Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Available for sale securities
Total Investment Securities	$ 1,794,322	$ 1,740,420
Gross Unrealized Gains	96,436	51,816
Gross Unrealized Losses	165	2,211
Fair Value	1,890,593	1,790,025
Held to maturity securities
Total Investment Securities	898,786	806,038
Gross Unrealized Gains	42,714	22,073
Gross Unrealized Losses	74	545
Fair Value	941,426	827,566
Amortized Cost	2,693,108	2,546,458
Gross Unrealized Gains	139,150	73,889
Gross Unrealized Losses	239	2,756
Fair Value	2,832,019	2,617,591
U.S. Treasury
Available for sale securities
Total Investment Securities	598
Gross Unrealized Gains	2
Gross Unrealized Losses	0
Fair Value	600
U.S. Government-sponsored agency securities
Available for sale securities
Total Investment Securities	2,385	38,529
Gross Unrealized Gains	68	346
Gross Unrealized Losses	0	0
Fair Value	2,453	38,875
Held to maturity securities
Total Investment Securities	10,100	15,619
Gross Unrealized Gains	3	1
Gross Unrealized Losses	0	37
Fair Value	10,103	15,583
State and municipal
Available for sale securities
Total Investment Securities	1,026,917	859,511
Gross Unrealized Gains	67,439	41,092
Gross Unrealized Losses	138	807
Fair Value	1,094,218	899,796
Held to maturity securities
Total Investment Securities	374,214	354,115
Gross Unrealized Gains	23,779	15,151
Gross Unrealized Losses	43	107
Fair Value	397,950	369,159
U.S. Government-sponsored mortgage-backed securities
Available for sale securities
Total Investment Securities	761,391	842,349
Gross Unrealized Gains	28,870	10,378
Gross Unrealized Losses	27	1,404
Fair Value	790,234	851,323
Held to maturity securities
Total Investment Securities	512,972	434,804
Gross Unrealized Gains	18,932	6,921
Gross Unrealized Losses	31	401
Fair Value	531,873	441,324
Corporate obligations
Available for sale securities
Total Investment Securities	3,031	31
Gross Unrealized Gains	57	0
Gross Unrealized Losses	0	0
Fair Value	3,088	31
Foreign investment
Held to maturity securities
Total Investment Securities	1,500	1,500
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 1,500	$ 1,500

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