Investment Securities - Amortized Cost and Approximate Fair Value of Securities (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Available for sale securities
Total Investment Securities	$ 1,732,279,000	$ 1,740,420,000
Gross Unrealized Gains	83,903,000	51,816,000
Gross Unrealized Losses	407,000	2,211,000
Fair Value	1,815,775,000	1,790,025,000
Held to maturity securities
Total Investment Securities	882,179,000	806,038,000
Gross Unrealized Gains	38,027,000	22,073,000
Gross Unrealized Losses	28,000	545,000
Fair Value	920,178,000	827,566,000
Amortized Cost	2,614,458,000	2,546,458,000
Gross Unrealized Gains	121,930,000	73,889,000
Gross Unrealized Losses	435,000	2,756,000
Fair Value	2,735,953,000	2,617,591,000
U.S. Treasury
Available for sale securities
Total Investment Securities	596,000
Gross Unrealized Gains	4,000
Gross Unrealized Losses	0
Fair Value	600,000
U.S. Government-sponsored agency securities
Available for sale securities
Total Investment Securities	12,398,000	38,529,000
Gross Unrealized Gains	80,000	346,000
Gross Unrealized Losses	0	0
Fair Value	12,478,000	38,875,000
Held to maturity securities
Total Investment Securities	35,050,000	15,619,000
Gross Unrealized Gains	8,000	1,000
Gross Unrealized Losses	0	37,000
Fair Value	35,058,000	15,583,000
State and municipal
Available for sale securities
Total Investment Securities	898,991,000	859,511,000
Gross Unrealized Gains	50,523,000	41,092,000
Gross Unrealized Losses	306,000	807,000
Fair Value	949,208,000	899,796,000
Held to maturity securities
Total Investment Securities	358,479,000	354,115,000
Gross Unrealized Gains	18,030,000	15,151,000
Gross Unrealized Losses	28,000	107,000
Fair Value	376,481,000	369,159,000
U.S. Government-sponsored mortgage-backed securities
Available for sale securities
Total Investment Securities	820,263,000	842,349,000
Gross Unrealized Gains	33,296,000	10,378,000
Gross Unrealized Losses	101,000	1,404,000
Fair Value	853,458,000	851,323,000
Held to maturity securities
Total Investment Securities	487,150,000	434,804,000
Gross Unrealized Gains	19,989,000	6,921,000
Gross Unrealized Losses	0	401,000
Fair Value	507,139,000	441,324,000
Corporate obligations
Available for sale securities
Total Investment Securities	31,000	31,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	31,000	31,000
Foreign investment
Held to maturity securities
Total Investment Securities	1,500,000	1,500,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 1,500,000	$ 1,500,000

Debt Securities, Available-for-sale and Held-to-maturity, Accumulated Gross Unrealized Gain, before Tax

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