Loss And Loss Adjustment Expense Reserves (Schedule Of Financial Guarantees And Related Claim Liability) (Detail) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2016 USD ($) issue policy	Dec. 31, 2015 USD ($) issue policy
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		421	411
Number of issues | issue	[1]	146	140
Remaining weighted average contract period (in years)		7 years 2 months 12 days	7 years 4 months 24 days
Principal		$ 10,620	$ 11,160
Interest		5,817	6,247
Total		16,437	17,407
Gross claim liability		670	797
Less: Gross potential recoveries		686	752
Discount, net	[2]	14	116
Net claim liability (recoverable)		(30)	(71)
Unearned premium revenue		$ 88	$ 98
Caution List Low [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		82	57
Number of issues | issue	[1]	14	12
Remaining weighted average contract period (in years)		7 years 3 months 18 days	7 years 7 months 6 days
Principal		$ 3,013	$ 2,591
Interest		2,843	2,733
Total		5,856	5,324
Gross claim liability			0
Less: Gross potential recoveries			0
Discount, net	[2]		0
Net claim liability (recoverable)			0
Unearned premium revenue		$ 9	$ 8
Caution List Medium [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		7	18
Number of issues | issue	[1]	5	6
Remaining weighted average contract period (in years)		5 years	6 years 8 months 12 days
Principal		$ 64	$ 147
Interest		17	57
Total		81	204
Gross claim liability			0
Less: Gross potential recoveries			0
Discount, net	[2]		0
Net claim liability (recoverable)			0
Unearned premium revenue		$ 1	$ 2
Caution List High [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		5	171
Number of issues | issue	[1]	3	5
Remaining weighted average contract period (in years)		7 years 3 months 18 days	9 years 7 months 6 days
Principal		$ 338	$ 1,996
Interest		117	1,038
Total		455	3,034
Gross claim liability			0
Less: Gross potential recoveries			0
Discount, net	[2]		0
Net claim liability (recoverable)			0
Unearned premium revenue		$ 8	$ 33
Classified List [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		327	165
Number of issues | issue	[1]	124	117
Remaining weighted average contract period (in years)		7 years 1 month 6 days	6 years 8 months 12 days
Principal		$ 7,205	$ 6,426
Interest		2,840	2,419
Total		10,045	8,845
Gross claim liability		670	797
Less: Gross potential recoveries		686	752
Discount, net	[2]	14	116
Net claim liability (recoverable)		(30)	(71)
Unearned premium revenue		$ 70	$ 55

[1] An issue represents the aggregate of financial guarantee policies that share the same revenue source for purposes of making debt service payments.

[2] Represents discount related to Gross Claim Liability and Gross Potential Recoveries.