Loss And Loss Adjustment Expense Reserves (Schedule Of Financial Guarantees And Related Claim Liability) (Detail) $ in Millions		12 Months Ended
		Dec. 31, 2016 USD ($) issue policy	Dec. 31, 2015 USD ($) issue policy
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		430	411
Number of issues | issue	[1]	149	140
Remaining weighted average contract period (in years)		7 years 1 month 6 days	7 years 4 months 24 days
Principal		$ 10,285	$ 11,160
Interest		5,683	6,247
Total		15,968	17,407
Gross claim liability		718	797
Less: Gross potential recoveries		770	752
Discount, net	[2]	(75)	116
Net claim liability (recoverable)		23	(71)
Unearned premium revenue		$ 85	$ 98
Caution List Low [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		90	57
Number of issues | issue	[1]	17	12
Remaining weighted average contract period (in years)		7 years 6 months	7 years 7 months 6 days
Principal		$ 2,917	$ 2,591
Interest		2,795	2,733
Total		5,712	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		6	18
Number of issues | issue	[1]	4	6
Remaining weighted average contract period (in years)		3 years 4 months 24 days	6 years 8 months 12 days
Principal		$ 17	$ 147
Interest		4	57
Total		21	204
Gross claim liability			0
Less: Gross potential recoveries			0
Discount, net	[2]		0
Net claim liability (recoverable)			0
Unearned premium revenue		$ 0	$ 2
Caution List High [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Number of policies | policy		3	171
Number of issues | issue	[1]	2	5
Remaining weighted average contract period (in years)		7 years 2 months 12 days	9 years 7 months 6 days
Principal		$ 320	$ 1,996
Interest		107	1,038
Total		427	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		331	165
Number of issues | issue	[1]	126	117
Remaining weighted average contract period (in years)		7 years	6 years 8 months 12 days
Principal		$ 7,031	$ 6,426
Interest		2,777	2,419
Total		9,808	8,845
Gross claim liability		718	797
Less: Gross potential recoveries		770	752
Discount, net	[2]	(75)	116
Net claim liability (recoverable)		23	(71)
Unearned premium revenue		$ 68	$ 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.