Goodwill and Other Intangible Assets - Narrative (Details) - USD ($)		3 Months Ended		9 Months Ended
	Jul. 01, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Amortization expense		$ 14,600,000	$ 6,200,000	$ 25,200,000	$ 18,600,000
Goodwill [Line Items]
Goodwill		1,510,431,000	1,432,787,000	1,510,431,000	1,432,787,000	$ 1,432,787,000	$ 1,432,787,000
Purchase accounting gain		532,150,000	$ 0	$ 532,150,000	0
Core deposit intangibles
Goodwill [Line Items]
Finite-lived intangible asset, estimated useful life				10 years
SunTrust Banks, Inc. Branches
Goodwill [Line Items]
Goodwill		77,644,000		$ 77,644,000
SunTrust Banks, Inc. Branches | Core deposit intangibles
Goodwill [Line Items]
Finite-Lived Intangible Assets, Net		7,000,000.0		7,000,000.0
IBERIABANK (IBKC)
Goodwill [Line Items]
Goodwill		(532,150,000)		(532,150,000)
Purchase accounting gain	$ 532,200,000
IBERIABANK (IBKC) | Core deposit intangibles
Goodwill [Line Items]
Finite-Lived Intangible Assets, Net		206,700,000		206,700,000
IBERIABANK (IBKC) | Customer relationships
Goodwill [Line Items]
Finite-Lived Intangible Assets, Net		14,000,000.0		14,000,000.0
IBERIABANK (IBKC) | State Banking Licenses [Member]
Goodwill [Line Items]
Finite-Lived Intangible Assets, Net		6,800,000		6,800,000
IBERIABANK (IBKC) | Purchased Credit Card Intangible Assets [Member]
Goodwill [Line Items]
Finite-Lived Intangible Assets, Net		10,300,000		10,300,000
Non-Strategic
Goodwill [Line Items]
Gross goodwill		200,000,000.0		200,000,000.0		200,000,000.0
Accumulated impairments		114,100,000		114,100,000		114,100,000
Accumulated divestiture related write-offs		85,900,000		85,900,000		85,900,000
Goodwill		0		0		$ 0
Purchase accounting gain		$ 0		$ 0	$ 0