Financial Guarantees in Force - Summary of International Finance Guaranteed Portfolio (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 79,346,000	$ 108,299,000
International Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	17,333,000	21,049,000
International Finance [Member] | United Kingdom [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	12,798,000	15,494,000
International Finance [Member] | Australia [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,393,000	1,851,000
International Finance [Member] | Italy [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	898,000	948,000
International Finance [Member] | Austria [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	696,000	737,000
International Finance [Member] | FRANCE
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	286,000	288,000
International Finance [Member] | Internationally Diversified [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	648,000	974,000
International Finance [Member] | Other International [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 614,000	$ 757,000