Investment Securities - Amortized Cost and Approximate Fair Values of Securities (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Total available for sale
Amortized Cost	$ 1,807,389,000	$ 1,740,420,000
Gross Unrealized Gains	112,079,000	51,816,000
Gross Unrealized Losses	349,000	2,211,000
Fair Value	1,919,119,000	1,790,025,000
Total held to maturity
Amortized Cost	1,227,668,000	806,038,000
Gross Unrealized Gains	52,877,000	22,073,000
Gross Unrealized Losses	252,000	545,000
Fair Value	1,280,293,000	827,566,000
Total Investment Securities
Amortized Cost	3,035,057,000	2,546,458,000
Gross Unrealized Gains	164,956,000	73,889,000
Gross Unrealized Losses	601,000	2,756,000
Fair Value	3,199,412,000	2,617,591,000
U.S. Government-sponsored agency securities
Total available for sale
Amortized Cost	2,380,000	38,529,000
Gross Unrealized Gains	50,000	346,000
Gross Unrealized Losses	0	0
Fair Value	2,430,000	38,875,000
Total held to maturity
Amortized Cost	31,087,000	15,619,000
Gross Unrealized Gains	10,000	1,000
Gross Unrealized Losses	113,000	37,000
Fair Value	30,984,000	15,583,000
State and municipal
Total available for sale
Amortized Cost	1,168,711,000	859,511,000
Gross Unrealized Gains	89,420,000	41,092,000
Gross Unrealized Losses	246,000	807,000
Fair Value	1,257,885,000	899,796,000
Total held to maturity
Amortized Cost	619,927,000	354,115,000
Gross Unrealized Gains	34,978,000	15,151,000
Gross Unrealized Losses	32,000	107,000
Fair Value	654,873,000	369,159,000
U.S. Government-sponsored mortgage-backed securities
Total available for sale
Amortized Cost	632,267,000	842,349,000
Gross Unrealized Gains	22,505,000	10,378,000
Gross Unrealized Losses	103,000	1,404,000
Fair Value	654,669,000	851,323,000
Total held to maturity
Amortized Cost	575,154,000	434,804,000
Gross Unrealized Gains	17,889,000	6,921,000
Gross Unrealized Losses	107,000	401,000
Fair Value	592,936,000	441,324,000
Corporate obligations
Total available for sale
Amortized Cost	4,031,000	31,000
Gross Unrealized Gains	104,000	0
Gross Unrealized Losses	0	0
Fair Value	4,135,000	31,000
Foreign investment
Total held to maturity
Amortized Cost	1,500,000	1,500,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 1,500,000	$ 1,500,000

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