Investment Securities - Amortized cost and fair value for available for sale securities (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2022	Mar. 31, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
investment securities held-to-maturity
Amortized cost	$ 2,738,178,000			$ 2,738,178,000		$ 1,819,901,000
Gross Unrealized Gains	691,000			691,000		318,000
Gross Unrealized Losses	(439,707,000)			(439,707,000)		(42,155,000)
Fair Value	2,299,162,000			2,299,162,000		1,778,064,000
Investment securities available for sale
Amortized cost	6,319,896,000			6,319,896,000		5,221,296,000
Gross Unrealized Gains	3,136,000			3,136,000		35,980,000
Gross Unrealized Losses	(953,422,000)			(953,422,000)		(63,798,000)
Fair Value	5,369,610,000			5,369,610,000		5,193,478,000
Realized gains or losses		$ 0
Gross realized gains	103,000		$ 680,000	103,000	$ 748,000
Gross realized losses	73,000		616,000	73,000	648,000
Securities gains, net	30,000		$ 64,000	30,000	$ 100,000
Atlantic Capital
Investment securities available for sale
Sold securities		$ 414,400,000
U.S. Treasuries
Investment securities available for sale
Amortized cost	271,783,000			271,783,000
Gross Unrealized Losses	(7,358,000)			(7,358,000)
Fair Value	264,425,000			264,425,000
U.S. Government agencies
investment securities held-to-maturity
Amortized cost	197,261,000			197,261,000		112,913,000
Gross Unrealized Losses	(30,091,000)			(30,091,000)		(2,627,000)
Fair Value	167,170,000			167,170,000		110,286,000
Investment securities available for sale
Amortized cost	245,943,000			245,943,000		98,882,000
Gross Unrealized Losses	(27,320,000)			(27,320,000)		(1,765,000)
Fair Value	218,623,000			218,623,000		97,117,000
Residential mortgage-backed securities issued by U.S. government agencies or sponsored enterprises
investment securities held-to-maturity
Amortized cost	1,632,761,000			1,632,761,000		1,120,104,000
Gross Unrealized Gains						14,000
Gross Unrealized Losses	(267,505,000)			(267,505,000)		(24,278,000)
Fair Value	1,365,256,000			1,365,256,000		1,095,840,000
Investment securities available for sale
Amortized cost	2,048,537,000			2,048,537,000		1,851,700,000
Gross Unrealized Gains						5,324,000
Gross Unrealized Losses	(318,470,000)			(318,470,000)		(25,985,000)
Fair Value	1,730,067,000			1,730,067,000		1,831,039,000
Residential collateralized mortgage-obligations issued by U.S. government agencies or sponsored enterprises
investment securities held-to-maturity
Amortized cost	483,436,000			483,436,000		174,178,000
Gross Unrealized Gains	691,000			691,000
Gross Unrealized Losses	(64,602,000)			(64,602,000)		(4,937,000)
Fair Value	419,525,000			419,525,000		169,241,000
Investment securities available for sale
Amortized cost	729,712,000			729,712,000		730,949,000
Gross Unrealized Gains	6,000			6,000		5,957,000
Gross Unrealized Losses	(101,906,000)			(101,906,000)		(10,911,000)
Fair Value	627,812,000			627,812,000		725,995,000
Commercial mortgage-backed securities issued by U.S. government agencies or sponsored enterprises
investment securities held-to-maturity
Amortized cost	366,227,000			366,227,000		350,116,000
Gross Unrealized Gains						304,000
Gross Unrealized Losses	(66,366,000)			(66,366,000)		(8,021,000)
Fair Value	299,861,000			299,861,000		342,399,000
Investment securities available for sale
Amortized cost	1,224,869,000			1,224,869,000		1,220,233,000
Gross Unrealized Gains	2,551,000			2,551,000		5,438,000
Gross Unrealized Losses	(199,644,000)			(199,644,000)		(18,430,000)
Fair Value	1,027,776,000			1,027,776,000		1,207,241,000
State and municipal obligations
Investment securities available for sale
Amortized cost	1,256,045,000			1,256,045,000		798,211,000
Gross Unrealized Gains	23,000			23,000		16,697,000
Gross Unrealized Losses	(252,750,000)			(252,750,000)		(2,219,000)
Fair Value	1,003,318,000			1,003,318,000		812,689,000
Small Business Administration loan-backed securities
investment securities held-to-maturity
Amortized cost	58,493,000			58,493,000		62,590,000
Gross Unrealized Losses	(11,143,000)			(11,143,000)		(2,292,000)
Fair Value	47,350,000			47,350,000		60,298,000
Investment securities available for sale
Amortized cost	507,412,000			507,412,000		502,812,000
Gross Unrealized Gains	546,000			546,000		2,330,000
Gross Unrealized Losses	(43,722,000)			(43,722,000)		(4,479,000)
Fair Value	464,236,000			464,236,000		500,663,000
Corporate securities.
Investment securities available for sale
Amortized cost	35,595,000			35,595,000		18,509,000
Gross Unrealized Gains	10,000			10,000		234,000
Gross Unrealized Losses	(2,252,000)			(2,252,000)		(9,000)
Fair Value	$ 33,353,000			$ 33,353,000		$ 18,734,000