Fair Value Measurement of Assets and Liabilities - Quantitative Information about Level 3 Inputs (Details) - Significant Unobservable Inputs (Level 3) - Residential mortgage-backed securities - Discounted cash flow		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Minimum
Fair Value Assets Measured On Recurring Basis
Prepayment rate	1.80%
Default rate	3.10%
Loss severity	45.40%
Maximum
Fair Value Assets Measured On Recurring Basis
Prepayment rate	28.80%
Default rate	47.40%
Loss severity	62.00%
Weighted Average
Fair Value Assets Measured On Recurring Basis
Prepayment rate		13.00%
Default rate		8.90%
Loss severity		57.10%