Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Available for sale securities
Amortized Cost	$ 878,694,000	$ 696,189,000
Gross Unrealized Gains	15,990,000	10,347,000
Gross Unrealized Losses	3,963,000	9,674,000
Fair Value	890,721,000	696,862,000
Held to maturity securities
Total Investment Securities	578,166,000	607,643,000
Gross Unrealized Gains	10,802,000	8,092,000
Gross Unrealized Losses	1,698,000	3,802,000
Fair Value	587,270,000	611,933,000
Amortized Cost	1,456,860,000	1,303,832,000
Gross Unrealized Gains	26,792,000	18,439,000
Gross Unrealized Losses	5,661,000	13,476,000
Fair Value	1,477,991,000	1,308,795,000
U.S. Treasury
Available for sale securities
Amortized Cost	250,000
Fair Value	250,000
U.S. Government-sponsored agency securities
Available for sale securities
Amortized Cost		100,000
Gross Unrealized Gains
Gross Unrealized Losses
Fair Value		100,000
State and municipal
Available for sale securities
Amortized Cost	452,666,000	360,779,000
Gross Unrealized Gains	13,748,000	8,443,000
Gross Unrealized Losses	2,006,000	5,564,000
Fair Value	464,408,000	363,658,000
Held to maturity securities
Total Investment Securities	237,257,000	224,811,000
Gross Unrealized Gains	5,935,000	3,136,000
Gross Unrealized Losses	575,000	1,796,000
Fair Value	242,617,000	226,151,000
U.S. Government-sponsored mortgage-backed securities
Available for sale securities
Amortized Cost	423,390,000	313,459,000
Gross Unrealized Gains	2,242,000	1,904,000
Gross Unrealized Losses	1,957,000	3,071,000
Fair Value	423,675,000	312,292,000
Held to maturity securities
Total Investment Securities	317,291,000	360,213,000
Gross Unrealized Gains	4,867,000	4,956,000
Gross Unrealized Losses	812,000	1,527,000
Fair Value	321,346,000	363,642,000
Corporate obligations
Available for sale securities
Amortized Cost	31,000	31,000
Gross Unrealized Gains
Gross Unrealized Losses
Fair Value	31,000	31,000
Equity securities
Available for sale securities
Amortized Cost	2,357,000	21,820,000
Gross Unrealized Gains
Gross Unrealized Losses		1,039,000
Fair Value	2,357,000	20,781,000
U.S. Government-sponsored agency securities
Available for sale securities
Gross Unrealized Losses	0
Held to maturity securities
Total Investment Securities	22,618,000	22,619,000
Gross Unrealized Gains
Gross Unrealized Losses	311,000	479,000
Fair Value	22,307,000	$ 22,140,000
Foreign Investments
Held to maturity securities
Total Investment Securities	1,000,000
Fair Value	$ 1,000,000

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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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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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