Outstanding Exposure - Expected Amortization of Net Par Outstanding (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Expected Amortization of Net Par Outstanding [Abstract]
0 to 5 years	$ 84,966
5 to 10 years	54,173
10 to 15 years	47,352
15 to 20 years	36,866
20 years and above	41,595
Net Par Outstanding	264,952	$ 296,318
Public Finance [Member]
Expected Amortization of Net Par Outstanding [Abstract]
0 to 5 years	78,860
5 to 10 years	51,541
10 to 15 years	45,634
15 to 20 years	34,974
20 years and above	41,305
Net Par Outstanding	252,314	271,179
Structured Finance [Member]
Expected Amortization of Net Par Outstanding [Abstract]
0 to 5 years	6,106
5 to 10 years	2,632
10 to 15 years	1,718
15 to 20 years	1,892
20 years and above	290
Net Par Outstanding	$ 12,638	$ 25,139