Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available for sale securities
Total Investment Securities	$ 1,014,230	$ 986,565
Gross Unrealized Gains	6,847	17,896
Gross Unrealized Losses	17,278	4,514
Fair Value	1,003,799	999,947
Held to maturity securities
Total Investment Securities	540,021	560,655
Gross Unrealized Gains	3,772	9,636
Gross Unrealized Losses	5,394	2,083
Fair Value	538,399	568,208
Amortized Cost	1,554,251	1,547,220
Gross Unrealized Gains	10,619	27,532
Gross Unrealized Losses	22,672	6,597
Fair Value	1,542,198	1,568,155
U.S. Government-sponsored agency securities
Available for sale securities
Total Investment Securities	6,491
Gross Unrealized Gains	8
Gross Unrealized Losses	1
Fair Value	6,498
Held to maturity securities
Total Investment Securities	22,619	22,618
Gross Unrealized Gains
Gross Unrealized Losses	715	435
Fair Value	21,904	22,183
State and municipal
Available for sale securities
Total Investment Securities	510,884	510,852
Gross Unrealized Gains	6,588	16,932
Gross Unrealized Losses	7,238	1,091
Fair Value	510,234	526,693
Held to maturity securities
Total Investment Securities	228,620	235,594
Gross Unrealized Gains	2,784	6,295
Gross Unrealized Losses	911	244
Fair Value	230,493	241,645
U.S. Government-sponsored mortgage-backed securities
Available for sale securities
Total Investment Securities	496,824	473,325
Gross Unrealized Gains	251	964
Gross Unrealized Losses	10,039	3,423
Fair Value	487,036	470,866
Held to maturity securities
Total Investment Securities	287,782	301,443
Gross Unrealized Gains	988	3,341
Gross Unrealized Losses	3,762	1,404
Fair Value	285,008	303,380
Corporate obligations
Available for sale securities
Total Investment Securities	31	31
Gross Unrealized Gains
Gross Unrealized Losses
Fair Value	31	31
Equity securities
Available for sale securities
Total Investment Securities		2,357
Gross Unrealized Gains
Gross Unrealized Losses
Fair Value		2,357
Foreign Investments
Held to maturity securities
Total Investment Securities	1,000	1,000
Gross Unrealized Losses	6
Fair Value	$ 994	$ 1,000

Debt Securities, Available-for-sale and Held-to-maturity, Accumulated Gross Unrealized Gain, before Tax

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Debt Securities, Available-for-sale and Held-to-maturity,Amortized Cost

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Debt Securities, Available-for-sale and Held-to-maturity, Amortized Cost, Accumulated Gross Unrealized Loss, before Tax

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in equity securities classified as available-for-sale.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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