SECURITIES (Securities available for sale) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	$ 108,020	$ 29,765
Gross Unrealized Losses	(26,853)	(28,402)
Equity securities with a readily determinable fair value	19,569	18,435
Amortized, Total	4,869,531	3,311,433
Estimated Fair Value, Total	4,950,698	3,312,796
Total debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,849,962	3,292,998
Gross Unrealized Gains	108,020	29,765
Gross Unrealized Losses	(26,853)	(28,402)
Securities available for sale	4,931,129	3,294,361
U.S. government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,185	9,844
Gross Unrealized Gains	87	49
Gross Unrealized Losses	0	0
Securities available for sale	3,272	9,893
Mortgage and asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,719,747	2,579,081
Gross Unrealized Gains	46,959	17,200
Gross Unrealized Losses	(24,198)	(19,003)
Securities available for sale	3,742,508	2,577,278
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,127,030	704,073
Gross Unrealized Gains	60,974	12,516
Gross Unrealized Losses	(2,655)	(9,399)
Securities available for sale	$ 1,185,349	$ 707,190

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Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value.

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