Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Available for sale securities
Amortized Cost	$ 715,055,000	$ 696,189,000
Gross Unrealized Gains	22,835,000	10,347,000
Gross Unrealized Losses	7,102,000	9,674,000
Fair Value	730,788,000	696,862,000
Held to maturity securities
Total Investment Securities	596,429,000	607,643,000
Gross Unrealized Gains	9,037,000	8,092,000
Gross Unrealized Losses	3,375,000	3,802,000
Fair Value	602,091,000	611,933,000
Amortized Cost	1,311,484,000	1,303,832,000
Gross Unrealized Gains	31,872,000	18,439,000
Gross Unrealized Losses	10,477,000	13,476,000
Fair Value	1,332,879,000	1,308,795,000
U.S. Treasury
Available for sale securities
Amortized Cost	747,000
Fair Value	747,000
U.S. Government-sponsored agency securities
Available for sale securities
Amortized Cost		100,000
Gross Unrealized Gains
Gross Unrealized Losses	0
Fair Value		100,000
Held to maturity securities
Total Investment Securities		22,619,000
Gross Unrealized Losses	458,000	479,000
Fair Value		22,140,000
State and municipal
Available for sale securities
Amortized Cost	368,545,000	360,779,000
Gross Unrealized Gains	9,345,000	8,443,000
Gross Unrealized Losses	4,153,000	5,564,000
Fair Value	373,737,000	363,658,000
Held to maturity securities
Total Investment Securities	231,129,000	224,811,000
Gross Unrealized Gains	4,096,000	3,136,000
Gross Unrealized Losses	1,386,000	1,796,000
Fair Value	233,839,000	226,151,000
U.S. Government-sponsored mortgage-backed securities
Available for sale securities
Amortized Cost	323,912,000	313,459,000
Gross Unrealized Gains	1,736,000	1,904,000
Gross Unrealized Losses	2,949,000	3,071,000
Fair Value	322,699,000	312,292,000
Held to maturity securities
Total Investment Securities	342,682,000	360,213,000
Gross Unrealized Gains	4,941,000	4,956,000
Gross Unrealized Losses	1,531,000	1,527,000
Fair Value	346,092,000	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	21,820,000	21,820,000
Gross Unrealized Gains	11,754,000
Gross Unrealized Losses		1,039,000
Fair Value	33,574,000	$ 20,781,000
U.S. Government-sponsored agency securities
Held to maturity securities
Total Investment Securities	22,618,000
Gross Unrealized Gains
Gross Unrealized Losses	458,000
Fair Value	$ 22,160,000