Short Duration Contracts - Claims development - Mortgage (Details) $ in Millions	Dec. 31, 2024 USD ($) claims	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Claims Development [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance	$ 21,118
Mortgage
Claims Development [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	873
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	565
All outstanding liabilities before 2012, net of reinsurance	13
Liabilities for losses and loss adjustment expenses, net of reinsurance	321
Mortgage | 2015
Claims Development [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	188	$ 188	$ 189	$ 191	$ 191	$ 189	$ 195	$ 198	$ 197	$ 223
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	186	186	185	184	183	180	171	151	92	$ 16
Total of IBNR liabilities plus expected development on reported claims	$ 0
Cumulative number of paid claims | claims	4,686
Mortgage | 2016
Claims Development [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 136	136	137	142	142	141	149	171	184
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	134	133	132	132	131	127	113	72	$ 11
Total of IBNR liabilities plus expected development on reported claims	$ 0
Cumulative number of paid claims | claims	3,557
Mortgage | 2017
Claims Development [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 99	99	102	109	108	107	132	179
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	95	93	92	90	87	79	48	$ 9
Total of IBNR liabilities plus expected development on reported claims	$ 0
Cumulative number of paid claims | claims	2,707
Mortgage | 2018
Claims Development [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 69	69	72	88	89	96	132
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	63	60	59	56	50	31	$ 4
Total of IBNR liabilities plus expected development on reported claims	$ 0
Cumulative number of paid claims | claims	1,966
Mortgage | 2019
Claims Development [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 52	51	63	110	119	108
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	42	39	34	29	20	$ 3
Total of IBNR liabilities plus expected development on reported claims	$ 0
Cumulative number of paid claims | claims	1,435
Mortgage | 2020
Claims Development [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 31	33	78	374	420
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	19	13	8	4	$ 1
Total of IBNR liabilities plus expected development on reported claims	$ 0
Cumulative number of paid claims | claims	835
Mortgage | 2021
Claims Development [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 17	20	77	144
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	8	5	2	$ 0
Total of IBNR liabilities plus expected development on reported claims	$ 0
Cumulative number of paid claims | claims	381
Mortgage | 2022
Claims Development [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 30	55	173
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	10	3	$ 0
Total of IBNR liabilities plus expected development on reported claims	$ 0
Cumulative number of paid claims | claims	469
Mortgage | 2023
Claims Development [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 71	182
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	7	$ 0
Total of IBNR liabilities plus expected development on reported claims	$ 0
Cumulative number of paid claims | claims	366
Mortgage | 2024
Claims Development [Line Items]
Incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 180
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	1
Total of IBNR liabilities plus expected development on reported claims	$ 1
Cumulative number of paid claims | claims	40