Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Available for sale securities
Amortized Cost	$ 579,152	$ 529,212
Gross Unrealized Gains	19,257	20,581
Gross Unrealized Losses	570	250
Fair Value	597,839	549,543
Held to maturity securities
Amortized Cost	610,954	631,088
Gross Unrealized Gains	17,273	17,243
Gross Unrealized Losses	379	608
Fair Value	627,848	647,723
Amortized Cost	1,190,106	1,160,300
Gross Unrealized Gains	36,530	37,824
Gross Unrealized Losses	949	858
Fair Value	1,225,687	1,197,266
U.S. Government-sponsored agency securities
Available for sale securities
Amortized Cost	100	100
Gross Unrealized Gains	$ 5	$ 9
Gross Unrealized Losses
Fair Value	$ 105	$ 109
State and municipal
Available for sale securities
Amortized Cost	289,998	216,915
Gross Unrealized Gains	11,617	11,801
Gross Unrealized Losses	497	123
Fair Value	301,118	228,593
Held to maturity securities
Amortized Cost	218,794	204,443
Gross Unrealized Gains	5,707	5,716
Gross Unrealized Losses	166	96
Fair Value	224,335	210,063
U.S. Government-sponsored mortgage-backed securities
Available for sale securities
Amortized Cost	287,317	310,460
Gross Unrealized Gains	7,635	8,771
Gross Unrealized Losses	73	127
Fair Value	294,879	319,104
Held to maturity securities
Amortized Cost	392,160	426,645
Gross Unrealized Gains	11,566	11,527
Gross Unrealized Losses	213	512
Fair Value	403,513	437,660
Corporate obligations
Available for sale securities
Amortized Cost	$ 31	$ 31
Gross Unrealized Gains
Gross Unrealized Losses
Fair Value	$ 31	$ 31
Equity securities
Available for sale securities
Amortized Cost	$ 1,706	1,706
Gross Unrealized Gains
Gross Unrealized Losses
Fair Value	$ 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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Amount of accumulated unrealized loss 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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