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, 2019 USD ($)
Future premiums expected to be collected, September 30, 2019	$ 12,324
Future premiums expected to be collected, December 31, 2019	11,060
Future premiums expected to be collected, December 31, 2020	43,319
Future premiums expected to be collected, December 31, 2021	37,827
Future premiums expected to be collected, December 31, 2022	35,997
Future premiums expected to be collected, December 31, 2023	34,530
Future premiums expected to be collected, December 31, 2028	151,837
Future premiums expected to be collected, December 31, 2033	113,812
Future premiums expected to be collected, December 31, 2038	61,127
Future premiums expected to be collected, December 31, 2043	26,437
Future premiums expected to be collected, December 31, 2048	11,782
Future premiums expected to be collected, December 31, 2053	2,252
Future premiums expected to be collected, December 31, 2058	31
Future premiums expected to be collected, Total	542,335
Future expected premiums to be earned, net of reinsurance, September 30, 2019	11,927
Future expected premiums to be earned, net of reinsurance, December 31, 2019	11,541
Future expected premiums to be earned, net of reinsurance, December 31, 2020	45,409
Future expected premiums to be earned, net of reinsurance, December 31, 2021	41,186
Future expected premiums to be earned, net of reinsurance, December 31, 2022	38,351
Future expected premiums to be earned, net of reinsurance, December 31, 2023	35,714
Future expected premiums to be earned, net of reinsurance, December 31, 2028	146,197
Future expected premiums to be earned, net of reinsurance, December 31, 2033	96,407
Future expected premiums to be earned, net of reinsurance, December 31, 2038	48,749
Future expected premiums to be earned, net of reinsurance, December 31, 2043	16,643
Future expected premiums to be earned, net of reinsurance, December 31, 2048	6,885
Future expected premiums to be earned, net of reinsurance, December 31, 2053	1,577
Future expected premiums to be earned, net of reinsurance, December 31, 2058	50
Future expected premiums to be earned, net of reinsurance, Total	$ 500,636