Mergers and Acquisitions- CSFL (Details)		3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 07, 2020 USD ($) $ / shares shares	Jun. 30, 2020 USD ($) $ / shares	Jun. 30, 2019 USD ($) $ / shares	Jun. 30, 2020 USD ($) $ / shares	Jun. 30, 2019 USD ($) $ / shares	Dec. 31, 2019 USD ($)
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Deposit Liabilities [Abstract]
Goodwill		$ 1,603,383,000	$ 1,002,900,000	$ 1,603,383,000	$ 1,002,900,000	$ 1,002,900,000
Additional information
Reversal of CSFL's ending allowance for credit losses		2,431,000	2,862,000	5,654,000	4,481,000
Proforma amounts
Total revenues (net interest income plus noninterest income), Pro Forma		415,049,000	353,372,000	785,210,000	678,648,000
Net interest income		266,431,000	277,811,000	536,670,000	541,729,000
Net adjusted income available to the common shareholder		$ 116,891,000	$ 101,120,000	$ 167,968,000	$ 214,400,000
EPS - basic | $ / shares		$ 1.64	$ 1.37	$ 2.37	$ 3.10
EPS - diluted | $ / shares		$ 1.64	$ 1.36	$ 2.36	$ 3.08
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	12,920,749,000
Premises and equipment	326,788,000
Intangible assets	130,862,000
OREO and repossessed assets	10,058,000
Bank owned life insurance	333,053,000
Deferred tax asset.	45,442,000
Other assets	1,060,532,000
Total assets	19,041,422,000
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	1,083,456,000
Total liabilities	17,380,016,000
Net identifiable assets acquired over (under) liabilities assumed	1,661,406,000
Goodwill	600,483,000		$ 1,000,000.0		$ 1,000,000.0	1,000,000.0
Net assets acquired over liabilities assumed	$ 2,261,889,000
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	8,080,000
Restricted stock conversion	7,407,000
Common stock issued	2,246,327,000
Price per fractional share	74
Cash paid for stock options outstanding	8,080,000
Fair value of total consideration transferred	2,261,888,000
Merger-related charges/costs		$ 40,200,000	0	$ 44,300,000	0
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	$ 3,800,000
Fair value adjustment of lease receivable	116,000
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
Merger-related charges/costs		$ 40,200,000	$ 0	$ 44,300,000	$ 0
Provision for loan losses						3,800,000
Provisional income tax expense						$ 119,100,000
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	324,396,000
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	1,061,136,000
Total assets	20,255,290,000
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	1,088,048,000
Total liabilities	17,372,307,000
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)