Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Available for sale securities
Amortized Cost	$ 2,265,457	$ 2,268,655
Gross Unrealized Gains	18,278	89,536
Gross Unrealized Losses	119,538	13,640
Fair Value	2,164,197	2,344,551
Held to maturity securities
Amortized Cost	2,325,311	2,180,047
Allowance for Credit Losses	245	245
Net Carrying Amount	2,325,066	2,179,802
Gross Unrealized Gains	6,697	37,776
Gross Unrealized Losses	186,014	15,320
Fair Value	2,145,994	2,202,503
U.S. Treasury
Available for sale securities
Amortized Cost	1,964	1,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	5	1
Fair Value	1,959	999
U.S. Government-sponsored agency securities
Available for sale securities
Amortized Cost	85,494	96,244
Gross Unrealized Gains	7	437
Gross Unrealized Losses	6,403	1,545
Fair Value	79,098	95,136
Held to maturity securities
Amortized Cost	400,126	371,457
Allowance for Credit Losses	0	0
Net Carrying Amount	400,126	371,457
Gross Unrealized Gains	0	226
Gross Unrealized Losses	29,998	7,268
Fair Value	370,128	364,415
State and municipal
Available for sale securities
Amortized Cost	1,543,969	1,495,696
Gross Unrealized Gains	17,890	81,734
Gross Unrealized Losses	65,326	898
Fair Value	1,496,533	1,576,532
Held to maturity securities
Amortized Cost	1,082,257	1,057,301
Allowance for Credit Losses	245	245
Net Carrying Amount	1,082,012	1,057,056
Gross Unrealized Gains	5,718	29,593
Gross Unrealized Losses	105,731	2,170
Fair Value	982,244	1,084,724
U.S. Government-sponsored mortgage-backed securities
Available for sale securities
Amortized Cost	629,999	671,684
Gross Unrealized Gains	305	7,109
Gross Unrealized Losses	47,780	11,188
Fair Value	582,524	667,605
Held to maturity securities
Amortized Cost	841,428	749,789
Allowance for Credit Losses	0	0
Net Carrying Amount	841,428	749,789
Gross Unrealized Gains	979	7,957
Gross Unrealized Losses	50,285	5,881
Fair Value	792,122	751,865
Corporate obligations
Available for sale securities
Amortized Cost	4,031	4,031
Gross Unrealized Gains	76	256
Gross Unrealized Losses	24	8
Fair Value	4,083	4,279
Foreign investment
Held to maturity securities
Amortized Cost	1,500	1,500
Allowance for Credit Losses	0	0
Net Carrying Amount	1,500	1,500
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	1
Fair Value	$ 1,500	$ 1,499

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).

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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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