Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Available for sale securities
Amortized Cost	$ 611,972	$ 640,499
Gross Unrealized Gains	24,960	18,479
Gross Unrealized Losses	147	578
Fair Value	636,785	658,400
Held to maturity securities
Amortized Cost	633,992	618,599
Gross Unrealized Gains	21,527	14,583
Gross Unrealized Losses	74	802
Fair Value	655,445	632,380
Amortized Cost	1,245,964	1,259,098
Gross Unrealized Gains	46,487	33,062
Gross Unrealized Losses	221	1,380
Fair Value	1,292,230	1,290,780
U.S. Government-sponsored agency securities
Available for sale securities
Amortized Cost	100	100
Gross Unrealized Gains	$ 3	$ 4
Gross Unrealized Losses
Fair Value	$ 103	$ 104
State and municipal
Available for sale securities
Amortized Cost	312,516	291,730
Gross Unrealized Gains	17,711	14,241
Gross Unrealized Losses	107	60
Fair Value	330,120	305,911
Held to maturity securities
Amortized Cost	218,374	219,767
Gross Unrealized Gains	9,244	6,982
Gross Unrealized Losses	49	15
Fair Value	227,569	226,734
U.S. Government-sponsored mortgage-backed securities
Available for sale securities
Amortized Cost	295,443	342,550
Gross Unrealized Gains	7,183	4,234
Gross Unrealized Losses	40	518
Fair Value	302,586	346,266
Held to maturity securities
Amortized Cost	401,640	398,832
Gross Unrealized Gains	12,261	7,601
Gross Unrealized Losses	25	787
Fair Value	413,876	405,646
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	3,882	$ 3,912
Gross Unrealized Gains	$ 63
Gross Unrealized Losses
Fair Value	$ 3,945	$ 3,912
Federal agencies
Held to maturity securities
Amortized Cost	13,978
Gross Unrealized Gains	$ 22
Gross Unrealized Losses
Fair Value	$ 14,000
Certificates of deposit
Available for sale securities
Amortized Cost		$ 2,176
Gross Unrealized Gains
Gross Unrealized Losses
Fair Value		$ 2,176

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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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