Preferred stock - Narrative (Details) - $ / shares	9 Months Ended
	Sep. 30, 2020	May 31, 2020	Dec. 31, 2019
Class of Stock [Line Items]
Preferred stock, shares authorized (shares)	100,000,000		100,000,000
Preferred stock, par value (usd per share)	$ 0.01		$ 0.01
Series G Noncumulative Perpetual Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares authorized (shares)		1,000,000
Per annum dividend rate	4.70%
LIBOR | Series G Noncumulative Perpetual Preferred Stock
Class of Stock [Line Items]
Preferred stock, basis spread on variable rate	4.358%