Net income per common share - Computation of net income (loss) per common share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Earnings Per Share, Basic and Diluted [Abstract]
Net income	$ 218,079	$ 127,628	$ 480,711	$ 161,930
Preferred stock dividends	(353)	(353)	(706)	(1,053)
Net income applicable to common stock	$ 217,726	$ 127,275	$ 480,005	$ 160,877
Average common shares outstanding	81,609,435	85,135,522	82,748,275	87,962,040
Average potential dilutive common shares	163,354	26,139	140,103	77,672
Average common shares outstanding - assuming dilution	81,772,789	85,161,661	82,888,378	88,039,712
Basic EPS	$ 2.67	$ 1.49	$ 5.80	$ 1.83
Diluted EPS	$ 2.66	$ 1.49	$ 5.79	$ 1.83