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		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017
Minimum
Fair Value Assets Measured On Recurring Basis
Prepayment rate	1.10%
Default rate	1.40%
Loss severity	47.70%
Maximum
Fair Value Assets Measured On Recurring Basis
Prepayment rate	34.10%
Default rate	30.90%
Loss severity	63.80%
Weighted Average
Fair Value Assets Measured On Recurring Basis
Prepayment rate		18.00%
Default rate		6.70%
Loss severity		58.30%