Investment Securities - Amortized cost and fair value for available for sale securities (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
investment securities held-to-maturity
Amortized cost	$ 1,641,485,000		$ 1,641,485,000		$ 955,542,000
Gross Unrealized Gains	83,000		83,000		2,764,000
Gross Unrealized Losses	(34,384,000)		(34,384,000)		(1,123,000)
Fair Value	1,607,184,000		1,607,184,000		957,183,000
Investment securities available for sale
Amortized cost	4,632,039,000		4,632,039,000		3,268,037,000
Gross Unrealized Gains	42,766,000		42,766,000		66,800,000
Gross Unrealized Losses	(43,251,000)		(43,251,000)		(4,165,000)
Fair Value	4,631,554,000		4,631,554,000		3,330,672,000
Gross realized gains	680,000	$ 469,000	748,000	$ 469,000
Gross realized losses	615,000	454,000	648,000	454,000
Net realized gain (loss)	65,000	$ 15,000	100,000	$ 15,000
U.S. Government agencies
investment securities held-to-maturity
Amortized cost	87,912,000		87,912,000		25,000,000
Gross Unrealized Gains					1,000
Gross Unrealized Losses	(2,162,000)		(2,162,000)
Fair Value	85,750,000		85,750,000		25,001,000
Investment securities available for sale
Amortized cost	74,336,000		74,336,000		29,882,000
Gross Unrealized Gains					16,000
Gross Unrealized Losses	(1,493,000)		(1,493,000)		(642,000)
Fair Value	72,843,000		72,843,000		29,256,000
Residential mortgage-backed securities issued by U.S. government agencies or sponsored enterprises
investment securities held-to-maturity
Amortized cost	1,028,847,000		1,028,847,000		632,269,000
Gross Unrealized Gains					1,827,000
Gross Unrealized Losses	(19,505,000)		(19,505,000)		(1,032,000)
Fair Value	1,009,342,000		1,009,342,000		633,064,000
Investment securities available for sale
Amortized cost	1,639,495,000		1,639,495,000		1,351,506,000
Gross Unrealized Gains	8,307,000		8,307,000		16,657,000
Gross Unrealized Losses	(17,354,000)		(17,354,000)		(1,031,000)
Fair Value	1,630,448,000		1,630,448,000		1,367,132,000
Agencies or sponsored enterprises
investment securities held-to-maturity
Amortized cost	144,540,000		144,540,000		75,767,000
Gross Unrealized Gains					405,000
Gross Unrealized Losses	(4,029,000)		(4,029,000)
Fair Value	140,511,000		140,511,000		76,172,000
Investment securities available for sale
Amortized cost	703,245,000		703,245,000		739,797,000
Gross Unrealized Gains	9,924,000		9,924,000		16,579,000
Gross Unrealized Losses	(5,041,000)		(5,041,000)		(825,000)
Fair Value	708,128,000		708,128,000		755,551,000
Commercial mortgage-backed securities issued by U.S. government agencies or sponsored enterprises
investment securities held-to-maturity
Amortized cost	316,330,000		316,330,000		174,506,000
Gross Unrealized Gains	83,000		83,000		300,000
Gross Unrealized Losses	(6,586,000)		(6,586,000)		(91,000)
Fair Value	309,827,000		309,827,000		174,715,000
Investment securities available for sale
Amortized cost	1,003,259,000		1,003,259,000		229,219,000
Gross Unrealized Gains	7,294,000		7,294,000		10,939,000
Gross Unrealized Losses	(11,265,000)		(11,265,000)		(50,000)
Fair Value	999,288,000		999,288,000		240,108,000
State and municipal obligations
Investment securities available for sale
Amortized cost	720,417,000		720,417,000		502,575,000
Gross Unrealized Gains	13,218,000		13,218,000		17,491,000
Gross Unrealized Losses	(4,033,000)		(4,033,000)		(27,000)
Fair Value	729,602,000		729,602,000		520,039,000
Small Business Administration loan-backed securities
investment securities held-to-maturity
Amortized cost	63,856,000		63,856,000		48,000,000
Gross Unrealized Gains					231,000
Gross Unrealized Losses	(2,102,000)		(2,102,000)
Fair Value	61,754,000		61,754,000		48,231,000
Investment securities available for sale
Amortized cost	477,765,000		477,765,000		401,496,000
Gross Unrealized Gains	3,700,000		3,700,000		4,978,000
Gross Unrealized Losses	(4,065,000)		(4,065,000)		(1,590,000)
Fair Value	477,400,000		477,400,000		404,884,000
Corporate securities.
Investment securities available for sale
Amortized cost	13,522,000		13,522,000		13,562,000
Gross Unrealized Gains	323,000		323,000		140,000
Fair Value	$ 13,845,000		$ 13,845,000		$ 13,702,000