Investments - Securities pledged under repurchase and lending agreements (Details) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Investments [Line Items]
Securities pledged under repurchase agreements	$ 1,197	$ 1,149
Securities pledged under securities lending agreements	2,324	3,256
Non-U.S. governments
Investments [Line Items]
Securities pledged under repurchase agreements	35	59
Securities pledged under securities lending agreements	64	57
Corporate debt
Investments [Line Items]
Securities pledged under repurchase agreements	1,162	1,090
Securities pledged under securities lending agreements	2,260	3,199
Overnight and Continuous
Investments [Line Items]
Securities pledged under repurchase agreements	5	6
Securities pledged under securities lending agreements	0	0
Overnight and Continuous \| Non-U.S. governments
Investments [Line Items]
Securities pledged under repurchase agreements	0	0
Securities pledged under securities lending agreements	0	0
Overnight and Continuous \| Corporate debt
Investments [Line Items]
Securities pledged under repurchase agreements	5	6
Securities pledged under securities lending agreements	0	0
Up to 30 Days
Investments [Line Items]
Securities pledged under repurchase agreements	105	623
Securities pledged under securities lending agreements	1,932	3,256
Up to 30 Days \| Non-U.S. governments
Investments [Line Items]
Securities pledged under repurchase agreements	0	25
Securities pledged under securities lending agreements	48	57
Up to 30 Days \| Corporate debt
Investments [Line Items]
Securities pledged under repurchase agreements	105	598
Securities pledged under securities lending agreements	1,884	3,199
31 - 90 Days
Investments [Line Items]
Securities pledged under repurchase agreements	1,087	520
Securities pledged under securities lending agreements	392	0
31 - 90 Days \| Non-U.S. governments
Investments [Line Items]
Securities pledged under repurchase agreements	35	34
Securities pledged under securities lending agreements	16	0
31 - 90 Days \| Corporate debt
Investments [Line Items]
Securities pledged under repurchase agreements	1,052	486
Securities pledged under securities lending agreements	376	0
91 - 364 Days
Investments [Line Items]
Securities pledged under repurchase agreements	0	0
Securities pledged under securities lending agreements	0	0
91 - 364 Days \| Non-U.S. governments
Investments [Line Items]
Securities pledged under repurchase agreements	0	0
Securities pledged under securities lending agreements	0	0
91 - 364 Days \| Corporate debt
Investments [Line Items]
Securities pledged under repurchase agreements	0	0
Securities pledged under securities lending agreements	0	0
365 Days or Greater
Investments [Line Items]
Securities pledged under repurchase agreements	0	0
Securities pledged under securities lending agreements	0	0
365 Days or Greater \| Non-U.S. governments
Investments [Line Items]
Securities pledged under repurchase agreements	0	0
Securities pledged under securities lending agreements	0	0
365 Days or Greater \| Corporate debt
Investments [Line Items]
Securities pledged under repurchase agreements	0	0
Securities pledged under securities lending agreements	$ 0	$ 0