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		Mar. 31, 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
Assets held for sale			0
Total assets		891	892
Investment agreements		330	337
Medium-term notes		644	650
Long-term debt		2,154	2,121
Payable for investments purchased	[2]		0
Total liabilities		4,357	4,328
Gross		1,188	1,220
Ceded		39	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		1,229	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
Assets held for sale			0
Total assets		902	918
Investment agreements		411	433
Medium-term notes		408	406
Long-term debt		929	1,002
Payable for investments purchased	[2]		0
Total liabilities		3,003	3,109
Gross		1,630	1,785
Ceded		56	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		1,255	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
Assets held for sale			0
Total assets		1	2
Long-term debt		929	1,002
Payable for investments purchased	[2]		0
Total liabilities		1,283	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		354	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		411	433
Medium-term notes		408	406
Total liabilities		1,720	1,755
Gross		1,630	1,785
Ceded		56	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		$ 901	$ 916

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

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