Accumulated Other Comprehensive Loss - Reclassifications (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Gain on sale of investment securities	$ 77	$ 18	$ 167	$ 37
Income tax (expense)/benefit	(23,036)	(10,532)	(35,293)	(20,289)
Net income	52,014	25,775	99,618	49,380
Interest on short-term borrowings (interest expense)	4,659	226	6,317	326
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	77	18	167	37
Income tax (expense)/benefit	(27)	(6)	(58)	(13)
Net income	50	12	109	24
Amortization of defined benefit pension plan items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries, wages and employee benefits (operating expenses)	(539)	577	(1,076)	1,154
Income tax (expense)/benefit	189	(202)	377	(404)
Net Income (loss)	(350)	375	(699)	750
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	(409)	0	(409)	0
Interest on short-term borrowings (interest expense)	$ 409	$ 0	$ 409	$ 0