INVESTMENT SECURITIES - Narrative (Details)		3 Months Ended		9 Months Ended
		Sep. 30, 2020 USD ($) security	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) security	Sep. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($) security	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, realized gain | $		$ 0	$ 0	$ 0	$ 0
Debt securities, available-for-sale, realized loss | $		0	0	0	0
Equity securities, at fair value | $		12,070,000		12,070,000			$ 11,782,000
Cumulative effect of change in accounting principal | $		$ 608,833,000	605,372,000	$ 608,833,000	605,372,000	$ 596,682,000	$ 614,464,000	$ 597,467,000	$ 581,145,000
Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Available-for-sale [Line Items]
Cumulative effect of change in accounting principal | $	[1]								0
Residential MBS
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, number of positions | security		86		86			76
Debt securities, available-for-sale, unrealized loss position, number of positions | security		13		13			45
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, number of positions | security		6		6			25
Held-to-maturity securities, number of positions | security		7		7			7
Held-to-maturity securities, unrealized loss position, number of positions | security		0		0
Held-to-maturity securities, continuous unrealized loss position, 12 months or longer, number of positions | security							2
Commercial MBS and CMOs
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, number of positions | security		46		46			42
Debt securities, available-for-sale, unrealized loss position, number of positions | security		22		22			19
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, number of positions | security		9		9			8
Agency bonds
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, number of positions | security		3		3			15
Debt securities, available-for-sale, unrealized loss position, number of positions | security		2		2			12
Debt securities, available-for-sale, continuous unrealized loss position, 12 months or longer, number of positions | security		0		0			9
CRA Qualified Investment Fund
Debt Securities, Available-for-sale [Line Items]
Equity securities, at fair value | $		$ 12,100,000		$ 12,100,000			$ 11,800,000
Changes in fair value recognized in other noninterest income | $		0	89,000	288,000	399,000
Accumulated Other Comprehensive (Loss) Income (Net of Taxes) Available for Sale Securities
Debt Securities, Available-for-sale [Line Items]
Unrealized gain on available-for-sale investment securities tax liability | $		2,700,000		2,700,000			593,000
Cumulative effect of change in accounting principal | $		6,571,000	1,967,000	6,571,000	1,967,000	6,566,000	1,444,000	1,128,000	(4,935,000)
Accumulated Other Comprehensive (Loss) Income (Net of Taxes) Available for Sale Securities | Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Available-for-sale [Line Items]
Cumulative effect of change in accounting principal | $	[1]								399,000
Retained Earnings
Debt Securities, Available-for-sale [Line Items]
Cumulative effect of change in accounting principal | $		$ 187,147,000	$ 156,016,000	$ 187,147,000	$ 156,016,000	$ 175,852,000	$ 165,236,000	$ 146,514,000	129,806,000
Retained Earnings | Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Available-for-sale [Line Items]
Cumulative effect of change in accounting principal | $	[1]								$ (399,000)

[1] Represents the impact of adopting Accounting Standards Update ("ASU") 2016-01. See Note 3 to the unaudited consolidated financial statements for further information.