Preferred stock - Summary (Details) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Class of Stock [Line Items]
Total shares issued and outstanding (shares)	45,826	35,826
Carrying value	$ 4,532	$ 3,542
Series A Noncumulative Perpetual Preferred Stock
Class of Stock [Line Items]
Per annum dividend rate description	Greater of (i) three-month LIBOR plus 0.565% for the related distribution period; or (ii) 4.000%
Per annum dividend rate	4.00%
Total shares issued and outstanding (shares)	5,001	5,001
Carrying value	$ 500	$ 500
Liquidation preference per share (usd per share)	$ 100,000
Series A Noncumulative Perpetual Preferred Stock | LIBOR
Class of Stock [Line Items]
Preferred stock, basis spread on variable rate	0.565%
Series C Noncumulative Perpetual Preferred Stock
Class of Stock [Line Items]
Per annum dividend rate	5.20%
Total shares issued and outstanding (shares)	5,825	5,825
Carrying value	$ 568	$ 568
Liquidation preference per share (usd per share)	$ 100,000
Series D Noncumulative Perpetual Preferred Stock
Class of Stock [Line Items]
Per annum dividend rate description	4.50% to but excluding June 20, 2023, then a floating rate equal to the three-month LIBOR plus 2.46%
Per annum dividend rate	4.50%
Total shares issued and outstanding (shares)	5,000	5,000
Carrying value	$ 494	$ 494
Liquidation preference per share (usd per share)	$ 100,000
Series D Noncumulative Perpetual Preferred Stock | LIBOR
Class of Stock [Line Items]
Preferred stock, basis spread on variable rate	2.46%
Series E Noncumulative Perpetual Preferred Stock
Class of Stock [Line Items]
Per annum dividend rate description	4.95% to but excluding June 20, 2020, then a floating rate equal to the three-month LIBOR plus 3.42%
Per annum dividend rate	4.95%
Total shares issued and outstanding (shares)	10,000	10,000
Carrying value	$ 990	$ 990
Liquidation preference per share (usd per share)	$ 100,000
Series E Noncumulative Perpetual Preferred Stock | LIBOR
Class of Stock [Line Items]
Preferred stock, basis spread on variable rate	3.42%
Series F Noncumulative Perpetual Preferred Stock
Class of Stock [Line Items]
Per annum dividend rate description	4.625% to but excluding Sept. 20, 2026, then a floating rate equal to the three-month LIBOR plus 3.131%
Per annum dividend rate	4.625%
Total shares issued and outstanding (shares)	10,000	10,000
Carrying value	$ 990	$ 990
Liquidation preference per share (usd per share)	$ 100,000
Series F Noncumulative Perpetual Preferred Stock | LIBOR
Class of Stock [Line Items]
Preferred stock, basis spread on variable rate	3.131%
Series G Noncumulative Perpetual Preferred Stock
Class of Stock [Line Items]
Per annum dividend rate description	4.70% to but excluding Sept. 20, 2025, then a floating rate equal to the five-year treasury rate plus 4.358%
Per annum dividend rate	4.70%
Total shares issued and outstanding (shares)	10,000	0
Carrying value	$ 990	$ 0
Liquidation preference per share (usd per share)	$ 100,000
Series G Noncumulative Perpetual Preferred Stock | LIBOR
Class of Stock [Line Items]
Preferred stock, basis spread on variable rate	4.358%