Fair Value Assets Measured On Recurring Basis
Prepayment rate	15.30%
Default rate	3.40%
Loss severity	47.20%
Maximum
Fair Value Assets Measured On Recurring Basis
Prepayment rate	25.00%
Default rate	36.40%
Loss severity	66.00%
Weighted Average
Fair Value Assets Measured On Recurring Basis
Prepayment rate	21.50%
Default rate	9.30%
Loss severity	60.50%