Preferred stock - Summary of Preferred Dividends Paid (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Class of Stock [Line Items]
Preferred dividends paid	$ 66	$ 31	$ 61	$ 166	$ 146
Series A Preferred Stock
Class of Stock [Line Items]
Dividend paid per share (usd per share)	$ 1,011.11	$ 1,011.11	$ 1,011.11	$ 3,033.33	$ 3,044.44
Preferred dividends paid	$ 5	$ 5	$ 5	$ 15	$ 15
Series C Preferred Stock
Class of Stock [Line Items]
Dividend paid per share (usd per share)			$ 1,300		$ 3,900
Preferred dividends paid			$ 7		$ 23
Series D Preferred Stock
Class of Stock [Line Items]
Dividend paid per share (usd per share)		$ 2,250		$ 2,250	$ 2,250
Preferred dividends paid		$ 11		$ 11	$ 11
Series E Preferred Stock
Class of Stock [Line Items]
Dividend paid per share (usd per share)	$ 898.50	$ 911.68	$ 962.65	$ 2,735	$ 3,437.65
Preferred dividends paid	$ 10	$ 9	$ 10	$ 28	$ 35
Series F Preferred Stock
Class of Stock [Line Items]
Dividend paid per share (usd per share)	$ 2,312.5		$ 2,312.5	$ 4,625	$ 4,625
Preferred dividends paid	$ 23		$ 23	$ 46	$ 46
Series G Preferred Stock
Class of Stock [Line Items]
Dividend paid per share (usd per share)	$ 2,350		$ 1,579.72	$ 4,700	$ 1,579.72
Preferred dividends paid	$ 23		$ 16	$ 47	$ 16
Series H Preferred Stock
Class of Stock [Line Items]
Dividend paid per share (usd per share)	$ 925.00	$ 925.00		$ 3,258.06
Preferred dividends paid	$ 5	$ 6		$ 19