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	Dec. 31, 2014
Insurance [Abstract]
Future premiums expected to be collected, March 31, 2015	$ 25,413
Future premiums expected to be collected, June 30. 2015	22,681
Future premiums expected to be collected, September 30, 2015	23,415
Future premiums expected to be collected, December 31, 2015	22,739
Future premiums expected to be collected, December 31, 2016	87,454
Future premiums expected to be collected, December 31, 2017	81,156
Future premiums expected to be collected, December 31, 2018	76,245
Future premiums expected to be collected, December 31, 2019	72,404
Future premiums expected to be collected, December 31, 2024	320,870
Future premiums expected to be collected, December 31, 2029	267,513
Future premiums expected to be collected, December 31, 2034	194,711
Future premiums expected to be collected, December 31, 2039	76,970
Future premiums expected to be collected, December 31, 2044	24,577
Future premiums expected to be collected, December 31, 2049	9,570
Future premiums expected to be collected, December 31, 2054	1,244
Future premiums expected to be collected, December 31, 2059	5
Future premiums expected to be collected, Total	1,306,967
Future expected premium to be earned, net of reinsurance, March 31, 2015	38,161
Future expected premiums to be earned, net of reinsurance, June 30, 2015	37,272
Future expected premiums to be earned, net of reinsurance, September 30, 2015	35,279
Future expected premiums to be earned, net of reinsurance, December 31, 2015	33,840
Future expected premiums to be earned, net of reinsurance, December 31, 2016	125,754
Future expected premiums to be earned, net of reinsurance, December 31, 2017	113,937
Future expected premiums to be earned, net of reinsurance, December 31, 2018	105,115
Future expected premiums to be earned, net of reinsurance, December 31, 2019	97,864
Future expected premiums to be earned, net of reinsurance, December 31, 2024	401,094
Future expected premiums to be earned, net of reinsurance, December 31, 2029	282,294
Future expected premiums to be earned, net of reinsurance, December 31, 2034	175,689
Future expected premiums to be earned, net of reinsurance, December 31, 2039	69,030
Future expected premiums to be earned, net of reinsurance, December 31, 2044	23,197
Future expected premiums to be earned, net of reinsurance, December 31, 2049	9,797
Future expected premiums to be earned, net of reinsurance, December 31, 2054	2,184
Future expected premiums to be earned, net of reinsurance, December 31, 2059	2
Future expected premiums to be earned, net of reinsurance, Total	$ 1,550,509