Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Available for sale securities
Total Investment Securities	$ 1,229,729	$ 1,149,727
Gross Unrealized Gains	21,575	6,721
Gross Unrealized Losses	3,672	14,253
Investment securities available for sale	1,247,632	1,142,195
Held to maturity securities
Total Investment Securities	615,027	490,387
Gross Unrealized Gains	9,312	3,571
Gross Unrealized Losses	1,704	4,741
Fair Value	622,635	489,217
Amortized Cost	1,844,756	1,640,114
Gross Unrealized Gains	30,887	10,292
Gross Unrealized Losses	5,376	18,994
Fair Value	1,870,267	1,631,412
U.S. Government-sponsored agency securities
Available for sale securities
Total Investment Securities	33,494	13,493
Gross Unrealized Gains	210	92
Gross Unrealized Losses	1	3
Investment securities available for sale	33,703	13,582
Held to maturity securities
Total Investment Securities	22,619	22,618
Gross Unrealized Gains	0	0
Gross Unrealized Losses	350	545
Fair Value	22,269	22,073
State and municipal
Available for sale securities
Total Investment Securities	633,617	605,994
Gross Unrealized Gains	18,773	5,995
Gross Unrealized Losses	532	5,854
Investment securities available for sale	651,858	606,135
Held to maturity securities
Total Investment Securities	254,520	197,909
Gross Unrealized Gains	6,682	2,858
Gross Unrealized Losses	117	872
Fair Value	261,085	199,895
U.S. Government-sponsored mortgage-backed securities
Available for sale securities
Total Investment Securities	562,587	530,209
Gross Unrealized Gains	2,592	634
Gross Unrealized Losses	3,139	8,396
Investment securities available for sale	562,040	522,447
Held to maturity securities
Total Investment Securities	336,888	268,860
Gross Unrealized Gains	2,630	713
Gross Unrealized Losses	1,237	3,323
Fair Value	338,281	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	1,000	1,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	1
Fair Value	$ 1,000	$ 999

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