Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Gain (Loss) on Investments [Line Items]
Amortized Cost - Available for sale securities	$ 587,021	$ 498,104
Gross Unrealized Gains - Available for sale securities	10,748	21,184
Gross Unrealized Losses - Available for sale securities	13,176	5,945
Fair Value - Available for sale securities	584,593	513,343
Amortized Cost - Held to maturity securities	324,399	361,020
Gross Unrealized Gains - Held to maturity securities	7,598	17,170
Gross Unrealized Losses - Held to maturity securities	232	16
Fair Value - Held to maturity securities	331,765	378,174
Amortized Cost - Total Investment Securities	911,420	859,124
Gross Unrealized Gains - Total Investment Securities	18,346	38,354
Gross Unrealized Losses - Total Investment Securities	13,408	5,961
Fair Value - Total Investment Securities	916,358	891,517
U.S. Government-sponsored agency securities
Gain (Loss) on Investments [Line Items]
Amortized Cost - Available for sale securities	4,022	4,475
Gross Unrealized Gains - Available for sale securities	40	165
Fair Value - Available for sale securities	4,062	4,640
State and municipal
Gain (Loss) on Investments [Line Items]
Amortized Cost - Available for sale securities	184,912	148,187
Gross Unrealized Gains - Available for sale securities	5,351	10,025
Gross Unrealized Losses - Available for sale securities	3,169	18
Fair Value - Available for sale securities	187,094	158,194
Amortized Cost - Held to maturity securities	115,943	117,227
Gross Unrealized Gains - Held to maturity securities	552	5,489
Gross Unrealized Losses - Held to maturity securities	11	1
Fair Value - Held to maturity securities	116,484	122,715
U.S. Government-sponsored mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Amortized Cost - Available for sale securities	390,114	337,631
Gross Unrealized Gains - Available for sale securities	5,357	10,994
Gross Unrealized Losses - Available for sale securities	4,488	46
Fair Value - Available for sale securities	390,983	348,579
Amortized Cost - Held to maturity securities	208,456	243,793
Gross Unrealized Gains - Held to maturity securities	7,046	11,681
Gross Unrealized Losses - Held to maturity securities	221	15
Fair Value - Held to maturity securities	215,281	255,459
Corporate obligations
Gain (Loss) on Investments [Line Items]
Amortized Cost - Available for sale securities	6,267	6,105
Gross Unrealized Losses - Available for sale securities	5,519	5,881
Fair Value - Available for sale securities	748	224
Equity securities
Gain (Loss) on Investments [Line Items]
Amortized Cost - Available for sale securities	1,706	1,706
Fair Value - Available for sale securities	$ 1,706	$ 1,706

Securities, Amortized Cost

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

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As of the balance sheet date, the excess of fair value over carrying value (amortized cost) of a debt security categorized as held-to-maturity, exclusive of dividend or interest income recognized but not yet received and exclusive of any write-downs for other-than-temporary impairment. Such item represents the gross unrecognized holding gain.

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