Preferred stock - Summary of Preferred Dividends Paid (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Class of Stock [Line Items]
Preferred dividends paid	$ 36	$ 49	$ 36
Series A Preferred Stock
Class of Stock [Line Items]
Depositary shares per share	1.00%
Dividend paid per share (usd per share)	$ 1,011.11	$ 1,011.11	$ 1,000.00
Preferred dividends paid	$ 5	$ 5	$ 5
Series C Preferred Stock
Class of Stock [Line Items]
Depositary shares per share	0.025%
Dividend paid per share (usd per share)	$ 1,300.00	$ 1,300.00	$ 1,300.00
Preferred dividends paid	$ 8	$ 8	$ 8
Series D Preferred Stock
Class of Stock [Line Items]
Depositary shares per share	1.00%
Dividend paid per share (usd per share)		$ 2,250.00
Preferred dividends paid	$ 0	$ 11	0
Series E Preferred Stock
Class of Stock [Line Items]
Depositary shares per share	1.00%
Dividend paid per share (usd per share)		$ 2,475.00
Preferred dividends paid	$ 0	$ 25	$ 0
Series F Preferred Stock
Class of Stock [Line Items]
Depositary shares per share	1.00%
Dividend paid per share (usd per share)	$ 2,312.50		$ 2,312.50
Preferred dividends paid	$ 23	$ 0	$ 23