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, 2020	Mar. 31, 2019
Earnings Per Share, Basic and Diluted [Abstract]
Net income	$ 34,302	$ 167,925
Preferred stock dividends	(670)	(931)
Net income applicable to common stock	$ 33,632	$ 166,994
Average common shares outstanding	90,788,557	98,581,743
Average potential dilutive common shares	104,404	177,155
Average common shares outstanding - assuming dilution	90,892,961	98,758,898
Net Income per Common Share - Basic	$ 0.37	$ 1.69
Net Income per Common Share - Diluted	$ 0.37	$ 1.69