Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Available for sale securities
Total Investment Securities	$ 1,360,946	$ 1,149,727
Gross Unrealized Gains	39,820	6,721
Gross Unrealized Losses	807	14,253
Investment securities available for sale	1,399,959	1,142,195
Held to maturity securities
Total Investment Securities	692,965	490,387
Gross Unrealized Gains	16,264	3,571
Gross Unrealized Losses	314	4,741
Fair Value	708,915	489,217
Amortized Cost	2,053,911	1,640,114
Gross Unrealized Gains	56,084	10,292
Gross Unrealized Losses	1,121	18,994
Fair Value	2,108,874	1,631,412
U.S. Government-sponsored agency securities
Available for sale securities
Total Investment Securities	33,494	13,493
Gross Unrealized Gains	453	92
Gross Unrealized Losses	0	3
Investment securities available for sale	33,947	13,582
Held to maturity securities
Total Investment Securities	33,865	22,618
Gross Unrealized Gains	20	0
Gross Unrealized Losses	66	545
Fair Value	33,819	22,073
State and municipal
Available for sale securities
Total Investment Securities	685,123	605,994
Gross Unrealized Gains	30,835	5,995
Gross Unrealized Losses	200	5,854
Investment securities available for sale	715,758	606,135
Held to maturity securities
Total Investment Securities	310,076	197,909
Gross Unrealized Gains	10,987	2,858
Gross Unrealized Losses	22	872
Fair Value	321,041	199,895
U.S. Government-sponsored mortgage-backed securities
Available for sale securities
Total Investment Securities	642,298	530,209
Gross Unrealized Gains	8,532	634
Gross Unrealized Losses	607	8,396
Investment securities available for sale	650,223	522,447
Held to maturity securities
Total Investment Securities	347,524	268,860
Gross Unrealized Gains	5,257	713
Gross Unrealized Losses	226	3,323
Fair Value	352,555	266,250
Corporate obligations
Available for sale securities
Total Investment Securities	31	31
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Investment securities available for sale	31	31
Foreign investment
Held to maturity securities
Total Investment Securities	1,500	1,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	1
Fair Value	$ 1,500	$ 999

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