CHANGE IN ACCUMULATED OTHER COMPREHENSIVE (LOSS) INCOME - Components of Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions	$ 329	$ 693
Income taxes	(21,407)	(1,288)
Income before taxes	86,548	11,855
Net income attributable to WSFS	65,082	10,927
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income attributable to WSFS	(393)	(624)
Reclassification out of Accumulated Other Comprehensive Income | Net change in defined benefit plan
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes	5	10
Prior service costs (credits)	(22)	(19)
Actuarial losses (gains)	1	(23)
Income before taxes	(21)	(42)
Net income attributable to WSFS	(16)	(32)
Reclassification out of Accumulated Other Comprehensive Income | Net unrealized gains on terminated cash flow hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes	35	0
Amortization of net unrealized gains to income during the period	(146)	0
Net income attributable to WSFS	(111)	0
Reclassification out of Accumulated Other Comprehensive Income | Realized Gains on Securities Transactions | Net change in investment securities available-for-sale
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions	(329)	(693)
Income taxes	79	166
Net income attributable to WSFS	(250)	(527)
Reclassification out of Accumulated Other Comprehensive Income | Amortization of Net Unrealized Gains to Income During Period | Net change in investment securities available-for-sale
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes	5	20
Amortization of net unrealized gains to income during the period	(21)	(85)
Net income attributable to WSFS	$ (16)	$ (65)