Mergers and Acquisitions- CSFL (Details)			3 Months Ended		6 Months Ended	9 Months Ended
	Feb. 01, 2021 USD ($)	Jun. 07, 2020 USD ($) $ / shares shares	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($) $ / shares	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Deposit Liabilities [Abstract]
Goodwill			$ 1,581,085,000			$ 1,581,085,000		$ 1,563,942,000
Additional information
Reversal of CSFL's ending allowance for credit losses			3,847,000	$ 4,642,000		12,883,000	$ 10,296,000
Proforma amounts
Total revenues (net interest income plus noninterest income)					$ 1,163,057,000
Net interest income					799,727,000
Net adjusted income available to the common shareholder					$ 240,838,000
EPS - basic | $ / shares					$ 3.40
EPS - diluted | $ / shares					$ 3.38
CSFL
Business Combination Recognized Identifiable Assets Acquired [Abstract]
Cash and cash equivalents		$ 2,566,450,000
Investment securities		1,193,910,000
Loans held for sale		453,578,000
Loans, net of allowance and mark		12,950,583,000
Premises and equipment		313,435,000
Intangible assets		140,862,000
OREO and repossessed assets		10,009,000
Bank owned life insurance		333,053,000
Deferred tax asset		37,622,000
Other assets		965,386,000
Total assets		18,964,888,000
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Deposit Liabilities [Abstract]
Noninterest-bearing		5,291,443,000
Interest-bearing		10,332,072,000
Total deposits		15,623,515,000
Federal funds purchased and securities sold under agreements to repurchase		401,546,000
Other borrowings		271,499,000
Other liabilities		973,990,000
Total liabilities		17,270,550,000
Net identifiable assets acquired over (under) liabilities assumed		1,694,338,000
Goodwill		562,370,000	562,400,000			562,400,000		562,400,000
Net assets acquired over liabilities assumed		$ 2,256,708,000
Consideration:
Number of shares issued as consideration | shares		37,271,069
Purchase price per share of the Company's common stock | $ / shares		$ 60.27
Company common stock issued ($2,246,327) and cash exchanged for fractional shares ($74)		$ 2,246,401,000
Stock option conversion		2,900,000
Restricted stock conversion		7,407,000
Value of common stock issued for fractional shares		2,246,327,000
Price per fractional share		74,000
Cash paid for stock options outstanding		2,900,000
Fair value of total consideration transferred		2,256,708,000
Noncash Merger Related Costs		2,900,000
Merger-related charges/costs			16,600,000	21,600,000		59,600,000	65,900,000
Additional information
Reversal of CSFL's existing fair value adjustments		40,700,000
Adjustment to record securities at fair value (premium)		46,200,000
Reclassification of securities held as HTM to AFS		175,700,000
Total mark of the loan portfolio		$ 269,100,000
Total mark of the loan portfolio (as a percent)		2.04%
Credit mark of the loan portfolio		$ 259,700,000
Credit mark of the loan portfolio (as a percent)		1.97%
Reversal of CSFL's ending allowance for credit losses		$ 158,200,000
Fair value adjustments of allowance for credit losses		62,600,000
MTM adjustment on leased assets		4,000,000.0
Deminimus fixed assets written off		$ 1,600,000
Core deposit intangible from a third party valuation (as a percent)		1.28
Core deposit intangible from a third party valuation		$ 125,900,000
Existing core deposit intangible from prior transactions that was reversed		84,900,000
Existing goodwill from prior transactions that was reversed		1,200,000,000
Reversal of prior valuation reserves		878,000
New valuation reserves		$ 1,700,000
Deferred tax assets related to fair value adjustments with effective tax rate		22.00%
Fair value adjustment of bank property held for sale		$ 4,400,000
Fair value adjustment of lease receivable		116,000
Fair value adjustment of interest receivable		501,000
Fair value negative adjustment for investment in low income housing		3,300,000
Estimated premium for fixed maturity time deposits		20,200,000
Reversal of existing CSFL fair value adjustments related to time deposit marks from other merger transactions		546,000
Discount on TRUPs from a third party valuation		12,500,000
Reversal of the existing CSFL discount on TRUPs and other debt		5,100,000
Reversal of an existing unfunded commitment reserve at purchase date		7,100,000
Fair value adjustment to increase lease liabilities associated with rental facilities		2,500,000
Fair value adjustments to employee benefit plans		5,200,000
Merger-related charges/costs			16,600,000	21,600,000		59,600,000	65,900,000
CSFL | As Recorded by CSFL
Business Combination Recognized Identifiable Assets Acquired [Abstract]
Cash and cash equivalents		2,566,450,000
Investment securities		1,188,403,000
Loans held for sale		453,578,000
Loans, net of allowance and mark		12,969,091,000
Premises and equipment		308,150,000
Intangible assets		1,294,211,000
OREO and repossessed assets		10,849,000
Bank owned life insurance		333,053,000
Deferred tax asset		54,123,000
Other assets		967,059,000
Total assets		20,144,967,000
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Deposit Liabilities [Abstract]
Noninterest-bearing		5,291,443,000
Interest-bearing		10,312,370,000
Total deposits		15,603,813,000
Federal funds purchased and securities sold under agreements to repurchase		401,546,000
Other borrowings		278,900,000
Other liabilities		977,725,000
Total liabilities		17,261,984,000
Net identifiable assets acquired over (under) liabilities assumed		2,882,983,000
Net assets acquired over liabilities assumed		2,882,983,000
CSFL | Initial Fair Value Adjustments
Business Combination Recognized Identifiable Assets Acquired [Abstract]
Investment securities		5,507,000
Loans, net of allowance and mark		(48,342,000)
Premises and equipment		2,392,000
Intangible assets		(1,163,349,000)
OREO and repossessed assets		(791,000)
Deferred tax asset		(8,681,000)
Other assets		(604,000)
Total assets		(1,213,868,000)
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Deposit Liabilities [Abstract]
Interest-bearing		19,702,000
Total deposits		19,702,000
Other borrowings		(7,401,000)
Other liabilities		(4,592,000)
Total liabilities		7,709,000
Net identifiable assets acquired over (under) liabilities assumed		(1,221,577,000)
Goodwill		600,483,000
Net assets acquired over liabilities assumed		(621,094,000)
CSFL | Subsequent Fair Value Adjustments
Business Combination Recognized Identifiable Assets Acquired [Abstract]
Loans, net of allowance and mark		29,834,000
Premises and equipment		2,893,000
Intangible assets		10,000,000		$ 10,000,000.0			$ 10,000,000.0
OREO and repossessed assets		(49,000)
Deferred tax asset		(7,820,000)
Other assets		(1,069,000)
Total assets		33,789,000
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Deposit Liabilities [Abstract]
Other liabilities		857,000
Total liabilities		857,000
Net identifiable assets acquired over (under) liabilities assumed		32,932,000
Goodwill		(38,113,000)	$ 38,100,000			$ 38,100,000		$ 38,100,000
Net assets acquired over liabilities assumed		(5,181,000)
Additional information
Deferred tax liability		7,800,000
CSFL | Subsequent Fair Value Adjustments
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Deposit Liabilities [Abstract]
Other liabilities		$ 857,000
Additional information
Deferred tax assets related to fair value adjustments with effective tax rate		23.78%
Duncan-Williams, Inc.
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Deposit Liabilities [Abstract]
Goodwill	$ 15,800,000
Consideration:
Fair value of total consideration transferred	48,300,000
Additional premium paid after three years	$ 8,000,000.0