Reclassifications Out of Accumulated Other Comprehensive Income - Summary of reclassifications (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Securities (losses) gains, net	$ (1,021)	$ (656)	$ 42
Investment securities interest revenue	552,706	500,080	389,720
Other expense	(15,092)	(17,194)	(12,962)
Deposit interest expense	(66,856)	(39,543)	(17,062)
Federal Home Loan Bank advances interest expense	(2,697)	(6,345)	(6,095)
Salaries and employee benefits expense	(196,440)	(181,015)	(153,098)
Merger-related and other charges	(6,907)	(5,414)	(13,901)
Income tax benefit (expense)	(52,991)	(49,815)	(105,013)
Net of tax	184,346	164,927	67,250
Reclassifications Out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax	(2,997)	(2,113)	(4,962)
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 (losses) gains, net	(1,021)	(656)	42
Income tax benefit (expense)	247	132	(14)
Net of tax	(774)	(524)	28
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	(383)	(739)	(1,069)
Income tax benefit (expense)	92	180	401
Net of tax	(291)	(559)	(668)
Reclassifications Out of Accumulated Other Comprehensive Income | Amounts included in net income on derivative financial instruments accounted for as cash flow hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other expense	(235)	0	0
Deposit interest expense	(102)	(499)	(599)
Federal Home Loan Bank advances interest expense	0	0	(292)
Total before tax	(337)	(499)	(891)
Income tax benefit (expense)	86	129	346
Net of tax	(251)	(370)	(545)
Reclassifications Out of Accumulated Other Comprehensive Income | Reclassification of disproportionate tax effect related to terminated cash flow hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax benefit (expense)	0	0	(3,289)
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	(640)	(666)	(560)
Reclassifications Out of Accumulated Other Comprehensive Income | Actuarial losses
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other expense	(59)	(241)	0
Salaries and employee benefits expense	0	0	(238)
Reclassifications Out of Accumulated Other Comprehensive Income | Termination of funded plan
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Merger-related and other charges	(1,558)	0	0
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	(2,257)	(907)	(798)
Income tax benefit (expense)	576	247	310
Net of tax	$ (1,681)	$ (660)	$ (488)