Financial Guarantees in Force - Summary of International Finance Guaranteed Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 144,734,000	$ 179,092,000
International Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	24,952,000	31,618,000
International Finance [Member] | United Kingdom [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	17,998,000	21,282,000
International Finance [Member] | Australia [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	2,168,000	3,331,000
International Finance [Member] | Italy [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,415,000	2,412,000
International Finance [Member] | Austria [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	841,000	967,000
International Finance [Member] | New Zealand [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	459,000	526,000
International Finance [Member] | Internationally Diversified [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,225,000	1,918,000
International Finance [Member] | Other International [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 846,000	$ 1,182,000