Financial Guarantee Insurance Contracts - Summarized Future Gross Undiscounted Premiums Expected to be Collected, and Future Expected Premiums Earned, Net of Reinsurance (Detail) $ in Millions	Dec. 31, 2019 USD ($)
Future premiums expected to be collected, March 31, 2018	$ 14
Future premiums expected to be collected, June 30. 2018	11
Future premiums expected to be collected, September 30, 2018	10
Future premiums expected to be collected, December 31, 2018	9
Future premiums expected to be collected, December 31, 2019	37
Future premiums expected to be collected, December 31, 2020	36
Future premiums expected to be collected, December 31, 2021	34
Future premiums expected to be collected, December 31, 2022	33
Future premiums expected to be collected, December 31, 2027	143
Future premiums expected to be collected, December 31, 2032	102
Future premiums expected to be collected, December 31, 2037	47
Future premiums expected to be collected, December 31, 2042	22
Future premiums expected to be collected, December 31, 2047	9
Future premiums expected to be collected, December 31, 2052	1
Future premiums expected to be collected, Total	508
Future expected premium to be earned, net of reinsurance, March 31, 2018	10
Future expected premiums to be earned, net of reinsurance, June 30, 2018	10
Future expected premiums to be earned, net of reinsurance, September 30, 2018	10
Future expected premiums to be earned, net of reinsurance, December 31, 2018	10
Future expected premiums to be earned, net of reinsurance, December 31, 2019	36
Future expected premiums to be earned, net of reinsurance, December 31, 2020	34
Future expected premiums to be earned, net of reinsurance, December 31, 2021	32
Future expected premiums to be earned, net of reinsurance, December 31, 2022	30
Future expected premiums to be earned, net of reinsurance, December 31, 2027	124
Future expected premiums to be earned, net of reinsurance, December 31, 2032	82
Future expected premiums to be earned, net of reinsurance, December 31, 2037	38
Future expected premiums to be earned, net of reinsurance, December 31, 2042	14
Future expected premiums to be earned, net of reinsurance, December 31, 2047	5
Future expected premiums to be earned, net of reinsurance, December 31, 2052	1
Future expected premiums to be earned, net of reinsurance, Total	$ 436