Investment Securities - Amortized cost and fair value for available for sale securities (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Mar. 31, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Investment securities held-to-maturity
Amortized cost	$ 2,533,713,000			$ 2,533,713,000		$ 2,683,241,000
Gross Unrealized Losses	(521,313,000)			(521,313,000)		(433,073,000)
Fair Value	2,012,400,000			2,012,400,000		2,250,168,000
Investment securities available for sale
Amortized cost	5,709,270,000			5,709,270,000		6,216,141,000
Gross Unrealized Gains	125,000			125,000		4,054,000
Gross Unrealized Losses	(1,085,777,000)			(1,085,777,000)		(893,373,000)
Fair Value	4,623,618,000			4,623,618,000		5,326,822,000
Gross realized gains		$ 103,000		1,300,000	$ 103,000
Gross realized losses		73,000		1,300,000	73,000
Securities gains, net	0	$ 30,000		45,000	$ 30,000
Atlantic Capital
Investment securities available for sale
Realized gains or losses			$ 0
Sold securities			$ 414,400,000
U.S. Treasuries
Investment securities available for sale
Amortized cost	174,322,000			174,322,000		272,416,000
Gross Unrealized Losses	(2,063,000)			(2,063,000)		(6,778,000)
Fair Value	172,259,000			172,259,000		265,638,000
U.S. Government agencies
Investment securities held-to-maturity
Amortized cost	197,266,000			197,266,000		197,262,000
Gross Unrealized Losses	(33,016,000)			(33,016,000)		(29,787,000)
Fair Value	164,250,000			164,250,000		167,475,000
Investment securities available for sale
Amortized cost	246,059,000			246,059,000		245,972,000
Gross Unrealized Losses	(29,305,000)			(29,305,000)		(26,884,000)
Fair Value	216,754,000			216,754,000		219,088,000
Residential mortgage pass-through securities issued or guaranteed by U.S. government agencies or sponsored enterprises
Investment securities held-to-maturity
Amortized cost	1,474,360,000			1,474,360,000		1,591,646,000
Gross Unrealized Losses	(303,022,000)			(303,022,000)		(255,093,000)
Fair Value	1,171,338,000			1,171,338,000		1,336,553,000
Investment securities available for sale
Amortized cost	1,851,803,000			1,851,803,000		1,996,405,000
Gross Unrealized Losses	(363,097,000)			(363,097,000)		(298,052,000)
Fair Value	1,488,706,000			1,488,706,000		1,698,353,000
Residential collateralized mortgage-obligations issued by U.S. government agencies or sponsored enterprises
Investment securities held-to-maturity
Amortized cost	451,928,000			451,928,000		474,660,000
Gross Unrealized Losses	(85,938,000)			(85,938,000)		(69,664,000)
Fair Value	365,990,000			365,990,000		404,996,000
Investment securities available for sale
Amortized cost	644,581,000			644,581,000		708,337,000
Gross Unrealized Losses	(125,154,000)			(125,154,000)		(107,292,000)
Fair Value	519,427,000			519,427,000		601,045,000
Commercial mortgage-backed securities issued by U.S. government agencies or sponsored enterprises
Investment securities held-to-maturity
Amortized cost	355,664,000			355,664,000		362,586,000
Gross Unrealized Losses	(85,773,000)			(85,773,000)		(66,304,000)
Fair Value	269,891,000			269,891,000		296,282,000
Investment securities available for sale
Amortized cost	1,199,936,000			1,199,936,000		1,196,700,000
Gross Unrealized Gains	3,000			3,000		2,542,000
Gross Unrealized Losses	(250,407,000)			(250,407,000)		(198,844,000)
Fair Value	949,532,000			949,532,000		1,000,398,000
State and municipal obligations
Investment securities available for sale
Amortized cost	1,132,238,000			1,132,238,000		1,269,525,000
Gross Unrealized Gains	1,000			1,000		1,210,000
Gross Unrealized Losses	(259,232,000)			(259,232,000)		(205,883,000)
Fair Value	873,007,000			873,007,000		1,064,852,000
Small Business Administration loan-backed securities
Investment securities held-to-maturity
Amortized cost	54,495,000			54,495,000		57,087,000
Gross Unrealized Losses	(13,564,000)			(13,564,000)		(12,225,000)
Fair Value	40,931,000			40,931,000		44,862,000
Investment securities available for sale
Amortized cost	429,785,000			429,785,000		491,203,000
Gross Unrealized Gains	120,000			120,000		302,000
Gross Unrealized Losses	(52,435,000)			(52,435,000)		(46,695,000)
Fair Value	377,470,000			377,470,000		444,810,000
Corporate securities.
Investment securities available for sale
Amortized cost	30,546,000			30,546,000		35,583,000
Gross Unrealized Gains	1,000			1,000
Gross Unrealized Losses	(4,084,000)			(4,084,000)		(2,945,000)
Fair Value	$ 26,463,000			$ 26,463,000		$ 32,638,000