SECURITIES - Schedule of Debt Securities Components (Details) - USD ($)		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AVAILABLE FOR SALE:
Amortized cost		$ 2,437,513,000				$ 2,306,741,000				$ 2,437,513,000	$ 2,306,741,000
Gross unrealized gain		150,074,000				54,426,000				150,074,000	54,426,000
Gross unrealized loss		282,000				2,570,000				282,000	2,570,000
Securities available for sale, at estimated fair value		2,587,305,000				2,358,597,000				2,587,305,000	2,358,597,000
HELD TO MATURITY:
Amortized cost		108,998,000								108,998,000
Amortized cost						134,863,000					134,863,000
Gross unrealized gains		9,200,000				4,238,000				9,200,000	4,238,000
Gross unrealized losses		0				222,000				0	222,000
Estimated fair value		118,198,000				138,879,000				118,198,000	138,879,000
Debt Securities, Available-for-sale, Allowance for Credit Loss, Period Increase (Decrease)										0
Debt Securities, Available-for-sale, Allowance for Credit Loss		0								0
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net, Available-for-sale Securities											0
Debt Securities, Held-to-maturity, Credit Loss Expense (Reversal)										0
Transferred Securities, Unrealized Loss, Before Tax		2,900,000				3,700,000				2,900,000	3,700,000
Securities Transferred, Unrealized Loss, Net Of Tax		2,300,000				2,900,000				2,300,000	2,900,000
Transfer of available for sale securities to held to maturity securities										0	0
Interest receivable		38,708,000				28,452,000				38,708,000	28,452,000
Debt Securities, Held-to-maturity, Past Due		0								0
Debt Securities, Held-to-maturity, Nonaccrual		0								0
Realized net (loss) gain on sale of securities		(24,000)	$ 78,000	$ 2,662,000	$ 5,541,000	42,000	$ 42,000	$ 416,000	$ 256,000	8,257,000	756,000	$ (1,839,000)
Gross realized gains on AFS securities										8,400,000	5,700,000	2,000,000.0
Gross realized loss on AFS securities										129,000	4,900,000	3,800,000
Proceeds from sale of held-to-maturity securities										0	0	$ 0
State and Political Subdivisions
AVAILABLE FOR SALE:
Amortized cost		1,475,030,000				780,376,000				1,475,030,000	780,376,000
Gross unrealized gain		105,601,000				23,832,000				105,601,000	23,832,000
Gross unrealized loss		37,000				1,406,000				37,000	1,406,000
Securities available for sale, at estimated fair value		1,580,594,000				802,802,000				1,580,594,000	802,802,000
HELD TO MATURITY:
Amortized cost		907,000								907,000
Amortized cost						2,888,000					2,888,000
Gross unrealized gains		13,000				30,000				13,000	30,000
Gross unrealized losses		0				0				0	0
Estimated fair value		920,000				2,918,000				920,000	2,918,000
Other Stocks and Bonds
AVAILABLE FOR SALE:
Amortized cost		77,224,000				10,000,000				77,224,000	10,000,000
Gross unrealized gain		1,053,000				137,000				1,053,000	137,000
Gross unrealized loss		22,000				0				22,000	0
Securities available for sale, at estimated fair value		78,255,000				10,137,000				78,255,000	10,137,000
Residential
AVAILABLE FOR SALE:
Amortized cost	[1]	771,409,000				1,286,110,000				771,409,000	1,286,110,000
Gross unrealized gain	[1]	38,674,000				25,662,000				38,674,000	25,662,000
Gross unrealized loss	[1]	73,000				1,130,000				73,000	1,130,000
Securities available for sale, at estimated fair value	[1]	810,010,000				1,310,642,000				810,010,000	1,310,642,000
HELD TO MATURITY:
Amortized cost	[1]	47,948,000								47,948,000
Amortized cost	[1]					59,701,000					59,701,000
Gross unrealized gains	[1]	4,187,000				2,586,000				4,187,000	2,586,000
Gross unrealized losses	[1]	0				139,000				0	139,000
Estimated fair value	[1]	52,135,000				62,148,000				52,135,000	62,148,000
Commercial
AVAILABLE FOR SALE:
Amortized cost	[1]	113,850,000				230,255,000				113,850,000	230,255,000
Gross unrealized gain	[1]	4,746,000				4,795,000				4,746,000	4,795,000
Gross unrealized loss	[1]	150,000				34,000				150,000	34,000
Securities available for sale, at estimated fair value	[1]	118,446,000				235,016,000				118,446,000	235,016,000
HELD TO MATURITY:
Amortized cost	[1]	60,143,000								60,143,000
Amortized cost	[1]					72,274,000					72,274,000
Gross unrealized gains	[1]	5,000,000				1,622,000				5,000,000	1,622,000
Gross unrealized losses	[1]	0				83,000				0	83,000
Estimated fair value	[1]	$ 65,143,000				$ 73,813,000				$ 65,143,000	$ 73,813,000

[1] All MBS issued and/or guaranteed by U.S. government agencies or U.S. GSEs.