Net income per common share - Computation of net income (loss) per common share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Earnings Per Share, Basic and Diluted [Abstract]
Net income	$ 262,632	$ 34,302
Preferred stock dividends	(353)	(700)
Net income applicable to common stock	$ 262,279	$ 33,602
Average common shares outstanding	83,899,769	90,788,557
Average potential dilutive common shares	152,166	104,404
Average common shares outstanding - assuming dilution	84,051,935	90,892,961
Basic EPS	$ 3.13	$ 0.37
Diluted EPS	$ 3.12	$ 0.37