Loans and Allowance for Loan Losses - Allowance for loan losses by portfolio (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		$ 1,754,352	$ 977,660	$ 920,993	$ 885,242
Provision (credit) for loan losses		150,889	140,629	1,034,169	477,939
Loans charged-off		(121,721)	(201,683)	(402,891)	(493,967)
Loan recoveries		20,903	25,585	67,221	72,977
Net (charge-offs) recoveries		(100,818)	(176,098)	(335,670)	(420,990)
Balance at end of period		1,804,423	942,191	1,804,423	942,191
Ending balance of allowance attributable to loans:
Individually evaluated for impairment						$ 126,832
Collectively evaluated for impairment						794,161
Total allowance for loan losses		1,804,423		1,804,423		920,993
Ending balance of loans:
Individually evaluated for impairment						486,044
Collectively evaluated for impairment						63,460,813
Total		66,180,612		66,180,612		63,946,857
Commercial, Financial and Agricultural
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period		757,512	456,054	408,197	393,315
Provision (credit) for loan losses		7,128	28,787	386,106	142,185
Loans charged-off		(54,187)	(73,178)	(109,364)	(132,006)
Loan recoveries		3,398	3,236	10,523	11,405
Net (charge-offs) recoveries		(50,789)	(69,942)	(98,841)	(120,601)
Balance at end of period		713,851	414,899	713,851	414,899
Ending balance of allowance attributable to loans:
Individually evaluated for impairment						88,164
Collectively evaluated for impairment						320,033
Total allowance for loan losses						408,197
Ending balance of loans:
Individually evaluated for impairment						238,653
Collectively evaluated for impairment						24,193,585
Total		26,940,173		26,940,173		24,432,238
Commercial Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	[1]	227,875	117,786	118,633	112,437
Provision (credit) for loan losses	[1]	89,086	6,410	241,910	9,906
Loans charged-off	[1]	(372)	(2,270)	(9,512)	(2,407)
Loan recoveries	[1]	121	79	713	2,069
Net (charge-offs) recoveries	[1]	(251)	(2,191)	(8,799)	(338)
Balance at end of period	[1]	316,710	122,005	316,710	122,005
Ending balance of allowance attributable to loans:
Individually evaluated for impairment	[1]					13,255
Collectively evaluated for impairment	[1]					105,378
Total allowance for loan losses	[1]					118,633
Ending balance of loans:
Individually evaluated for impairment	[1]					78,301
Collectively evaluated for impairment	[1]					15,811,859
Total	[1]					15,890,160
Residential Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	[2]	183,485	100,351	99,089	101,929
Provision (credit) for loan losses	[2]	65,974	3,214	103,142	5,147
Loans charged-off	[2]	(962)	(4,835)	(4,056)	(14,526)
Loan recoveries	[2]	1,041	3,183	3,973	9,363
Net (charge-offs) recoveries	[2]	79	(1,652)	(83)	(5,163)
Balance at end of period	[2]	249,538	101,913	249,538	101,913
Ending balance of allowance attributable to loans:
Individually evaluated for impairment	[2]					22,775
Collectively evaluated for impairment	[2]					76,314
Total allowance for loan losses	[2]					99,089
Ending balance of loans:
Individually evaluated for impairment	[2]					155,728
Collectively evaluated for impairment	[2]					16,215,874
Total	[2]					16,371,602
Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	[3]	585,480	303,469	295,074	277,561
Provision (credit) for loan losses	[3]	(11,299)	102,218	303,011	320,701
Loans charged-off	[3]	(66,200)	(121,400)	(279,959)	(345,028)
Loan recoveries	[3]	16,343	19,087	52,012	50,140
Net (charge-offs) recoveries	[3]	(49,857)	(102,313)	(227,947)	(294,888)
Balance at end of period	[3]	$ 524,324	$ 303,374	524,324	$ 303,374
Ending balance of allowance attributable to loans:
Individually evaluated for impairment	[3]					2,638
Collectively evaluated for impairment	[3]					292,436
Total allowance for loan losses	[3]					295,074
Ending balance of loans:
Individually evaluated for impairment	[3]					13,362
Collectively evaluated for impairment	[3]					7,239,495
Total	[3]					$ 7,252,857
Impact of ASC 326 Adoption
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				184,931
Impact of ASC 326 Adoption | Commercial, Financial and Agricultural
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				18,389
Impact of ASC 326 Adoption | Commercial Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	[1]			(35,034)
Impact of ASC 326 Adoption | Residential Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	[2]			47,390
Impact of ASC 326 Adoption | Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	[3]			154,186
As Reported Under ASC 326
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				1,105,924
As Reported Under ASC 326 | Commercial, Financial and Agricultural
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period				426,586
As Reported Under ASC 326 | Commercial Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	[1]			83,599
As Reported Under ASC 326 | Residential Real Estate
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	[2]			146,479
As Reported Under ASC 326 | Consumer
Allowance for Loan and Lease Losses [Roll Forward]
Balance at beginning of period	[3]			$ 449,260

[1] Includes commercial real estate – mortgage and real estate – construction loans.

[2] Includes residential real estate – mortgage, equity lines of credit and equity loans.

[3] Includes credit card, consumer direct and consumer indirect loans.