CHANGE IN ACCUMULATED OTHER COMPREHENSIVE INCOME - Components of Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions	$ 3,322	$ 0	$ 5,923	$ 78
Income taxes	(15,140)	(15,902)	(14,181)	(32,253)
Income before taxes	65,963	69,497	67,760	134,747
Net income attributable to WSFS	51,145	53,882	54,961	103,105
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income attributable to WSFS	(2,760)	(96)	(4,805)	(383)
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	26	8	41	25
Prior service costs (credits)	(95)	(19)	(133)	(57)
Actuarial gains	(14)	(16)	(39)	(47)
Income before taxes	(109)	(35)	(172)	(104)
Net income attributable to WSFS	(83)	(27)	(131)	(79)
Reclassification out of Accumulated Other Comprehensive Income | Defined benefit pension plan settlement
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes			(67)	0
Actuarial gains			279	0
Net income attributable to WSFS			212	0
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	36	0	71	0
Amortization of net unrealized gains to income during the period	(149)	0	(295)	0
Net income attributable to WSFS	(113)	0	(224)	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	(3,322)	0	(5,923)	(78)
Income taxes	797	0	1,422	19
Net income attributable to WSFS	(2,525)	0	(4,501)	(59)
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	12	22	50	77
Amortization of net unrealized gains to income during the period	(51)	(91)	(211)	(322)
Net income attributable to WSFS	$ (39)	$ (69)	$ (161)	$ (245)