Financial Guarantee Insurance Contracts - Summarized Future Gross Undiscounted Premiums Expected to be Collected, and Future Expected Premiums Earned, Net of Reinsurance (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013
Insurance [Abstract]
Future premiums expected to be collected, December 31, 2013	$ 33,020
Future premiums expected to be collected, December 31, 2014	125,309
Future premiums expected to be collected, December 31, 2015	125,159
Future premiums expected to be collected, December 31, 2016	118,632
Future premiums expected to be collected, December 31, 2017	112,526
Future premiums expected to be collected, December 31, 2022	497,121
Future premiums expected to be collected, December 31, 2027	392,615
Future premiums expected to be collected, December 31, 2032	277,704
Future premiums expected to be collected, December 31, 2037	131,553
Future premiums expected to be collected, December 31, 2042	48,449
Future premiums expected to be collected, December 31, 2047	14,564
Future premiums expected to be collected, December 31, 2052	3,639
Future premiums expected to be collected, December 31, 2057	92
Future premiums expected to be collected, Total	1,880,383
Future expected premiums to be earned, net of reinsurance, December 31, 2013	51,560
Future expected premiums to be earned, net of reinsurance, December 31, 2014	186,459
Future expected premiums to be earned, net of reinsurance, December 31, 2015	167,106
Future expected premiums to be earned, net of reinsurance, December 31, 2016	154,135
Future expected premiums to be earned, net of reinsurance, December 31, 2017	143,323
Future expected premiums to be earned, net of reinsurance, December 31, 2022	591,581
Future expected premiums to be earned, net of reinsurance, December 31, 2027	414,028
Future expected premiums to be earned, net of reinsurance, December 31, 2032	257,483
Future expected premiums to be earned, net of reinsurance, December 31, 2037	124,457
Future expected premiums to be earned, net of reinsurance, December 31, 2042	42,002
Future expected premiums to be earned, net of reinsurance, December 31, 2047	14,429
Future expected premiums to be earned, net of reinsurance, December 31, 2052	4,692
Future expected premiums to be earned, net of reinsurance, December 31, 2057	296
Future expected premiums to be earned, net of reinsurance, Total	$ 2,151,551