CHANGE IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Components of Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income taxes	$ (15,902)	$ (9,893)		$ (32,253)	$ (27,569)
Realized gains on securities transactions	0	0		78	21
Interest and dividends on investment securities	141,262	75,415		383,742	215,179
Income before taxes	69,497	48,828		134,747	132,594
Net income attributable to WSFS	53,882	38,935		103,105	105,025
Prior service cost (credit), tax adjustment			$ 100
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net income attributable to WSFS	(96)	(144)		(383)	(375)
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	8	(7)		25	(3)
Prior service costs (credits)	(19)	(19)		(57)	21
Actuarial gains	(16)	(11)		(47)	(34)
Income before taxes	(35)	(30)		(104)	(13)
Net income attributable to WSFS	(27)	(37)		(79)	(16)
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]
Amortization of net unrealized gains to income during the period	(91)	(142)
Income taxes	22	35		77	108
Interest and dividends on investment securities				(322)	(451)
Net income attributable to WSFS	$ (69)	$ (107)		(245)	(343)
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]
Income taxes				19	5
Realized gains on securities transactions				(78)	(21)
Net income attributable to WSFS				$ (59)	$ (16)

Amount of unrealized and realized gain (loss) on investment in debt and equity securities.

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