Investment Securities - Amortized Cost and Approximate Fair Values of Securities (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Total available for sale
Total Investment Securities	$ 1,149,727,000	$ 986,565,000
Gross Unrealized Gains	6,721,000	17,896,000
Gross Unrealized Losses	14,253,000	4,514,000
Investment securities available for sale	1,142,195,000	999,947,000
Total held to maturity
Amortized Cost	490,387,000	560,655,000
Gross Unrealized Gains	3,571,000	9,636,000
Gross Unrealized Losses	4,741,000	2,083,000
Fair Value	489,217,000	568,208,000
Total Investment Securities
Amortized Cost	1,640,114,000	1,547,220,000
Gross Unrealized Gains	10,292,000	27,532,000
Gross Unrealized Losses	18,994,000	6,597,000
Fair Value	1,631,412,000	1,568,155,000
U.S. Government-sponsored agency securities
Total available for sale
Total Investment Securities	13,493,000
Gross Unrealized Gains	92,000
Gross Unrealized Losses	3,000
Investment securities available for sale	13,582,000
Total held to maturity
Amortized Cost	22,618,000	22,618,000
Gross Unrealized Gains
Gross Unrealized Losses	545,000	435,000
Fair Value	22,073,000	22,183,000
State and municipal
Total available for sale
Total Investment Securities	605,994,000	510,852,000
Gross Unrealized Gains	5,995,000	16,932,000
Gross Unrealized Losses	5,854,000	1,091,000
Investment securities available for sale	606,135,000	526,693,000
Total held to maturity
Amortized Cost	197,909,000	235,594,000
Gross Unrealized Gains	2,858,000	6,295,000
Gross Unrealized Losses	872,000	244,000
Fair Value	199,895,000	241,645,000
U.S. Government-sponsored mortgage-backed securities
Total available for sale
Total Investment Securities	530,209,000	473,325,000
Gross Unrealized Gains	634,000	964,000
Gross Unrealized Losses	8,396,000	3,423,000
Investment securities available for sale	522,447,000	470,866,000
Total held to maturity
Amortized Cost	268,860,000	301,443,000
Gross Unrealized Gains	713,000	3,341,000
Gross Unrealized Losses	3,323,000	1,404,000
Fair Value	266,250,000	303,380,000
Corporate obligations
Total available for sale
Total Investment Securities	31,000	31,000
Gross Unrealized Gains
Gross Unrealized Losses
Investment securities available for sale	31,000	31,000
Foreign Investment
Total available for sale
Gross Unrealized Losses	0
Total held to maturity
Amortized Cost	1,000,000	1,000,000
Gross Unrealized Gains
Gross Unrealized Losses	1,000
Fair Value	$ 999,000	1,000,000
Equity securities
Total available for sale
Total Investment Securities		2,357,000
Gross Unrealized Gains
Gross Unrealized Losses
Investment securities available for sale		$ 2,357,000

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