Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Securities gains (losses), net	$ 746	$ 0	$ 746	$ (118)
Investment securities interest revenue	141,773	140,615	401,925	416,287
Long-term debt interest expense	(4,292)	(3,214)	(10,186)	(9,813)
Deposit interest expense	(8,998)	(17,113)	(35,344)	(50,185)
Salaries and employee benefits expense	(59,067)	(50,501)	(162,236)	(146,161)
Other expense	(3,603)	(4,017)	(10,670)	(11,077)
Merger-related and other charges	(3,361)	(2,541)	(4,566)	(6,981)
Total before tax	59,362	62,345	132,072	176,815
Income tax (expense) benefit	(11,755)	(13,983)	(27,485)	(40,106)
Net income	47,607	48,362	104,587	136,709
Reclassifications Out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income	(116)	(1,369)	(621)	(2,102)
Reclassifications Out of Accumulated Other Comprehensive Income | Realized gains (losses) on available-for-sale securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Securities gains (losses), net	746	0	746	(118)
Income tax (expense) benefit	(191)	0	(191)	30
Net income	555	0	555	(88)
Reclassifications Out of Accumulated Other Comprehensive Income | Amortization of losses included in net income on available-for-sale securities transferred to held to maturity
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Investment securities interest revenue	(544)	(105)	(723)	(282)
Income tax (expense) benefit	130	25	173	67
Net income	(414)	(80)	(550)	(215)
Reclassifications Out of Accumulated Other Comprehensive Income | Reclassifications related to derivative financial instruments accounted for as cash flow hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Long-term debt interest expense	(130)	0	(197)	0
Deposit interest expense	0	0	0	(102)
Other expense	0	0	0	(235)
Total before tax	(130)	0	(197)	(337)
Income tax (expense) benefit	33	0	50	86
Net income	(97)	0	(147)	(251)
Reclassifications Out of Accumulated Other Comprehensive Income | Reclassifications related to defined benefit pension plan activity
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	(215)	(1,732)	(643)	(2,079)
Income tax (expense) benefit	55	443	164	531
Net income	(160)	(1,289)	(479)	(1,548)
Reclassifications Out of Accumulated Other Comprehensive Income | Prior service cost
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits expense	(133)	(158)	(398)	(476)
Reclassifications Out of Accumulated Other Comprehensive Income | Actuarial losses
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other expense	(82)	(16)	(245)	(45)
Reclassifications Out of Accumulated Other Comprehensive Income | Termination of defined benefit pension plan
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Merger-related and other charges	$ 0	$ (1,558)	$ 0	$ (1,558)