Preferred stock - Summary of Preferred Dividends Paid (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Class of Stock [Line Items]
Preferred dividends paid	$ 61	$ 49	$ 36	$ 146	$ 120
Series A Preferred Stock
Class of Stock [Line Items]
Depositary shares per share	1.00%			1.00%
Dividend paid per share (usd per share)	$ 1,011.11	$ 1,022.22	$ 1,022.22	$ 3,044.44	$ 3,044.44
Preferred dividends paid	$ 5	$ 5	$ 5	$ 15	$ 15
Series C Preferred Stock
Class of Stock [Line Items]
Depositary shares per share	0.03%			0.03%
Dividend paid per share (usd per share)	$ 1,300.00	$ 1,300.00	$ 1,300.00	$ 3,900.00	$ 3,900.00
Preferred dividends paid	$ 7	$ 8	$ 8	$ 23	$ 23
Series D Preferred Stock
Class of Stock [Line Items]
Depositary shares per share	1.00%			1.00%
Dividend paid per share (usd per share)		$ 2,250.00		$ 2,250.00	$ 2,250.00
Preferred dividends paid	$ 0	$ 11	0	$ 11	$ 11
Series E Preferred Stock
Class of Stock [Line Items]
Depositary shares per share	1.00%			1.00%
Dividend paid per share (usd per share)	$ 962.65	$ 2,475.00		$ 3,437.65	$ 2,475.00
Preferred dividends paid	$ 10	$ 25	$ 0	$ 35	$ 25
Series F Preferred Stock
Class of Stock [Line Items]
Depositary shares per share	1.00%			1.00%
Dividend paid per share (usd per share)	$ 2,312.50		$ 2,312.50	$ 4,625.00	$ 4,625.00
Preferred dividends paid	$ 23	0	$ 23	$ 46	$ 46
Series G Noncumulative Perpetual Preferred Stock
Class of Stock [Line Items]
Depositary shares per share	1.00%			1.00%
Dividend paid per share (usd per share)	$ 1,579.72			$ 1,579.72
Preferred dividends paid	$ 16	$ 0	$ 0	$ 16	$ 0