Fair Value Of Financial Instruments (Fair Value Hierarchy Table Presents The Company's Assets And Liabilities At Fair Value Not Recorded On The Company's Consolidated Balance Sheet) (Detail) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Variable Interest Entity Primary Beneficiary [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Investments held-to-maturity, fair value		$ 901	$ 916
Carrying Reported Amount Fair Value Disclosure [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Other investments		1	2
Accrued investment income	[1]		0
Total assets		891	892
Investment agreements		314	337
Medium-term notes		615	650
Long-term debt		2,218	2,121
Payable for investments purchased	[2]		0
Total liabilities		4,398	4,328
Gross		100	1,220
Ceded		38	39
Carrying Reported Amount Fair Value Disclosure [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Variable Interest Entity Primary Beneficiary [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Investments held-to-maturity, fair value		890	890
Variable interest entity notes		1,251	1,220
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Other investments	[1]	1	2
Accrued investment income	[1]		0
Total assets		902	918
Investment agreements		381	433
Medium-term notes		416	406
Long-term debt		1,132	1,002
Payable for investments purchased	[2]		0
Total liabilities		3,211	3,109
Gross		1,231	1,785
Ceded		68	61
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Variable Interest Entity Primary Beneficiary [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Investments held-to-maturity, fair value		901	916
Variable interest entity notes		1,282	1,268
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Fair Value Inputs Level 2 [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Other investments	[1]	1	2
Accrued investment income	[1]		0
Total assets		1	2
Long-term debt		1,132	1,002
Payable for investments purchased	[2]		0
Total liabilities		1,514	1,354
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Fair Value Inputs Level 2 [Member] | Variable Interest Entity Primary Beneficiary [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Variable interest entity notes		382	352
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Fair Value Inputs Level 3 [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Total assets		901	916
Investment agreements		381	433
Medium-term notes		416	406
Total liabilities		1,697	1,755
Gross		1,231	1,785
Ceded		68	61
Fair Value [Member] | Value Disclosed At Fair Value Not Recorded At Fair Value [Member] | Fair Value Inputs Level 3 [Member] | Variable Interest Entity Primary Beneficiary [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Investments held-to-maturity, fair value		901	916
Variable interest entity notes		$ 900	$ 916

[1] Reported within "Other assets" on MBIA's consolidated balance sheets.

[2] Reported within "Other liabilities" on MBIA's consolidated balance sheets.