Allowance for Credit Losses - Roll Forward Of the Allowance for Loan Losses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Allowance for loan losses:
Beginning balance	$ 120,185	$ 49,835	$ 52,162	$ 49,026
Charge-offs	(3,566)	(1,451)	(8,915)	(2,988)
Recoveries	1,868	775	6,406	1,621
Net (charge-offs) recoveries	(1,698)	(676)	(2,509)	(1,367)
Provision for credit losses on loans	26,900	900	53,250	2,400
Ending balance	145,387	50,059	145,387	50,059
Individually evaluated	5,080	1,873	5,080	1,873
Collectively evaluated	140,307	45,520	140,307	45,520
Loans:
Individually evaluated	23,387		23,387		$ 51,179
Collectively evaluated	10,973,917		10,973,917		9,466,195
Ending balance	10,997,304		10,997,304		9,689,638
Nonaccruing loans with no allowance for credit losses	7,135		7,135
Purchased with deteriorated credit quality
Allowance for loan losses:
Ending balance		2,666		2,666
Loans:
Ending balance					172,264
Commercial, financial, agricultural
Allowance for loan losses:
Beginning balance	25,937	9,622	10,658	8,269
Charge-offs	(1,156)	(694)	(1,549)	(952)
Recoveries	108	241	298	615
Net (charge-offs) recoveries	(1,048)	(453)	(1,251)	(337)
Provision for credit losses on loans	5,796	365	9,927	1,602
Ending balance	30,685	9,534	30,685	9,534
Individually evaluated	3,882	1,191	3,882	1,191
Collectively evaluated	26,803	8,172	26,803	8,172
Loans:
Individually evaluated	9,736		9,736		8,460
Collectively evaluated	2,631,862		2,631,862		1,329,974
Ending balance	2,641,598		2,641,598		1,367,972
Nonaccruing loans with no allowance for credit losses	720		720
Commercial, financial, agricultural | Purchased with deteriorated credit quality
Allowance for loan losses:
Ending balance		171		171
Loans:
Ending balance					29,538
Real estate – construction:
Allowance for loan losses:
Beginning balance	10,924	4,778	5,029	4,755
Charge-offs	(532)	0	(532)	0
Recoveries	0	0	0	7
Net (charge-offs) recoveries	(532)	0	(532)	7
Provision for credit losses on loans	2,146	524	4,536	540
Ending balance	12,538	5,302	12,538	5,302
Individually evaluated	0	8	0	8
Collectively evaluated	12,538	5,294	12,538	5,294
Loans:
Individually evaluated	0		0		12,416
Collectively evaluated	791,108		791,108		813,204
Ending balance	791,108		791,108		826,483
Nonaccruing loans with no allowance for credit losses	0		0
Real estate – construction: | Purchased with deteriorated credit quality
Allowance for loan losses:
Ending balance		0		0
Loans:
Ending balance					863
Real estate – 1-4 family mortgage
Allowance for loan losses:
Beginning balance	27,320	9,491	9,814	10,139
Charge-offs	(142)	(378)	(363)	(875)
Recoveries	48	115	136	312
Net (charge-offs) recoveries	(94)	(263)	(227)	(563)
Provision for credit losses on loans	2,175	388	5,500	40
Ending balance	29,401	9,616	29,401	9,616
Individually evaluated	378	188	378	188
Collectively evaluated	29,023	8,913	29,023	8,913
Loans:
Individually evaluated	5,142		5,142		20,262
Collectively evaluated	2,783,371		2,783,371		2,810,808
Ending balance	2,788,513		2,788,513		2,866,613
Nonaccruing loans with no allowance for credit losses	2,503		2,503
Real estate – 1-4 family mortgage | Purchased with deteriorated credit quality
Allowance for loan losses:
Ending balance		515		515
Loans:
Ending balance					35,543
Real estate – commercial mortgage:
Allowance for loan losses:
Beginning balance	44,237	24,643	24,990	24,492
Charge-offs	0	(167)	(2,047)	(729)
Recoveries	41	366	1,740	611
Net (charge-offs) recoveries	41	199	(307)	(118)
Provision for credit losses on loans	15,783	(540)	31,085	(72)
Ending balance	60,061	24,302	60,061	24,302
Individually evaluated	550	482	550	482
Collectively evaluated	59,511	21,842	59,511	21,842
Loans:
Individually evaluated	7,883		7,883		9,550
Collectively evaluated	4,402,870		4,402,870		4,131,582
Ending balance	4,410,753		4,410,753		4,244,265
Nonaccruing loans with no allowance for credit losses	3,910		3,910
Real estate – commercial mortgage: | Purchased with deteriorated credit quality
Allowance for loan losses:
Ending balance		1,978		1,978
Loans:
Ending balance					103,133
Lease financing
Allowance for loan losses:
Beginning balance	1,588		910
Charge-offs	0		0
Recoveries	5		10
Net (charge-offs) recoveries	5		10
Provision for credit losses on loans	219		371
Ending balance	1,812		1,812
Individually evaluated	0		0
Collectively evaluated	1,812		1,812
Loans:
Individually evaluated	0		0
Collectively evaluated	80,779		80,779
Ending balance	80,779		80,779
Nonaccruing loans with no allowance for credit losses	0		0
Installment and Other
Allowance for loan losses:
Beginning balance	10,179	1,301	761	1,371
Charge-offs	(1,736)	(212)	(4,424)	(432)
Recoveries	1,666	53	4,222	76
Net (charge-offs) recoveries	(70)	(159)	(202)	(356)
Provision for credit losses on loans	781	163	1,831	290
Ending balance	10,890	1,305	10,890	1,305
Individually evaluated	270	4	270	4
Collectively evaluated	10,620	1,299	10,620	1,299
Loans:
Individually evaluated	626		626		491
Collectively evaluated	283,927		283,927		380,627
Ending balance	284,553		284,553		384,305
Nonaccruing loans with no allowance for credit losses	$ 2		2
Installment and Other | Purchased with deteriorated credit quality
Allowance for loan losses:
Ending balance		$ 2		$ 2
Loans:
Ending balance					$ 3,187
Cumulative Effect, Period Of Adoption, Adjustment
Allowance for loan losses:
Beginning balance			42,484
Cumulative Effect, Period Of Adoption, Adjustment | Commercial, financial, agricultural
Allowance for loan losses:
Beginning balance			11,351
Cumulative Effect, Period Of Adoption, Adjustment | Real estate – construction:
Allowance for loan losses:
Beginning balance			3,505
Cumulative Effect, Period Of Adoption, Adjustment | Real estate – 1-4 family mortgage
Allowance for loan losses:
Beginning balance			14,314
Cumulative Effect, Period Of Adoption, Adjustment | Real estate – commercial mortgage:
Allowance for loan losses:
Beginning balance			4,293
Cumulative Effect, Period Of Adoption, Adjustment | Lease financing
Allowance for loan losses:
Beginning balance			521
Cumulative Effect, Period Of Adoption, Adjustment | Installment and Other
Allowance for loan losses:
Beginning balance			$ 8,500