Securities - Significant Assumptions (Details) - Non-agency residential	Mar. 31, 2018
Weighted Average
Significant Assumptions:
Prepayment speed	11.70%
Loss severity	21.60%
Life default rate	20.10%
Weighted Average \| Prime
Significant Assumptions:
Loss severity	16.40%
Life default rate	15.10%
Weighted Average \| Alt-A
Significant Assumptions:
Loss severity	17.60%
Life default rate	20.00%
Weighted Average \| Subprime
Significant Assumptions:
Loss severity	27.90%
Life default rate	22.40%
10th Percentile
Significant Assumptions:
Prepayment speed	2.90%
Loss severity	9.10%
Life default rate	1.30%
90th Percentile
Significant Assumptions:
Prepayment speed	20.90%
Loss severity	40.60%
Life default rate	72.30%