Preferred stock - Summary of Preferred Dividends Paid (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Class of Stock [Line Items]
Preferred dividends paid	$ 49	$ 36	$ 48	$ 85	$ 84
Series A Preferred Stock
Class of Stock [Line Items]
Depositary shares per share	1.00%			1.00%
Dividend paid per share (usd per share)	$ 1,022.22	$ 1,011.11	$ 1,022.22	$ 2,033.33	$ 2,022.22
Preferred dividends paid	$ 5	$ 5	$ 5	$ 10	$ 10
Series C Preferred Stock
Class of Stock [Line Items]
Depositary shares per share	0.025%			0.025%
Dividend paid per share (usd per share)	$ 1,300.00	$ 1,300.00	$ 1,300.00	$ 2,600.00	$ 2,600.00
Preferred dividends paid	$ 8	$ 8	$ 7	$ 16	$ 15
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	$ 2,250.00
Preferred dividends paid	$ 11	0	$ 11	$ 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)	$ 2,475.00		$ 2,475.00	$ 2,475.00	$ 2,475.00
Preferred dividends paid	$ 25	$ 0	$ 25	$ 25	$ 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	$ 2,312.50
Preferred dividends paid	$ 0	$ 23	$ 0	$ 23	$ 23
Series G Noncumulative Perpetual Preferred Stock
Class of Stock [Line Items]
Depositary shares per share	1.00%			1.00%