Investments - Securities pledged under repurchase and lending agreements (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Investments [Line Items]
Securities pledged under repurchase agreements	$ 1,302	$ 1,149
Securities pledged under securities lending agreements	2,028	3,256
Non-U.S. governments
Investments [Line Items]
Securities pledged under repurchase agreements	27	59
Securities pledged under securities lending agreements	50	57
Corporate debt
Investments [Line Items]
Securities pledged under repurchase agreements	1,275	1,090
Securities pledged under securities lending agreements	1,978	3,199
Overnight and Continuous
Investments [Line Items]
Securities pledged under repurchase agreements	4	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	4	6
Securities pledged under securities lending agreements	0	0
Up to 30 Days
Investments [Line Items]
Securities pledged under repurchase agreements	455	623
Securities pledged under securities lending agreements	1,735	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	50	57
Up to 30 Days \| Corporate debt
Investments [Line Items]
Securities pledged under repurchase agreements	455	598
Securities pledged under securities lending agreements	1,685	3,199
31 - 90 Days
Investments [Line Items]
Securities pledged under repurchase agreements	843	520
Securities pledged under securities lending agreements	293	0
31 - 90 Days \| Non-U.S. governments
Investments [Line Items]
Securities pledged under repurchase agreements	27	34
Securities pledged under securities lending agreements	0	0
31 - 90 Days \| Corporate debt
Investments [Line Items]
Securities pledged under repurchase agreements	816	486
Securities pledged under securities lending agreements	293	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