Financial Guarantee Insurance Contracts - Summarized Future Gross Undiscounted Premiums Expected to be Collected, and Future Expected Premiums Earned, Net of Reinsurance (Detail) $ in Thousands	Jun. 30, 2015 USD ($)
Insurance [Abstract]
Future premiums expected to be collected, September 30, 2015	$ 22,273
Future premiums expected to be collected, December 31, 2015	21,471
Future premiums expected to be collected, December 31, 2016	84,330
Future premiums expected to be collected, December 31, 2017	78,696
Future premiums expected to be collected, December 31, 2018	73,574
Future premiums expected to be collected, December 31, 2019	69,741
Future premiums expected to be collected, December 31, 2024	308,352
Future premiums expected to be collected, December 31, 2029	256,512
Future premiums expected to be collected, December 31, 2034	180,354
Future premiums expected to be collected, December 31, 2039	66,338
Future premiums expected to be collected, December 31, 2044	25,295
Future premiums expected to be collected, December 31, 2049	9,838
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	1,198,023
Future expected premiums to be earned, net of reinsurance, September 30, 2015	37,587
Future expected premiums to be earned, net of reinsurance, December 31, 2015	34,681
Future expected premiums to be earned, net of reinsurance, December 31, 2016	125,263
Future expected premiums to be earned, net of reinsurance, December 31, 2017	109,613
Future expected premiums to be earned, net of reinsurance, December 31, 2018	99,333
Future expected premiums to be earned, net of reinsurance, December 31, 2019	92,422
Future expected premiums to be earned, net of reinsurance, December 31, 2024	378,905
Future expected premiums to be earned, net of reinsurance, December 31, 2029	267,686
Future expected premiums to be earned, net of reinsurance, December 31, 2034	167,874
Future expected premiums to be earned, net of reinsurance, December 31, 2039	67,332
Future expected premiums to be earned, net of reinsurance, December 31, 2044	23,340
Future expected premiums to be earned, net of reinsurance, December 31, 2049	9,800
Future expected premiums to be earned, net of reinsurance, December 31, 2054	2,183
Future expected premiums to be earned, net of reinsurance, December 31, 2059	3
Future expected premiums to be earned, net of reinsurance, Total	$ 1,416,022