Investment securities (Investments Classified by Credit Rating) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	$ 1,390	$ 1,568
Agency mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	1,376
Asset-backed
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	14
AAA Rating
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	14
AAA Rating | Agency mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	0
AAA Rating | Asset-backed
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	14
AA Rating
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	1,376
AA Rating | Agency mortgage-backed residential
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	1,376
AA Rating | Asset-backed
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	$ 0