Segment Information - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Segment Information [Line Items]
Number of reportable segments			2
Financial Guarantee [Member]
Segment Information [Line Items]
Net investment income	$ 81,037	$ 81,588	$ 276,251	$ 241,217
Interest expense	23,269	31,131	83,966	88,846
Financial Services [Member]
Segment Information [Line Items]
Net investment income	2,986	9,040	13,611	21,369
Interest expense	1,556	1,943	4,996	6,158
Corporate and Other [Member]
Segment Information [Line Items]
Net investment income	55	71	169	220
Interest expense	$ 0	$ 0	$ 0	$ 0

The cost of borrowed funds accounted for as interest that was charged against earnings during the period.

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This item represents investment income derived from investments in debt and equity securities consisting of interest income earned from investments in debt securities and on cash and cash equivalents, dividend income from investments in equity securities, and income or expense derived from the amortization of investment related discounts or premiums, respectively, net of related investment expenses. This item does not include realized or unrealized gains or losses on the sale or holding of investments in debt and equity securities required to be included in earnings for the period or for other than temporary losses related to investments in debt and equity securities which are included in realized losses in the period recognized, and does not include investment income from real or personal property, such as rental income.

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