Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Available for sale securities
Amortized Cost	$ 2,334,464	$ 1,807,389
Amortized Cost	2,334,464	1,807,389
Gross Unrealized Gains	100,325	112,079
Gross Unrealized Losses	7,889	349
Fair Value	2,426,900	1,919,119
Held to maturity securities
Amortized cost of investment securities held to maturity	1,721,659	1,227,668
Allowance for Credit Losses	245	0
Held to maturity securities	1,721,414	1,227,668
Gross Unrealized Gains	43,441	52,877
Gross Unrealized Losses	4,253	252
Fair Value	1,760,847	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	11,502	2,380
Gross Unrealized Gains	39	50
Gross Unrealized Losses	0	0
Fair Value	11,541	2,430
Held to maturity securities
Amortized cost of investment securities held to maturity	60,677	31,087
Allowance for Credit Losses	0	0
Held to maturity securities	60,677	31,087
Gross Unrealized Gains	1	10
Gross Unrealized Losses	715	113
Fair Value	59,963	30,984
State and municipal
Available for sale securities
Amortized Cost	1,365,211	1,168,711
Gross Unrealized Gains	87,152	89,420
Gross Unrealized Losses	518	246
Fair Value	1,451,845	1,257,885
Held to maturity securities
Amortized cost of investment securities held to maturity	863,426	619,927
Allowance for Credit Losses	245	0
Held to maturity securities	863,181	619,927
Gross Unrealized Gains	31,276	34,978
Gross Unrealized Losses	977	32
Fair Value	893,725	654,873
U.S. Government-sponsored mortgage-backed securities
Available for sale securities
Amortized Cost	952,720	632,267
Gross Unrealized Gains	12,937	22,505
Gross Unrealized Losses	7,371	103
Fair Value	958,286	654,669
Held to maturity securities
Amortized cost of investment securities held to maturity	796,056	575,154
Allowance for Credit Losses	0	0
Held to maturity securities	796,056	575,154
Gross Unrealized Gains	12,164	17,889
Gross Unrealized Losses	2,561	107
Fair Value	805,659	592,936
Corporate obligations
Available for sale securities
Amortized Cost	4,031	4,031
Gross Unrealized Gains	197	104
Gross Unrealized Losses	0	0
Fair Value	4,228	4,135
Foreign investment
Held to maturity securities
Amortized cost of investment securities held to maturity	1,500	1,500
Allowance for Credit Losses	0	0
Held to maturity securities	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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Amortized cost 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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