CHANGE IN ACCUMULATED OTHER COMPREHENSIVE INCOME - Components of Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gains on securities transactions	$ 1,908	$ 63	$ 2,601	$ 78
Income taxes	2,247	(10,091)	959	(16,351)
(Loss) income before taxes	(10,058)	46,060	1,797	65,250
Net (loss) income attributable to WSFS	(7,111)	36,200	3,816	49,223
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net (loss) income attributable to WSFS	(1,439)	(176)	(2,063)	(320)
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	10	(9)	21	(17)
Prior service costs (credits)	(19)	(19)	(38)	(38)
Actuarial gains	(24)	(16)	(48)	(31)
(Loss) income before taxes	(43)	(35)	(86)	(69)
Net (loss) income attributable to WSFS	(33)	(44)	(65)	(86)
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	(67)	0
Actuarial gains	279	0	279	0
Net (loss) income attributable to WSFS	212	0	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	35	0	35	0
Amortization of net unrealized gains to income during the period	(146)	0	(146)	0
Net (loss) income attributable to WSFS	(111)	0	(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	(1,908)	(63)	(2,601)	(78)
Income taxes	458	15	624	19
Net (loss) income attributable to WSFS	(1,450)	(48)	(1,977)	(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	18	27	38	56
Amortization of net unrealized gains to income during the period	(75)	(111)	(160)	(231)
Net (loss) income attributable to WSFS	$ (57)	$ (84)	$ (122)	$ (175)