COMPENSATION AND BENEFIT PLANS (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) item shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Contribution by employer (as a percent)	9.30%
Employee's contribution (as a percent)	5.70%
Benefit plan liability	$ 1,915,000	$ 1,876,000	$ 1,837,000	$ 1,536,000
Gain on settlements	43,000
Contributions expenses	$ 2,500,000	$ 2,300,000	2,100,000
Allocated shares | shares	178,078	213,152
Shares committed to be allocated | shares	230,724	106,575
Unallocated shares | shares	4,064,008	1,811,788
Total shares | shares	4,472,810	2,131,515
Fair value of unallocated shares, end of period	$ 44,663,448	$ 16,035,000
ESOP compensation expense	$ 2,655,000	1,097,000	1,143,000
ESOP restoration benefit payable period	six months and a day
Restoration accrual	$ 210,000	24,000	182,000
Through 2035
Defined Benefit Plan Disclosure [Line Items]
Shares committed to be allocated | shares	230,724
From 2036 to 2038
Defined Benefit Plan Disclosure [Line Items]
Shares committed to be allocated | shares	124,148
Executive
Defined Benefit Plan Disclosure [Line Items]
Expense amount	$ 82,000
Split-dollar Life Insurance Arrangements
Defined Benefit Plan Disclosure [Line Items]
Expense amount	$ 38,000	73,000	106,000
Deferred Compensation Plan
Defined Benefit Plan Disclosure [Line Items]
Number of executive officers | item	1
Interest rate (as a percent)	1.50%
Expense amount	$ 31,000	29,000	28,000
Eligible age for medical insurance plan	65 years
Deferred Compensation Plan | Coastway
Defined Benefit Plan Disclosure [Line Items]
Expense amount		46,000
Deferred Compensation Plan | Other Liabilities And Accrued Expenses
Defined Benefit Plan Disclosure [Line Items]
Employer obligation	$ 353,000	322,000	293,000
Deferred Compensation Plan | Other Liabilities And Accrued Expenses | Coastway
Defined Benefit Plan Disclosure [Line Items]
Benefit plan liability		1,500,000
Deferred Compensation Plan | Other assets | Coastway
Defined Benefit Plan Disclosure [Line Items]
Plan Assets		1,500,000
Supplemental medical and prescription drug
Defined Benefit Plan Disclosure [Line Items]
Number of executive officers | item	2
Expense amount	$ 66,000	3,000	10,000
Supplemental medical and prescription drug | Other Liabilities And Accrued Expenses
Defined Benefit Plan Disclosure [Line Items]
Employer obligation	297,000	229,000	226,000
Management Incentive Program
Defined Benefit Plan Disclosure [Line Items]
Expense amount	2,100,000	3,300,000	2,000,000
Supplemental Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Contributions expenses	$ 1,800,000	658,000	933,000
Number of SERP agreements | item	2
Supplemental Retirement Plans | Coastway
Defined Benefit Plan Disclosure [Line Items]
Expense amount		90,000
Supplemental Retirement Plans | Other Liabilities And Accrued Expenses
Defined Benefit Plan Disclosure [Line Items]
Employer obligation	$ 7,100,000	5,300,000	4,700,000
Supplemental Retirement Plans | Other Liabilities And Accrued Expenses | Coastway
Defined Benefit Plan Disclosure [Line Items]
Employer obligation		3,400,000
Supplemental Retirement Plans | Executive
Defined Benefit Plan Disclosure [Line Items]
Number of executive officers | item	2
Long-Term Incentive Plan
Defined Benefit Plan Disclosure [Line Items]
Expense amount		56,000	453,000
Long-Term Incentive Plan | Other Liabilities And Accrued Expenses
Defined Benefit Plan Disclosure [Line Items]
Employer obligation	$ 838,000	1,300,000	1,300,000
Post-Retirement Life Insurance
Defined Benefit Plan Disclosure [Line Items]
Expense amount	17,000	29,000	7,000
Post-Retirement Life Insurance | Other Liabilities And Accrued Expenses
Defined Benefit Plan Disclosure [Line Items]
Employer obligation	$ 20,000	$ 171,000	$ 142,000
Directors Retirement Plan | Maximum
Defined Benefit Plan Disclosure [Line Items]
Vesting period		6 years
Defined benefit pension plan | Coastway
Defined Benefit Plan Disclosure [Line Items]
Plan Assets		$ 1,400,000
Benefit plan liability		1,600,000
Defined benefit pension plan | Unfunded Plan | Other Liabilities And Accrued Expenses | Coastway
Defined Benefit Plan Disclosure [Line Items]
Benefit plan liability		$ 202,000