Other comprehensive income (loss) - Reclassification out of accumulated other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Reclassification out of accumulated other comprehensive loss
Personnel costs	$ (147,682)	$ (139,757)	$ (432,298)	$ (389,941)
Income tax (benefit) expense	(41,370)	(42,018)	(131,923)	(35,613)
Net income	165,319	140,648	504,350	511,755
Reclassification out of accumulated other comprehensive income
Reclassification out of accumulated other comprehensive loss
Net income	(4,409)	(2,845)	(12,955)	(7,738)
Reclassification out of accumulated other comprehensive income | Adjustment of pension and postretirement benefit plan
Reclassification out of accumulated other comprehensive loss
Income (loss) before tax	(5,877)	(4,518)	(17,629)	(13,554)
Income tax (benefit) expense	(2,204)	(1,762)	(6,611)	(5,285)
Net income	(3,673)	(2,756)	(11,018)	(8,269)
Reclassification out of accumulated other comprehensive income | Adjustment of pension and postretirement benefit plan | Amortization of net losses | Personnel costs
Reclassification out of accumulated other comprehensive loss
Income (loss) before tax	(5,877)	(5,386)	(17,629)	(16,157)
Reclassification out of accumulated other comprehensive income | Adjustment of pension and postretirement benefit plan | Amortization of prior service credit | Personnel costs
Reclassification out of accumulated other comprehensive loss
Income (loss) before tax	0	868	0	2,603
Reclassification out of accumulated other comprehensive income | Unrealized holding gains (losses) on debt securities
Reclassification out of accumulated other comprehensive loss
Income (loss) before tax	(20)	0	(20)	0
Income tax (benefit) expense	(4)	0	(4)	0
Net income	(16)	0	(16)	0
Reclassification out of accumulated other comprehensive income | Unrealized holding gains (losses) on debt securities | Realized gain (loss) on sale of securities
Reclassification out of accumulated other comprehensive loss
Net (loss) gain and valuation adjustments on debt securities	(20)	0	(20)	0
Reclassification out of accumulated other comprehensive income | Unrealized net (losses) gains on cash flow hedges
Reclassification out of accumulated other comprehensive loss
Income (loss) before tax	(1,221)	(147)	(3,142)	870
Income tax (benefit) expense	(501)	(58)	(1,221)	339
Net income	(720)	(89)	(1,921)	531
Reclassification out of accumulated other comprehensive income | Unrealized net (losses) gains on cash flow hedges | Forward contracts | Mortgage banking activities
Reclassification out of accumulated other comprehensive loss
Income (loss) before tax	(1,337)	(147)	(3,258)	870
Reclassification out of accumulated other comprehensive income | Unrealized net (losses) gains on cash flow hedges | Interest Rate Swap [Member]
Reclassification out of accumulated other comprehensive loss
Other Operating Income	$ 116	$ 0	$ 116	$ 0