SECURITIES AVAILABLE FOR SALE, HELD TO MATURITY, AND MARKETABLE EQUITY SECURITIES - Summary of Securities Available for Sale and Held to Maturity (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2020	Dec. 31, 2019	Jun. 30, 2020
Securities available for sale
Amortized Cost	$ 1,532,560	$ 1,297,339
Gross Unrealized Gains	43,636	16,503
Gross Unrealized Losses	(907)	(2,287)
Securities available for sale, at fair value	1,575,289	1,311,555
Allowance	0	0
Securities held to maturity
Amortized Cost	330,197	357,979
Gross Unrealized Gains	25,847	16,263
Gross Unrealized Losses	(9)	(965)
Fair Value	356,035	373,277
Allowance	(96)	0	$ (113)
Marketable equity securities, amortized cost	34,179	37,138
Marketable equity securities, gross unrealized gains	1,480	5,147
Marketable equity securities, gross unrealized losses	(3,666)	(729)
Marketable equity securities, fair value	31,993	41,556
Total, amortized cost basis	1,896,936	1,692,456
Total, gross unrealized gain	70,963	37,913
Total, gross unrealized loss	(4,582)	(3,981)
Total, fair value	1,963,317	1,726,388
Total, Allowance	96	0
Municipal bonds and obligations
Securities available for sale
Amortized Cost	97,463	104,325
Gross Unrealized Gains	7,057	5,813
Gross Unrealized Losses	(13)	0
Securities available for sale, at fair value	104,507	110,138
Allowance	0	0
Securities held to maturity
Amortized Cost	242,666	252,936
Gross Unrealized Gains	18,401	13,095
Gross Unrealized Losses	0	(5)
Fair Value	261,067	266,026
Allowance	(67)	0	(62)
Agency collateralized mortgage obligations
Securities available for sale
Amortized Cost	722,124	742,550
Gross Unrealized Gains	19,469	6,431
Gross Unrealized Losses	(124)	(169)
Securities available for sale, at fair value	741,469	748,812
Allowance	0	0
Securities held to maturity
Amortized Cost	68,124	69,667
Gross Unrealized Gains	6,535	2,870
Gross Unrealized Losses	0	(50)
Fair Value	74,659	72,487
Allowance	0	0
Agency mortgage-backed securities
Securities available for sale
Amortized Cost	322,252	146,589
Gross Unrealized Gains	4,202	1,515
Gross Unrealized Losses	(437)	(360)
Securities available for sale, at fair value	326,017	147,744
Allowance	0	0
Securities held to maturity
Amortized Cost	5,280	6,271
Gross Unrealized Gains	208	29
Gross Unrealized Losses	0	0
Fair Value	5,488	6,300
Allowance	0	0
Agency commercial mortgage-backed securities
Securities available for sale
Amortized Cost	263,406	148,066
Gross Unrealized Gains	10,304	176
Gross Unrealized Losses	(102)	(1,146)
Securities available for sale, at fair value	273,608	147,096
Allowance	0	0
Securities held to maturity
Amortized Cost	10,305	10,353
Gross Unrealized Gains	671	51
Gross Unrealized Losses	0	0
Fair Value	10,976	10,404
Allowance	0	0
Corporate bonds
Securities available for sale
Amortized Cost	74,357	115,395
Gross Unrealized Gains	842	1,788
Gross Unrealized Losses	(222)	(607)
Securities available for sale, at fair value	74,977	116,576
Allowance	0	0
Tax advantaged economic development bonds
Securities held to maturity
Amortized Cost	3,527	18,456
Gross Unrealized Gains	32	218
Gross Unrealized Losses	(9)	(910)
Fair Value	3,550	17,764
Allowance	(29)	0	$ (51)
Other bonds and obligations
Securities available for sale
Amortized Cost	52,958	40,414
Gross Unrealized Gains	1,762	780
Gross Unrealized Losses	(9)	(5)
Securities available for sale, at fair value	54,711	41,189
Allowance	0	0
Securities held to maturity
Amortized Cost	295	296
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	295	296
Allowance	$ 0	$ 0

Debt Securities, Available-for-sale And Held-to-maturity, Allowance For Credit Losses

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Debt Securities, Available-for-sale and Held-to-maturity, and Equity Securities, FV-NI

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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