Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Available for sale securities
Total Investment Securities	$ 1,726,185	$ 1,740,420
Gross Unrealized Gains	98,162	51,816
Gross Unrealized Losses	187	2,211
Fair Value	1,824,160	1,790,025
Held to maturity securities
Total Investment Securities	1,109,126	806,038
Gross Unrealized Gains	44,670	22,073
Gross Unrealized Losses	1,278	545
Fair Value	1,152,518	827,566
Amortized Cost	2,835,311	2,546,458
Gross Unrealized Gains	142,832	73,889
Gross Unrealized Losses	1,465	2,756
Fair Value	2,976,678	2,617,591
U.S. Treasury
Available for sale securities
Total Investment Securities	300
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	300
U.S. Government-sponsored agency securities
Available for sale securities
Total Investment Securities	2,384	38,529
Gross Unrealized Gains	60	346
Gross Unrealized Losses	0	0
Fair Value	2,444	38,875
Held to maturity securities
Total Investment Securities	21,094	15,619
Gross Unrealized Gains	2	1
Gross Unrealized Losses	33	37
Fair Value	21,063	15,583
State and municipal
Available for sale securities
Total Investment Securities	1,074,381	859,511
Gross Unrealized Gains	72,752	41,092
Gross Unrealized Losses	187	807
Fair Value	1,146,946	899,796
Held to maturity securities
Total Investment Securities	573,538	354,115
Gross Unrealized Gains	25,880	15,151
Gross Unrealized Losses	1,240	107
Fair Value	598,178	369,159
U.S. Government-sponsored mortgage-backed securities
Available for sale securities
Total Investment Securities	645,089	842,349
Gross Unrealized Gains	25,306	10,378
Gross Unrealized Losses	0	1,404
Fair Value	670,395	851,323
Held to maturity securities
Total Investment Securities	512,994	434,804
Gross Unrealized Gains	18,788	6,921
Gross Unrealized Losses	3	401
Fair Value	531,779	441,324
Corporate obligations
Available for sale securities
Total Investment Securities	4,031	31
Gross Unrealized Gains	44	0
Gross Unrealized Losses	0	0
Fair Value	4,075	31
Foreign investment
Held to maturity securities
Total Investment Securities	1,500	1,500
Gross Unrealized Gains	0	0
Gross Unrealized Losses	2	0
Fair Value	$ 1,498	$ 1,500

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