SECURITIES (Schedule of Securities) (Details) - USD ($)	Sep. 30, 2020	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019
Schedule of Securities [Line Items]
Securities—trading, Amortized Cost	$ 27,203,000			$ 27,203,000
Securities—trading	23,276,000			25,636,000
Available-for-sale Securities, Amortized Cost	1,690,069,000			1,529,946,000
Available-for-sale Securities, Gross Unrealized Gains	70,397,000			25,858,000
Available-for-sale Securities, Gross Unrealized Losses	(2,082,000)			(4,247,000)
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions	0
Securities—available-for-sale	1,758,384,000			1,551,557,000
Held-to-maturity Securities, Amortized Cost	429,135,000			236,094,000
Held-to-maturity Securities, Gross Unrealized Gains	22,277,000			4,658,000
Held-to-maturity Securities, Gross Unrealized Losses	606,000			2,947,000
Securities—held-to-maturity	450,806,000			237,805,000
Impact of adopting ASC 326	(102,000)	$ (102,000)	$ 0	0
U.S. Government and agency obligations
Schedule of Securities [Line Items]
Available-for-sale Securities, Amortized Cost	169,740,000			90,468,000
Available-for-sale Securities, Gross Unrealized Gains	1,140,000			286,000
Available-for-sale Securities, Gross Unrealized Losses	(967,000)			(1,156,000)
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions	0
Securities—available-for-sale	169,913,000			89,598,000
Held-to-maturity Securities, Amortized Cost	341,000			385,000
Held-to-maturity Securities, Gross Unrealized Gains	10,000			4,000
Held-to-maturity Securities, Gross Unrealized Losses	0			0
Securities—held-to-maturity	351,000			389,000
Impact of adopting ASC 326	0	0	0	0
Municipal bonds
Schedule of Securities [Line Items]
Available-for-sale Securities, Amortized Cost	257,056,000			101,927,000
Available-for-sale Securities, Gross Unrealized Gains	15,707,000			5,233,000
Available-for-sale Securities, Gross Unrealized Losses	(355,000)			(3,000)
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions	0
Securities—available-for-sale	272,408,000			107,157,000
Held-to-maturity Securities, Amortized Cost	377,019,000			177,208,000
Held-to-maturity Securities, Gross Unrealized Gains	19,260,000			3,733,000
Held-to-maturity Securities, Gross Unrealized Losses	593,000			2,213,000
Securities—held-to-maturity	395,686,000			178,728,000
Impact of adopting ASC 326	(60,000)	(61,000)	0	0
Corporate bonds
Schedule of Securities [Line Items]
Securities—trading, Amortized Cost	27,203,000			27,203,000
Securities—trading	23,276,000			25,636,000
Available-for-sale Securities, Amortized Cost	53,756,000			4,357,000
Available-for-sale Securities, Gross Unrealized Gains	2,300,000			14,000
Available-for-sale Securities, Gross Unrealized Losses	(85,000)			(6,000)
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions	0
Securities—available-for-sale	55,971,000			4,365,000
Held-to-maturity Securities, Amortized Cost	3,254,000			3,353,000
Held-to-maturity Securities, Gross Unrealized Gains	0			0
Held-to-maturity Securities, Gross Unrealized Losses	13,000			11,000
Securities—held-to-maturity	3,241,000			3,342,000
Impact of adopting ASC 326	(42,000)	(41,000)	0	0
Mortgage-backed or related securities
Schedule of Securities [Line Items]
Available-for-sale Securities, Amortized Cost	1,199,816,000			1,324,999,000
Available-for-sale Securities, Gross Unrealized Gains	51,199,000			20,325,000
Available-for-sale Securities, Gross Unrealized Losses	(515,000)			(3,013,000)
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions	0
Securities—available-for-sale	1,250,500,000			1,342,311,000
Held-to-maturity Securities, Amortized Cost	48,521,000			55,148,000
Held-to-maturity Securities, Gross Unrealized Gains	3,007,000			921,000
Held-to-maturity Securities, Gross Unrealized Losses	0			723,000
Securities—held-to-maturity	51,528,000			55,346,000
Impact of adopting ASC 326	0	$ 0	$ 0	0
Asset-backed Securities
Schedule of Securities [Line Items]
Available-for-sale Securities, Amortized Cost	9,701,000			8,195,000
Available-for-sale Securities, Gross Unrealized Gains	51,000			0
Available-for-sale Securities, Gross Unrealized Losses	(160,000)			(69,000)
Debt Securities, Available-for-sale, Unrealized Loss Position, Number of Positions	0
Securities—available-for-sale	$ 9,592,000			$ 8,126,000