Mergers and Acquisitions - CSFL Narrative (Details) $ in Thousands			3 Months Ended			4 Months Ended	9 Months Ended
	Jul. 23, 2021	Jun. 07, 2020 USD ($) shares	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)
Mergers and Acquisitions
Goodwill			$ 1,581,085	$ 1,563,942			$ 1,581,085
Assets			40,903,708	37,789,873			40,903,708
Atlantic Capital
Mergers and Acquisitions
Assets			4,200,000				4,200,000
Loans			2,300,000				2,300,000
Deposits			3,700,000				3,700,000
CSFL
Mergers and Acquisitions
Shares of the common stock in exchange for each share		0.3001
Number of shares issued as consideration | shares		37,271,069
Purchase price		$ 2,256,708
Value of the conversion of outstanding stock options and restricted stock				10,300		$ 15,500	10,300
Purchased price decreased				5,200
Loans at fair value acquired after initial fair value adjustment		13,000,000
Estimated discount on loans		$ 239,500
Estimated discount on loans (as a percent)		1.82%
Fair value adjustment on loans acquired					$ 29,800
Loans acquired as a percentage of total loans		113.90%
Credit deficiencies that were identified as Purchased Credit Deteriorated ("PCD") loans.		$ 3,100,000
Estimated fair value of intangible asset		140,862
Goodwill		562,370	562,400	562,400			562,400
Acquisition costs			16,600		21,600		59,600	$ 65,900
CSFL | Core deposit intangibles
Mergers and Acquisitions
Estimated fair value of intangible asset		$ 125,900
Estimated economic life of intangible asset		10 years
Atlantic Capital
Mergers and Acquisitions
Shares of the common stock in exchange for each share	0.36
Subsequent Fair Value Adjustments | CSFL
Mergers and Acquisitions
Estimated fair value of intangible asset		$ 10,000			$ 10,000	$ 10,000		$ 10,000
Goodwill		$ (38,113)	$ 38,100	$ 38,100			$ 38,100