Securities - Amortized cost and fair value for available for sale securities (Details) - USD ($)	3 Months Ended
Mar. 31, 2024	Dec. 31, 2023
Investment securities held-to-maturity
Amortized cost	$ 2,446,589,000	$ 2,487,440,000
Gross Unrealized Losses	(430,022,000)	(402,704,000)
Fair Value	2,016,567,000	2,084,736,000
Investment securities available for sale
Amortized cost	5,428,594,000	5,561,006,000
Gross Unrealized Gains	986,000	1,898,000
Gross Unrealized Losses	(831,180,000)	(778,516,000)
Fair Value	4,598,400,000	4,784,388,000
Sold securities	0
Realized gains or losses	0
U.S. Treasuries
Investment securities available for sale
Amortized cost	24,920,000	74,720,000
Gross Unrealized Losses	(333,000)	(830,000)
Fair Value	24,587,000	73,890,000
U.S. Government agencies
Investment securities held-to-maturity
Amortized cost	197,269,000	197,267,000
Gross Unrealized Losses	(25,821,000)	(24,607,000)
Fair Value	171,448,000	172,660,000
Investment securities available for sale
Amortized cost	246,118,000	246,089,000
Gross Unrealized Losses	(22,313,000)	(21,383,000)
Fair Value	223,805,000	224,706,000
Residential mortgage pass-through securities issued or guaranteed by U.S. government agencies or sponsored enterprises
Investment securities held-to-maturity
Amortized cost	1,406,440,000	1,438,102,000
Gross Unrealized Losses	(246,764,000)	(227,312,000)
Fair Value	1,159,676,000	1,210,790,000
Investment securities available for sale
Amortized cost	1,780,838,000	1,822,104,000
Gross Unrealized Gains	161,000	294,000
Gross Unrealized Losses	(288,576,000)	(264,092,000)
Fair Value	1,492,423,000	1,558,306,000
Residential collateralized mortgage-obligations issued by U.S. government agencies or sponsored enterprises
Investment securities held-to-maturity
Amortized cost	437,718,000	444,883,000
Gross Unrealized Losses	(72,593,000)	(68,139,000)
Fair Value	365,125,000	376,744,000
Investment securities available for sale
Amortized cost	611,044,000	626,735,000
Gross Unrealized Gains	4,000
Gross Unrealized Losses	(102,936,000)	(99,313,000)
Fair Value	508,112,000	527,422,000
Commercial mortgage-backed securities issued by U.S. government agencies or sponsored enterprises
Investment securities held-to-maturity
Amortized cost	352,399,000	354,055,000
Gross Unrealized Losses	(73,294,000)	(71,327,000)
Fair Value	279,105,000	282,728,000
Investment securities available for sale
Amortized cost	1,213,549,000	1,217,125,000
Gross Unrealized Gains	767,000	1,516,000
Gross Unrealized Losses	(202,099,000)	(194,471,000)
Fair Value	1,012,217,000	1,024,170,000
State and municipal obligations
Investment securities available for sale
Amortized cost	1,127,834,000	1,129,750,000
Gross Unrealized Gains	2,000	2,000
Gross Unrealized Losses	(167,477,000)	(152,291,000)
Fair Value	960,359,000	977,461,000
Small Business Administration loan-backed securities
Investment securities held-to-maturity
Amortized cost	52,763,000	53,133,000
Gross Unrealized Losses	(11,550,000)	(11,319,000)
Fair Value	41,213,000	41,814,000
Investment securities available for sale
Amortized cost	393,770,000	413,950,000
Gross Unrealized Gains	52,000	86,000
Gross Unrealized Losses	(43,779,000)	(42,350,000)
Fair Value	350,043,000	371,686,000
Corporate securities.
Investment securities available for sale
Amortized cost	30,521,000	30,533,000
Gross Unrealized Losses	(3,667,000)	(3,786,000)
Fair Value	$ 26,854,000	$ 26,747,000

The value of debt securities available for sale, sold during the period.

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost excluding accrued interest, before allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of realized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount excluding accrued interest, before allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity).

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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