Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Available for sale securities
Amortized Cost	$ 598,706	$ 537,848
Gross Unrealized Gains	17,945	7,896
Gross Unrealized Losses	1,467	9,543
Investment securities available for sale	615,184	536,201
Held to maturity securities
Amortized Cost	598,903	559,378
Gross Unrealized Gains	14,118	5,282
Gross Unrealized Losses	859	3,813
Investment securities held to maturity - fair value	612,162	560,847
Amortized Cost	1,197,609	1,097,226
Gross Unrealized Gains	32,063	13,178
Gross Unrealized Losses	2,326	13,356
Fair Value	1,227,346	1,097,048
U.S. Treasury
Available for sale securities
Amortized Cost	15,921	15,914
Gross Unrealized Gains	92	80
Gross Unrealized Losses		21
Investment securities available for sale	16,013	15,973
U.S. Government-sponsored agency securities
Available for sale securities
Amortized Cost	3,219	3,550
Gross Unrealized Gains	58	12
Gross Unrealized Losses		17
Investment securities available for sale	3,277	3,545
State and municipal
Available for sale securities
Amortized Cost	237,709	231,005
Gross Unrealized Gains	9,454	3,878
Gross Unrealized Losses	456	3,896
Investment securities available for sale	246,707	230,987
Held to maturity securities
Amortized Cost	173,080	145,941
Gross Unrealized Gains	3,900	62
Gross Unrealized Losses	157	91
Investment securities held to maturity - fair value	176,823	145,912
U.S. Government-sponsored mortgage-backed securities
Available for sale securities
Amortized Cost	338,588	279,299
Gross Unrealized Gains	8,341	3,926
Gross Unrealized Losses	242	1,973
Investment securities available for sale	346,687	281,252
Held to maturity securities
Amortized Cost	425,823	413,437
Gross Unrealized Gains	10,218	5,220
Gross Unrealized Losses	702	3,722
Investment securities held to maturity - fair value	435,339	414,935
Corporate obligations
Available for sale securities
Amortized Cost	1,563	6,374
Gross Unrealized Gains
Gross Unrealized Losses	769	3,636
Investment securities available for sale	794	2,738
Held to maturity securities
Amortized Cost	1,500
Equity securities
Available for sale securities
Amortized Cost	1,706	1,706
Investment securities available for sale	$ 1,706	$ 1,706

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Securities, Fair Value

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Securities Gross Unrealized Gains

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Securities Gross Unrealized Losses

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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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