Accumulated Other Comprehensive Loss - Reclassifications (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gain on sale of investment securities	$ 1	$ 16	$ 168	$ 53
Income tax (expense)/benefit	(10,253)	(2,848)	(45,546)	(23,137)
Net income	40,459	11,484	140,077	60,864
Interest on short-term borrowings (interest expense)	6,591	459	12,908	785
Gains and losses on available-for-sale securities | Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gain on sale of investment securities	1	16	168	53
Income tax (expense)/benefit	0	(6)	(59)	(18)
Net income	1	10	109	35
Amortization of defined benefit pension plan items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries, wages and employee benefits (operating expenses)	(537)	(2,895)	(1,613)	(1,742)
Income tax (expense)/benefit	188	1,013	565	610
Net Income (loss)	(349)	(1,882)	(1,048)	(1,132)
Gains and losses on cash flow hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax (expense)/benefit	344	0	344	0
Gains and losses on cash flow hedges | Amounts Reclassified from Accumulated Other Comprehensive Income (Loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	(231)	0	(640)	0
Interest on short-term borrowings (interest expense)	$ (575)	$ 0	$ (984)	$ 0