Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
Sep. 30, 2013	Dec. 31, 2012
Gain (Loss) on Investments [Line Items]
Amortized Cost - Available for sale securities	$ 610,181	$ 498,104
Gross Unrealized Gains - Available for sale securities	11,137	21,184
Gross Unrealized Losses - Available for sale securities	14,977	5,945
Investment securities available for sale	606,341	513,343
Amortized Cost - Held to maturity securities	307,794	361,020
Gross Unrealized Gains - Held to maturity securities	7,098	17,170
Gross Unrealized Losses - Held to maturity securities	509	16
Investment securities held to maturity - fair value	314,383	378,174
Amortized Cost - Total Investment Securities	917,975	859,124
Gross Unrealized Gains - Total Investment Securities	18,235	38,354
Gross Unrealized Losses - Total Investment Securities	15,486	5,961
Fair Value - Total Investment Securities	920,724	891,517
U.S. Government-sponsored agency securities
Gain (Loss) on Investments [Line Items]
Amortized Cost - Available for sale securities	3,791	4,475
Gross Unrealized Gains - Available for sale securities	18	165
Gross Unrealized Losses - Available for sale securities	9
Investment securities available for sale	3,800	4,640
State and municipal
Gain (Loss) on Investments [Line Items]
Amortized Cost - Available for sale securities	200,662	148,187
Gross Unrealized Gains - Available for sale securities	4,983	10,025
Gross Unrealized Losses - Available for sale securities	3,463	18
Investment securities available for sale	202,182	158,194
Amortized Cost - Held to maturity securities	115,796	117,227
Gross Unrealized Gains - Held to maturity securities	498	5,489
Gross Unrealized Losses - Held to maturity securities	6	1
Investment securities held to maturity - fair value	116,288	122,715
U.S. Government-sponsored mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Amortized Cost - Available for sale securities	397,687	337,631
Gross Unrealized Gains - Available for sale securities	6,136	10,994
Gross Unrealized Losses - Available for sale securities	5,654	46
Investment securities available for sale	398,169	348,579
Amortized Cost - Held to maturity securities	191,998	243,793
Gross Unrealized Gains - Held to maturity securities	6,600	11,681
Gross Unrealized Losses - Held to maturity securities	503	15
Investment securities held to maturity - fair value	198,095	255,459
Corporate obligations
Gain (Loss) on Investments [Line Items]
Amortized Cost - Available for sale securities	6,335	6,105
Gross Unrealized Losses - Available for sale securities	5,851	5,881
Investment securities available for sale	484	224
Equity securities
Gain (Loss) on Investments [Line Items]
Amortized Cost - Available for sale securities	1,706	1,706
Investment securities available for sale	$ 1,706	$ 1,706

Securities, Amortized Cost

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Securities, Fair Value

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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 of unrealized gain before deducting unrealized loss on investments in debt and equity securities classified as available-for-sale securities.

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Amount of unrealized loss before deducting unrealized gain on investments in available-for-sale securities.

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