Investment Securities - Amortized cost and fair value for available for sale securities (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Mar. 31, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022
Investment securities held-to-maturity
Amortized cost			$ 2,585,155,000		$ 2,683,241,000
Gross Unrealized Losses			(440,641,000)		(433,073,000)
Fair Value			2,144,514,000		2,250,168,000
Investment securities available for sale
Amortized cost			5,830,576,000		6,216,141,000
Gross Unrealized Gains			160,000		4,054,000
Gross Unrealized Losses			(881,402,000)		(893,373,000)
Fair Value			4,949,334,000		5,326,822,000
Gross realized gains			1,300,000
Gross realized losses			1,300,000
Securities gains, net	$ 0		45,000	$ 0
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			223,658,000		272,416,000
Gross Unrealized Losses			(3,745,000)		(6,778,000)
Fair Value			219,913,000		265,638,000
U.S. Government agencies
Investment securities held-to-maturity
Amortized cost			197,265,000		197,262,000
Gross Unrealized Losses			(29,352,000)		(29,787,000)
Fair Value			167,913,000		167,475,000
Investment securities available for sale
Amortized cost			246,030,000		245,972,000
Gross Unrealized Losses			(27,685,000)		(26,884,000)
Fair Value			218,345,000		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			1,514,472,000		1,591,646,000
Gross Unrealized Losses			(253,666,000)		(255,093,000)
Fair Value			1,260,806,000		1,336,553,000
Investment securities available for sale
Amortized cost			1,903,472,000		1,996,405,000
Gross Unrealized Losses			(299,102,000)		(298,052,000)
Fair Value			1,604,370,000		1,698,353,000
Residential collateralized mortgage-obligations issued by U.S. government agencies or sponsored enterprises
Investment securities held-to-maturity
Amortized cost			460,574,000		474,660,000
Gross Unrealized Losses			(72,157,000)		(69,664,000)
Fair Value			388,417,000		404,996,000
Investment securities available for sale
Amortized cost			666,198,000		708,337,000
Gross Unrealized Losses			(106,422,000)		(107,292,000)
Fair Value			559,776,000		601,045,000
Commercial mortgage-backed securities issued by U.S. government agencies or sponsored enterprises
Investment securities held-to-maturity
Amortized cost			357,815,000		362,586,000
Gross Unrealized Losses			(73,410,000)		(66,304,000)
Fair Value			284,405,000		296,282,000
Investment securities available for sale
Amortized cost			1,173,907,000		1,196,700,000
Gross Unrealized Gains					2,542,000
Gross Unrealized Losses			(211,413,000)		(198,844,000)
Fair Value			962,494,000		1,000,398,000
State and municipal obligations
Investment securities available for sale
Amortized cost			1,135,189,000		1,269,525,000
Gross Unrealized Gains			3,000		1,210,000
Gross Unrealized Losses			(180,737,000)		(205,883,000)
Fair Value			954,455,000		1,064,852,000
Small Business Administration loan-backed securities
Investment securities held-to-maturity
Amortized cost			55,029,000		57,087,000
Gross Unrealized Losses			(12,056,000)		(12,225,000)
Fair Value			42,973,000		44,862,000
Investment securities available for sale
Amortized cost			451,564,000		491,203,000
Gross Unrealized Gains			157,000		302,000
Gross Unrealized Losses			(47,162,000)		(46,695,000)
Fair Value			404,559,000		444,810,000
Corporate securities.
Investment securities available for sale
Amortized cost			30,558,000		35,583,000
Gross Unrealized Losses			(5,136,000)		(2,945,000)
Fair Value			$ 25,422,000		$ 32,638,000