Net income per common share - Computation of net income (loss) per common share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share, Basic and Diluted [Abstract]
Net income	$ 248,114	$ 168,416	$ 728,825	$ 330,346
Preferred stock dividends	(353)	(352)	(1,059)	(1,405)
Net income applicable to common stock	$ 247,761	$ 168,064	$ 727,766	$ 328,941
Average common shares outstanding	80,126,166	83,809,272	81,864,634	86,567,680
Average potential dilutive common shares	148,776	26,879	149,479	78,011
Average common shares outstanding - assuming dilution	80,274,942	83,836,151	82,014,113	86,645,691
Basic EPS	$ 3.09	$ 2.01	$ 8.89	$ 3.80
Diluted EPS	$ 3.09	$ 2.00	$ 8.87	$ 3.80