INVESTMENT SECURITIES - Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 614,240	$ 623,037
Gross Unrealized Gains	10,259	3,666
Gross Unrealized Losses	(1,000)	(1,629)
Estimated Fair Value	623,499	625,074
Fair Value
Less than 12 Months	150,420	169,044
12 Months or More	63,878	199,494
Total	214,298	368,538
Unrealized Losses
Less than 12 Months	(532)	(606)
12 Months or More	(468)	(1,023)
Total	(1,000)	(1,629)
Residential mortgage backed securities ('MBS") and collateralized mortgage obligations ("CMOs")
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	231,925	145,333
Gross Unrealized Gains	3,977	340
Gross Unrealized Losses	(62)	(481)
Estimated Fair Value	235,840	145,192
Fair Value
Less than 12 Months	15,931	43,623
12 Months or More	4,620	54,870
Total	20,551	98,493
Unrealized Losses
Less than 12 Months	(59)	(181)
12 Months or More	(3)	(300)
Total	(62)	(481)
Commercial MBS and CMOs
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	366,948	353,727
Gross Unrealized Gains	6,224	3,267
Gross Unrealized Losses	(893)	(825)
Estimated Fair Value	372,279	356,169
Fair Value
Less than 12 Months	122,166	95,950
12 Months or More	59,258	57,219
Total	181,424	153,169
Unrealized Losses
Less than 12 Months	(428)	(339)
12 Months or More	(465)	(486)
Total	(893)	(825)
Agency bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	15,367	123,977
Gross Unrealized Gains	58	59
Gross Unrealized Losses	(45)	(323)
Estimated Fair Value	15,380	123,713
Fair Value
Less than 12 Months	12,323	29,471
12 Months or More	0	87,405
Total	12,323	116,876
Unrealized Losses
Less than 12 Months	(45)	(86)
12 Months or More	0	(237)
Total	$ (45)	$ (323)