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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share, Basic and Diluted [Abstract]
Net income	$ 165,319	$ 140,648	$ 504,350	$ 511,755
Preferred stock dividends	(930)	(930)	(2,792)	(2,792)
Net income applicable to common stock	$ 164,389	$ 139,718	$ 501,558	$ 508,963
Average common shares outstanding	96,357,117	101,067,300	97,073,177	101,549,711
Average potential dilutive common shares	121,210	181,854	139,219	182,219
Average common shares outstanding - assuming dilution	96,478,327	101,249,154	97,212,396	101,731,930
Net Income per Common Share - Basic	$ 1.71	$ 1.38	$ 5.17	$ 5.01
Net Income per Common Share - Diluted	$ 1.70	$ 1.38	$ 5.16	$ 5.00