STOCKHOLDERS' EQUITY AND REGULATORY CAPITAL - Schedule of Capital Position (Details) $ in Thousands	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Capital Unit [Line Items]
Total Capital (to risk-weighted assets), Consolidated Capital Amount	$ 2,426,577	$ 2,219,920
Tier 1 Capital (to risk-weighted assets), Consolidated Capital Amount	2,098,403	1,910,195
Common Equity Tier 1 Capital (to risk-weighted assets), Consolidated Bank Capital Amount	2,098,403	1,910,195
Tier 1 Capital (to adjusted tangible assets), Consolidated Capital Amount	$ 2,098,403	$ 1,910,195
Total Capital (to risk-weighted assets), Consolidated Capital Percent	0.1523	0.1420
Tier 1 Capital (to risk-weighted assets), Consolidated Capital Percent	0.1317	0.1222
Common Equity Tier 1 Capital (to risk-weighted assets), Consolidated Capital Percent	13.17%	12.22%
Tier 1 Leverage Capital , Consolidated Capital Percent	0.1048	0.0979
Total Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Amount	$ 1,274,611	$ 1,250,689
Tier 1 Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Amount	955,958	938,017
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Amount	716,969	703,512
Tier 1 Leverage Capital, Minimum For Capital Adequacy Purposes Amount	$ 800,934	$ 780,333
Total Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Percentage	0.0800	0.0800
Tier 1 Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Percentage	0.0600	0.0600
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Percentage	4.50%	4.50%
Tier 1 Leverage Capital, Minimum For Capital Adequacy Purposes Percentage	0.0400	0.0400
Total Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Amount	$ 1,593,264	$ 1,563,361
Tier 1 Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Amount	1,274,611	1,250,689
Common Equity Tier 1 Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Amount	1,035,621	1,016,185
Tier 1 Leverage Capital, To Be Well-Capitalized Under Prompt Corrective Action Provisions Amount	$ 1,001,168	$ 975,416
Total Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Percentage	0.1000	0.1000
Tier 1 Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Percentage	0.0800	0.0800
Common Equity Tier 1 Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Percentage	6.50%	6.50%
Tier 1 Leverage Capital, To Be Well-Capitalized Under Prompt Corrective Action Provisions Percentage	0.0500	0.0500
Wilmington Savings Fund Society, FSB
Capital Unit [Line Items]
Total Capital (to risk-weighted assets), Consolidated Capital Amount	$ 2,382,514	$ 2,157,846
Tier 1 Capital (to risk-weighted assets), Consolidated Capital Amount	2,184,193	2,003,779
Common Equity Tier 1 Capital (to risk-weighted assets), Consolidated Bank Capital Amount	2,184,193	2,003,779
Tier 1 Capital (to adjusted tangible assets), Consolidated Capital Amount	$ 2,184,193	$ 2,003,779
Total Capital (to risk-weighted assets), Consolidated Capital Percent	0.1496	0.1384
Tier 1 Capital (to risk-weighted assets), Consolidated Capital Percent	0.1372	0.1286
Common Equity Tier 1 Capital (to risk-weighted assets), Consolidated Capital Percent	13.72%	12.86%
Tier 1 Leverage Capital , Consolidated Capital Percent	0.1092	0.1029
Total Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Amount	$ 1,273,856	$ 1,246,886
Tier 1 Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Amount	955,392	935,165
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Amount	716,544	701,373
Tier 1 Leverage Capital, Minimum For Capital Adequacy Purposes Amount	$ 800,021	$ 779,288
Total Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Percentage	0.0800	0.0800
Tier 1 Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Percentage	0.0600	0.0600
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Percentage	4.50%	4.50%
Tier 1 Leverage Capital, Minimum For Capital Adequacy Purposes Percentage	0.0400	0.0400
Total Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Amount	$ 1,592,320	$ 1,558,608
Tier 1 Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Amount	1,273,856	1,246,886
Common Equity Tier 1 Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Amount	1,035,008	1,013,095
Tier 1 Leverage Capital, To Be Well-Capitalized Under Prompt Corrective Action Provisions Amount	$ 1,000,026	$ 974,110
Total Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Percentage	0.1000	0.1000
Tier 1 Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Percentage	0.0800	0.0800
Common Equity Tier 1 Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Percentage	6.50%	6.50%
Tier 1 Leverage Capital, To Be Well-Capitalized Under Prompt Corrective Action Provisions Percentage	0.0500	0.0500