Financial Guarantees in Force - Summary of International Finance Guaranteed Portfolio (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 62,716,000	$ 79,346,000
International Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	16,812,000	17,333,000
International Finance [Member] | United Kingdom [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	13,554,000	12,798,000
International Finance [Member] | Australia [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	608,000	1,393,000
International Finance [Member] | Italy [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	877,000	898,000
International Finance [Member] | Austria [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	770,000	696,000
International Finance [Member] | FRANCE
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	329,000	286,000
International Finance [Member] | Internationally Diversified [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	368,000	648,000
International Finance [Member] | Other International [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 306,000	$ 614,000