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, 2014
Insurance [Abstract]
Future premiums expected to be collected, December 31, 2014	$ 30,343
Future premiums expected to be collected, December 31, 2015	119,080
Future premiums expected to be collected, December 31, 2016	112,384
Future premiums expected to be collected, December 31, 2017	105,925
Future premiums expected to be collected, December 31, 2018	100,696
Future premiums expected to be collected, December 31, 2023	444,832
Future premiums expected to be collected, December 31, 2028	352,220
Future premiums expected to be collected, December 31, 2033	224,150
Future premiums expected to be collected, December 31, 2038	85,447
Future premiums expected to be collected, December 31, 2043	28,756
Future premiums expected to be collected, December 31, 2048	11,920
Future premiums expected to be collected, December 31, 2053	2,253
Future premiums expected to be collected, December 31, 2058	31
Future premiums expected to be collected, Total	1,618,037
Future expected premiums to be earned, net of reinsurance, December 31, 2014	44,483
Future expected premiums to be earned, net of reinsurance, December 31, 2015	161,776
Future expected premiums to be earned, net of reinsurance, December 31, 2016	145,435
Future expected premiums to be earned, net of reinsurance, December 31, 2017	133,804
Future expected premiums to be earned, net of reinsurance, December 31, 2018	124,570
Future expected premiums to be earned, net of reinsurance, December 31, 2023	514,973
Future expected premiums to be earned, net of reinsurance, December 31, 2028	360,470
Future expected premiums to be earned, net of reinsurance, December 31, 2033	209,229
Future expected premiums to be earned, net of reinsurance, December 31, 2038	88,255
Future expected premiums to be earned, net of reinsurance, December 31, 2043	27,365
Future expected premiums to be earned, net of reinsurance, December 31, 2048	12,028
Future expected premiums to be earned, net of reinsurance, December 31, 2053	3,324
Future expected premiums to be earned, net of reinsurance, December 31, 2058	84
Future expected premiums to be earned, net of reinsurance, Total	$ 1,825,796