Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Available for sale securities
Amortized Cost	$ 2,307,977	$ 1,807,389
Gross Unrealized Gains	82,762	112,079
Gross Unrealized Losses	16,161	349
Fair Value	2,374,578	1,919,119
Held to maturity securities
Amortized Cost	2,071,183	1,227,668
Allowance for Credit Losses	245	0
Net Carrying Amount	2,070,938	1,227,668
Gross Unrealized Gains	34,769	52,877
Gross Unrealized Losses	19,397	252
Fair Value	2,086,555	1,280,293
U.S. Treasury
Available for sale securities
Amortized Cost	1,000
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	1,000
U.S. Government-sponsored agency securities
Available for sale securities
Amortized Cost	11,500	2,380
Gross Unrealized Gains	57	50
Gross Unrealized Losses	0	0
Fair Value	11,557	2,430
Held to maturity securities
Amortized Cost	90,678	31,087
Allowance for Credit Losses	0	0
Net Carrying Amount	90,678	31,087
Gross Unrealized Gains	86	10
Gross Unrealized Losses	840	113
Fair Value	89,924	30,984
State and municipal
Available for sale securities
Amortized Cost	1,466,349	1,168,711
Gross Unrealized Gains	72,152	89,420
Gross Unrealized Losses	3,946	246
Fair Value	1,534,555	1,257,885
Held to maturity securities
Amortized Cost	996,247	619,927
Allowance for Credit Losses	245	0
Net Carrying Amount	996,002	619,927
Gross Unrealized Gains	24,330	34,978
Gross Unrealized Losses	9,618	32
Fair Value	1,010,959	654,873
U.S. Government-sponsored mortgage-backed securities
Available for sale securities
Amortized Cost	825,097	632,267
Gross Unrealized Gains	10,255	22,505
Gross Unrealized Losses	12,215	103
Fair Value	823,137	654,669
Held to maturity securities
Amortized Cost	982,758	575,154
Allowance for Credit Losses	0	0
Net Carrying Amount	982,758	575,154
Gross Unrealized Gains	10,353	17,889
Gross Unrealized Losses	8,939	107
Fair Value	984,172	592,936
Corporate obligations
Available for sale securities
Amortized Cost	4,031	4,031
Gross Unrealized Gains	298	104
Gross Unrealized Losses	0	0
Fair Value	4,329	4,135
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	0
Fair Value	$ 1,500	$ 1,500

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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