Investment securities - Schedule of Investments Classified by Credit Rating (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	$ 4,561	$ 4,346
Agency mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	1,310	935
Mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	3,184	3,323
Asset-backed
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	67	88
AAA
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	3,169	3,322
AAA | Agency mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	0	0
AAA | Mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	3,108	3,241
AAA | Asset-backed
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	61	81
AA
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	1,388	1,016
AA | Agency mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	1,310	935
AA | Mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	75	78
AA | Asset-backed
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	3	3
A
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	3	6
A | Agency mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	0	0
A | Mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	1	4
A | Asset-backed
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	2	2
BBB
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	1	2
BBB | Agency mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	0	0
BBB | Mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	0	0
BBB | Asset-backed
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	$ 1	$ 2