Investment Securities - Amortized Cost and Approximate Fair Values of Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Total available for sale
Amortized Cost	$ 1,740,420	$ 1,149,727
Gross Unrealized Gains	51,816	6,721
Gross Unrealized Losses	2,211	14,253
Fair Value	1,790,025	1,142,195
Total held to maturity
Amortized Cost	806,038	490,387
Gross Unrealized Gains	22,073	3,571
Gross Unrealized Losses	545	4,741
Fair Value	827,566	489,217
Total Investment Securities
Amortized Cost	2,546,458	1,640,114
Gross Unrealized Gains	73,889	10,292
Gross Unrealized Losses	2,756	18,994
Fair Value	2,617,591	1,631,412
U.S. Government-sponsored agency securities
Total available for sale
Amortized Cost	38,529	13,493
Gross Unrealized Gains	346	92
Gross Unrealized Losses	0	3
Fair Value	38,875	13,582
Total held to maturity
Amortized Cost	15,619	22,618
Gross Unrealized Gains	1	0
Gross Unrealized Losses	37	545
Fair Value	15,583	22,073
State and municipal
Total available for sale
Amortized Cost	859,511	605,994
Gross Unrealized Gains	41,092	5,995
Gross Unrealized Losses	807	5,854
Fair Value	899,796	606,135
Total held to maturity
Amortized Cost	354,115	197,909
Gross Unrealized Gains	15,151	2,858
Gross Unrealized Losses	107	872
Fair Value	369,159	199,895
U.S. Government-sponsored mortgage-backed securities
Total available for sale
Amortized Cost	842,349	530,209
Gross Unrealized Gains	10,378	634
Gross Unrealized Losses	1,404	8,396
Fair Value	851,323	522,447
Total held to maturity
Amortized Cost	434,804	268,860
Gross Unrealized Gains	6,921	713
Gross Unrealized Losses	401	3,323
Fair Value	441,324	266,250
Corporate obligations
Total available for sale
Amortized Cost	31	31
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	31	31
Foreign investment
Total held to maturity
Amortized Cost	1,500	1,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	1
Fair Value	$ 1,500	$ 999

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