Financial Guarantees in Force - Summary of Financial Guarantee Portfolio Diversification by Type of Guaranteed Bond (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 223,633,000	$ 272,504,000
Military housing [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 6,324,000	$ 6,373,243