Financial Guarantees in Force - Summary of Financial Guarantee Portfolio Diversification by Type of Guaranteed Bond (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 46,927,000	$ 62,716,000
Public Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	23,442,000	32,088,000
Public Finance [Member] | Lease and Tax-backed Revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	7,565,000	11,893,000
Public Finance [Member] | General Obligation [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	4,214,000	6,257,000
Public Finance [Member] | Utility Revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,178,000	2,212,000
Public Finance [Member] | Higher Education [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,168,000	1,642,000
Public Finance [Member] | Housing Revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	6,159,000	6,312,000
Public Finance [Member] | Health Care Revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	459,000	807,000
Public Finance [Member] | Other [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	945,000	963,000
Public Finance [Member] | Transportation [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,754,000	2,002,000
Structured Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	9,947,000	13,816,000
Structured Finance [Member] | Mortgage-backed and Home Equity [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	5,510,000	7,267,000
Structured Finance [Member] | Investor-owned Utilities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,754,000	3,274,000
Structured Finance [Member] | Structured Insurance [Domain]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,365,000	1,420,000
Structured Finance [Member] | Student Loan [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	934,000	1,238,000
Structured Finance [Member] | Asset-backed Securities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	237,000	443,000
Structured Finance [Member] | Other [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	147,000	174,000
International Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	13,538,000	16,812,000
International Finance [Member] | Mortgage-backed and Home Equity [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	0	246,000
International Finance [Member] | Asset-backed Securities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,550,000	2,609,000
International Finance [Member] | Investor-owned and Public Utilities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	4,499,000	5,696,000
International Finance [Member] | Sovereign/Sub-sovereign [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	5,250,000	5,664,000
International Finance [Member] | Transportation [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,613,000	1,777,000
International Finance [Member] | Other [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 626,000	$ 820,000