Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Available for sale securities
Total Investment Securities	$ 1,177,274	$ 986,565
Gross Unrealized Gains	2,991	17,896
Gross Unrealized Losses	31,103	4,514
Investment securities available for sale	1,149,162	999,947
Held to maturity securities
Total Investment Securities	476,089	560,655
Gross Unrealized Gains	1,461	9,636
Gross Unrealized Losses	9,098	2,083
Fair Value	468,452	568,208
Amortized Cost	1,653,363	1,547,220
Gross Unrealized Gains	4,452	27,532
Gross Unrealized Losses	40,201	6,597
Fair Value	1,617,614	1,568,155
U.S. Government-sponsored agency securities
Available for sale securities
Total Investment Securities	18,492
Gross Unrealized Gains
Gross Unrealized Losses	134
Investment securities available for sale	18,358
Held to maturity securities
Total Investment Securities	22,618	22,618
Gross Unrealized Gains
Gross Unrealized Losses	833	435
Fair Value	21,785	22,183
State and municipal
Available for sale securities
Total Investment Securities	611,528	510,852
Gross Unrealized Gains	2,920	16,932
Gross Unrealized Losses	14,280	1,091
Investment securities available for sale	600,168	526,693
Held to maturity securities
Total Investment Securities	192,124	235,594
Gross Unrealized Gains	1,161	6,295
Gross Unrealized Losses	1,748	244
Fair Value	191,537	241,645
U.S. Government-sponsored mortgage-backed securities
Available for sale securities
Total Investment Securities	547,223	473,325
Gross Unrealized Gains	71	964
Gross Unrealized Losses	16,689	3,423
Investment securities available for sale	530,605	470,866
Held to maturity securities
Total Investment Securities	260,347	301,443
Gross Unrealized Gains	300	3,341
Gross Unrealized Losses	6,514	1,404
Fair Value	254,133	303,380
Corporate obligations
Available for sale securities
Total Investment Securities	31	31
Gross Unrealized Gains
Gross Unrealized Losses
Investment securities available for sale	31	31
Equity securities
Available for sale securities
Total Investment Securities		2,357
Gross Unrealized Gains
Gross Unrealized Losses
Investment securities available for sale		2,357
Foreign Investments
Held to maturity securities
Total Investment Securities	1,000	1,000
Gross Unrealized Losses	3
Fair Value	$ 997	$ 1,000

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