Investment Securities - Amortized Cost and Approximate Fair Values of Securities (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Total available for sale
Amortized Cost	$ 986,565,000	$ 696,189,000
Gross Unrealized Gains	17,896,000	10,347,000
Gross Unrealized Losses	4,514,000	9,674,000
Investment securities available for sale	999,947,000	696,862,000
Total held to maturity
Amortized Cost	560,655,000	607,643,000
Gross Unrealized Gains	9,636,000	8,092,000
Gross Unrealized Losses	2,083,000	3,802,000
Fair Value	568,208,000	611,933,000
Total Investment Securities
Amortized Cost	1,547,220,000	1,303,832,000
Gross Unrealized Gains	27,532,000	18,439,000
Gross Unrealized Losses	6,597,000	13,476,000
Fair Value	1,568,155,000	1,308,795,000
State and municipal
Total available for sale
Amortized Cost	510,852,000	360,779,000
Gross Unrealized Gains	16,932,000	8,443,000
Gross Unrealized Losses	1,091,000	5,564,000
Investment securities available for sale	526,693,000	363,658,000
Total held to maturity
Amortized Cost	235,594,000	224,811,000
Gross Unrealized Gains	6,295,000	3,136,000
Gross Unrealized Losses	244,000	1,796,000
Fair Value	241,645,000	226,151,000
U.S. Government-sponsored mortgage-backed securities
Total available for sale
Amortized Cost	473,325,000	313,459,000
Gross Unrealized Gains	964,000	1,904,000
Gross Unrealized Losses	3,423,000	3,071,000
Investment securities available for sale	470,866,000	312,292,000
Total held to maturity
Amortized Cost	301,443,000	360,213,000
Gross Unrealized Gains	3,341,000	4,956,000
Gross Unrealized Losses	1,404,000	1,527,000
Fair Value	303,380,000	363,642,000
Corporate obligations
Total available for sale
Amortized Cost	31,000	31,000
Gross Unrealized Gains
Gross Unrealized Losses
Investment securities available for sale	31,000	31,000
Equity securities
Total available for sale
Amortized Cost	2,357,000	21,820,000
Gross Unrealized Gains
Gross Unrealized Losses		1,039,000
Investment securities available for sale	2,357,000	20,781,000
U.S. Government-sponsored agency securities
Total available for sale
Amortized Cost		100,000
Gross Unrealized Gains
Gross Unrealized Losses	0
Investment securities available for sale		100,000
Total held to maturity
Amortized Cost	22,618,000	22,619,000
Gross Unrealized Losses	435,000	479,000
Fair Value	22,183,000	$ 22,140,000
Foreign Investment
Total held to maturity
Amortized Cost	1,000,000
Fair Value	$ 1,000,000