STOCKHOLDERS' EQUITY AND REGULATORY CAPITAL - Capital Position (Details) $ in Thousands	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Capital Unit [Line Items]
Total Capital (to risk-weighted assets), Consolidated Capital Amount	$ 2,575,170	$ 2,426,577
Tier 1 Capital (to risk-weighted assets), Consolidated Capital Amount	2,254,907	2,098,403
Common Equity Tier 1 Capital (to risk-weighted assets), Consolidated Bank Capital Amount	2,254,907	2,098,403
Tier 1 Capital (to adjusted tangible assets), Consolidated Capital Amount	$ 2,254,907	$ 2,098,403
Total Capital (to risk-weighted assets), Consolidated Capital Percent	0.1577	0.1523
Tier 1 Capital (to risk-weighted assets), Consolidated Capital Percent	0.1381	0.1317
Common Equity Tier 1 Capital (to risk-weighted assets), Consolidated Capital Percent	13.81%	13.17%
Tier 1 Leverage Capital , Consolidated Capital Percent	0.1096	0.1048
Total Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Amount	$ 1,306,677	$ 1,274,611
Tier 1 Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Amount	980,008	955,958
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Amount	735,006	716,969
Tier 1 Leverage Capital, Minimum For Capital Adequacy Purposes Amount	$ 822,637	$ 800,934
Total Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Percent	0.0800	0.0800
Tier 1 Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Percent	0.0600	0.0600
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Percent	4.50%	4.50%
Tier 1 Leverage Capital, Minimum For Capital Adequacy Purposes Percent	0.0400	0.0400
Total Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Amount	$ 1,633,346	$ 1,593,264
Tier 1 Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Amount	1,306,677	1,274,611
Common Equity Tier 1 Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Amount	1,061,675	1,035,621
Tier 1 Leverage Capital, To Be Well-Capitalized Under Prompt Corrective Action Provisions Amount	$ 1,028,296	$ 1,001,168
Total Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Percent	0.1000	0.1000
Tier 1 Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Percent	0.0800	0.0800
Common Equity Tier 1 Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Percent	6.50%	6.50%
Tier 1 Leverage Capital, To Be Well-Capitalized Under Prompt Corrective Action Provisions Percent	0.0500	0.0500
Wilmington Savings Fund Society, FSB
Capital Unit [Line Items]
Total Capital (to risk-weighted assets), Consolidated Capital Amount	$ 2,470,183	$ 2,382,514
Tier 1 Capital (to risk-weighted assets), Consolidated Capital Amount	2,265,995	2,184,193
Common Equity Tier 1 Capital (to risk-weighted assets), Consolidated Bank Capital Amount	2,265,995	2,184,193
Tier 1 Capital (to adjusted tangible assets), Consolidated Capital Amount	$ 2,265,995	$ 2,184,193
Total Capital (to risk-weighted assets), Consolidated Capital Percent	0.1513	0.1496
Tier 1 Capital (to risk-weighted assets), Consolidated Capital Percent	0.1388	0.1372
Common Equity Tier 1 Capital (to risk-weighted assets), Consolidated Capital Percent	13.88%	13.72%
Tier 1 Leverage Capital , Consolidated Capital Percent	0.1103	0.1092
Total Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Amount	$ 1,306,507	$ 1,273,856
Tier 1 Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Amount	979,880	955,392
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Amount	734,910	716,544
Tier 1 Leverage Capital, Minimum For Capital Adequacy Purposes Amount	$ 822,045	$ 800,021
Total Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Percent	0.0800	0.0800
Tier 1 Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Percent	0.0600	0.0600
Common Equity Tier 1 Capital (to risk-weighted assets), Minimum For Capital Adequacy Purposes Percent	4.50%	4.50%
Tier 1 Leverage Capital, Minimum For Capital Adequacy Purposes Percent	0.0400	0.0400
Total Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Amount	$ 1,633,133	$ 1,592,320
Tier 1 Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Amount	1,306,507	1,273,856
Common Equity Tier 1 Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Amount	1,061,537	1,035,008
Tier 1 Leverage Capital, To Be Well-Capitalized Under Prompt Corrective Action Provisions Amount	$ 1,027,556	$ 1,000,026
Total Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Percent	0.1000	0.1000
Tier 1 Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Percent	0.0800	0.0800
Common Equity Tier 1 Capital (to risk-weighted assets), To Be Well-Capitalized Under Prompt Corrective Action Provisions Percent	6.50%	6.50%
Tier 1 Leverage Capital, To Be Well-Capitalized Under Prompt Corrective Action Provisions Percent	0.0500	0.0500