Investment securities - Schedule of Investments Classified by Credit Rating (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	$ 4,548	$ 4,680
Agency mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	966	999
Mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	3,472	3,603
Asset-backed
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	110	78
AAA
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	3,477	3,570
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	3,375	3,497
AAA | Asset-backed
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	102	73
AA
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	1,054	1,094
AA | Agency mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	966	999
AA | Mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	85	93
AA | Asset-backed
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	3	2
A
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	15	15
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	12	13
A | Asset-backed
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	3	2
BBB
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	2	1
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	$ 2	$ 1