Financial Guarantee Insurance Contracts - Summarized Future Gross Undiscounted Premiums Expected to be Collected, and Future Expected Premiums Earned, Net of Reinsurance (Details) $ in Millions	Sep. 30, 2020 USD ($)
Future premiums expected to be collected, December 31, 2020	$ 11
Future premiums expected to be collected, December 31, 2021	36
Future premiums expected to be collected, December 31, 2022	34
Future premiums expected to be collected, December 31, 2023	33
Future premiums expected to be collected, December 31, 2024	31
Future premiums expected to be collected, December 31, 2029	138
Future premiums expected to be collected, December 31, 2034	101
Future premiums expected to be collected, December 31, 2039	50
Future premiums expected to be collected, December 31, 2044	22
Future premiums expected to be collected, December 31, 2049	9
Future premiums expected to be collected, December 31, 2054	1
Future premiums expected to be collected, Total	467
Future expected premiums to be earned, net of reinsurance, December 31, 2020	10
Future expected premiums to be earned, net of reinsurance, December 31, 2021	35
Future expected premiums to be earned, net of reinsurance, December 31, 2022	33
Future expected premiums to be earned, net of reinsurance, December 31, 2023	30
Future expected premiums to be earned, net of reinsurance, December 31, 2024	29
Future expected premiums to be earned, net of reinsurance, December 31, 2029	120
Future expected premiums to be earned, net of reinsurance, December 31, 2034	81
Future expected premiums to be earned, net of reinsurance, December 31, 2039	37
Future expected premiums to be earned, net of reinsurance, December 31, 2044	14
Future expected premiums to be earned, net of reinsurance, December 31, 2049	5
Future expected premiums to be earned, net of reinsurance, December 31, 2054	1
Future expected premiums to be earned, net of reinsurance, Total	$ 395