Mortgage-Backed and Credit Risk Transfer Securities - Components of Senior Re-REMIC at Fair Value (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities
Percentage of Re-REMIC holdings at fair value	100.00%	100.00%
Percentage, Re-REMIC holdings that are not senior class	57.50%
0% - 10%
Schedule of Available-for-sale Securities
Percentage of Re-REMIC holdings at fair value	34.50%	17.60%
10% - 20%
Schedule of Available-for-sale Securities
Percentage of Re-REMIC holdings at fair value	3.70%	7.40%
20% - 30%
Schedule of Available-for-sale Securities
Percentage of Re-REMIC holdings at fair value	12.30%	13.50%
30% - 40%
Schedule of Available-for-sale Securities
Percentage of Re-REMIC holdings at fair value	18.40%	15.70%
40% - 50%
Schedule of Available-for-sale Securities
Percentage of Re-REMIC holdings at fair value	9.60%	27.00%
50% - 60%
Schedule of Available-for-sale Securities
Percentage of Re-REMIC holdings at fair value	19.70%	16.10%
60% - 70%
Schedule of Available-for-sale Securities
Percentage of Re-REMIC holdings at fair value	1.80%	2.70%