Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Available for sale securities
Amortized Cost	$ 562,589	$ 529,212
Gross Unrealized Gains	14,737	20,581
Gross Unrealized Losses	1,911	250
Investment securities available for sale	575,415	549,543
Held to maturity securities
Amortized Cost	637,101	631,088
Gross Unrealized Gains	12,737	17,243
Gross Unrealized Losses	2,028	608
Investment securities held to maturity - fair value	647,810	647,723
Amortized Cost	1,199,690	1,160,300
Gross Unrealized Gains	27,474	37,824
Gross Unrealized Losses	3,939	858
Fair Value	1,223,225	1,197,266
U.S. Government-sponsored agency securities
Available for sale securities
Amortized Cost	100	100
Gross Unrealized Gains	$ 6	$ 9
Gross Unrealized Losses
Investment securities available for sale	$ 106	$ 109
State and municipal
Available for sale securities
Amortized Cost	281,965	216,915
Gross Unrealized Gains	8,370	11,801
Gross Unrealized Losses	1,707	123
Investment securities available for sale	288,628	228,593
Held to maturity securities
Amortized Cost	223,615	204,443
Gross Unrealized Gains	4,047	5,716
Gross Unrealized Losses	1,262	96
Investment securities held to maturity - fair value	226,400	210,063
U.S. Government-sponsored mortgage-backed securities
Available for sale securities
Amortized Cost	278,787	310,460
Gross Unrealized Gains	6,361	8,771
Gross Unrealized Losses	204	127
Investment securities available for sale	284,944	319,104
Held to maturity securities
Amortized Cost	413,486	426,645
Gross Unrealized Gains	8,690	11,527
Gross Unrealized Losses	766	512
Investment securities held to maturity - fair value	421,410	437,660
Corporate obligations
Available for sale securities
Amortized Cost	$ 31	$ 31
Gross Unrealized Gains
Gross Unrealized Losses
Investment securities available for sale	$ 31	$ 31
Equity securities
Available for sale securities
Amortized Cost	$ 1,706	1,706
Gross Unrealized Gains
Gross Unrealized Losses
Investment securities available for sale	$ 1,706	$ 1,706

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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

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

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This item represents the cost of debt and equity 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 after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

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Fair value of investments in debt securities classified as held-to-maturity.

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