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, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share, Basic and Diluted [Abstract]
Net income	$ 168,416	$ 165,319	$ 330,346	$ 504,350
Preferred stock dividends	(352)	(930)	(1,405)	(2,792)
Net income applicable to common stock	$ 168,064	$ 164,389	$ 328,941	$ 501,558
Average common shares outstanding	83,809,272	96,357,117	86,567,680	97,073,177
Average potential dilutive common shares	26,879	121,210	78,011	139,219
Average common shares outstanding - assuming dilution	83,836,151	96,478,327	86,645,691	97,212,396
Net Income per Common Share - Basic	$ 2.01	$ 1.71	$ 3.80	$ 5.17
Net Income per Common Share - Diluted	$ 2.00	$ 1.70	$ 3.80	$ 5.16